Laser Spectroscopy

DOE/ER-0461P UC-414

LASERSPECTROSCOPY

Assessment of Research Needs for Laser Technologies Applied to Advanced Spectroscopic Methods

May 1990

Prepared for: U.S. Department of Energy Office of Energy Research Office of Program Analysis Washington, DC 20545 Under Contract No. DE-ACOI-88ER30131 DISCLAIMER

This repo~ was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof,nor any of theiremployees, make any warranty, express or implied,or assumes any legalliabilityor responsibilityfor the accuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed,or representsthat itsuse would not infringeprivatelyowned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer,or otherwise does not necessarilyconstitute or imply itsendorsement, recommendation, or favoringby the United States Government or any agency thereof.The views and opinions of authors expressed herein do not necessarilystate or reflectthose of the United States Government or any agency thereof. DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. EXPERT TECHNICAL PANEL

The following panel of scientists evaluated the research needs for laser technologies applicable to programs of the Office of Health and Environmental Research of the Office of Energy Research, Department of Energy. This project was conducted under contract with the Office of Program Analysis, Department of Energy, and was carried out under the leadership of G. Samuel Hurst (Principal Investigator) of Consultec Scientific, Inc.

Main Panel

Prof. Keith Boyer Dr. N. Omenetto Department of Physics Commission EC/JRC University of Illinois Environment Institute Chicago, IL 60680 Chemistry Division 1-21020 Ispra, ITALY Dr. Robert L. Byer Ginzton Laboratory of Physics Or. Gary Salzman Stanford University MS M880 Stanford, CA 94305 Los Alamos National Lab. P. O. BOX 1663 Or. Ira W. Levin Los Alamos, NM 87545 Bldg. 2, Room BI-27 National Institutes of Health Or.John Travis Bethesda, MD 20892 Bldg. 222, Room A223 National Institute of Dr. David M. Lubman Standards & Technology Chemistry Department Gaithersburg, MD 20899 University of Michigan 930 North University Or. Nicholas Winograd Ann Arbor, MI 48109 Department of Chemistry Pennsylvania State Univ. Or. Stephen R. Lundeen University Park, PA 16802 Department of Physics Univ. of Notre Dame Notre Dame, IN 46556

Soecial Consultants

Prof. N. Bloembergen Prof. A. L. Schawlow 231 Pierce Hall DAS Department of Physics Harvard University Stanford University 29 Oxford Street Stanford, CA 94305 Cambridge, MA 02138

Prof. Yuan T. Lee Chemistry Department University of California Berkeley, CA 94720

iii ACKNOWLEDGEMENTS

In carrying out the work described in this report, the

Consultec Scientific, Inc. staff and its expert panel interacted with a large number of researchers and research managers throughout the government, industry, and private sectors of the laser community. The assistance of these individuals in providing us with information and in cooperating with us in other ways is gratefully acknowledged.

Consultec Scientific, Inc. is pleased to acknowledge the expert panel and to thank those outstanding scientists for the considerable time which had to be taken from their busy schedules.

The conclusions and overall research priorities described in the executive summary reflect the consensus reached at the final meeting of the Expert Scientific Panel.

Special thanks are also dueto Profs. Nicolaas Bloembergen,

Yuan T. Lee, and Arthur L. Schawlow who served as special consultants to this project. Also, the many valuable comments and suggestions from a group of peers (see Appendix III) are gratefully acknowledged.

Finally, we thank Dr. Gerald Goldstein for briefing the panel on the OHER programs and Dr. Robert Rosenthal, DOE Project

Manager, for his insightful coordination of the work.

iv TABLE OF CONTENTS

Page No.

EXECUTIVE SUMMARY ...... viii

RECOMMENDATIONS ...... ● . . . . ,...... ix

1. INTRODUCTION . . . ✎ ✎ ✎ ✎ ✎ ● . . . . ,...... 1

2. SUMMARY OF THE OFFICE OF HEALTH AND RESEARCH (OHER) PROGRAMS: A PERSPECTIVE ON THE USE OF LASER SPECTROSCOPY ...... 3

2.1. Introduction ...... 3

2.2. Source and Dose Determination ...... 3

2.3. Environmental Processes and Effects ...... 3

2.4. Health Effects ...... 6

2.5. Nuclear Medicine Applications ...... 7

3. STATUS OF RELEVANT LASER TECHNOLOGIES ...... 8

3.1. Introduction ...... ● ...... 8

3.2. Sources . ..*. ..** ● . . ✎ ● ...... 8

3.2.1. Gas and Liquid Lasers . . . ✎ . . ✎ ✎ ...... 8

3.2.2. Solid-State Lasers . . ✎ ✎ ...... 11

3.2.3. Free-Electron Lasers . ✎ ● ...... 13

3.2.4. X-Ray Lasers . . . . . ✎ ✎ ...... 13

3.2.5. Nonlinear Frequency Extension ...... 14

3.3. Detectors for Laser Spectroscopy ...... 16

3.3.1. Photon Detectors ...... 16

3.3.2. Ionization Detectors ...... 17

3.4. Imaging ...... 18

3.5. Reservoirs ...... 18

3.5.1. Introduction ...... 18

v 3.5.2. Classical Atomic Spectroscopic Methods ...... 19

3.5.3. Glow Discharges ...... 19

3.5.4. Laser-Induced Evaporation ...... 20

3.5.5. Particle-InducedVaporization . . ● ● . . 20

3.5.6. Supercritical and Liquid Injection

in Jets ...... ● ☛✎✎ . . ✎ ✎ . . 21

3.5.7. Other Methods ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ . . ✎ ✎ . . 21

3.6. Systems ...... ✎ ✎ . . 21

3.6.1. Laser Mass Spectroscopy Systems . ✎ ✎ . . 21

3.6.2. Laser ChromatographicSystems . . ● ✎ . ● 23

4. STATUS OF LASER SPECTROSCOPY ...... ✎ ✎ . ✎ 27

4.1. Introduction ...... ✎ ✎ . ✎ 27

4.2. Fluorescence . . . . ● ...... ● ● ● . ● 28

4.2.1. Introduction . . . . ● . . ● . ✎ ✎ ✎ . ✎ 28

4.2.2. Atomic Fluorescence . . ● . ✎ . ✎ ✎ ● ● ✎ 28

4.2.3. Gas Phase Molecular Fluorescence ✎ ✎ ✎ ✎ 29

4.2.4. Condensed Phase Fluorescence . ✎ ● ✎ ✎ ✎ 30

4.3. Absorption Spectroscopy ...... ● . ✎ ✎ ✎ ✎ ✎ 31

4.4. Raman Spectroscopy ...... ● . ✎ ✎ ✎ ✎ ✎ 31

4.5. Resonant and Multiphoton Ionization

of Atoms ...... , ✎ . ✎ ● ✎ ● . 34

4.6. Multiphoton Ionization of Molecules . ✎ . ✎ ✎ ✎ ✎ . 37

4.7. Photoacoustic and Photothermal

Spectroscopy ...... ✎ . ✎ ✎ ✎ ✎ . 39

4.7.1. Introduction ...... 0 ✎ . ✎ ✎ ✎ ✎ . 39

4.7.2. PhotoacousticSpectroscopy ✎ . ✎ ✎ ✎ ✎ . 39

4.7.3. Photothermal Spectroscopy ✎ ● ✎ ✎ ✎ ✎ . 40

vi 5. POTENTIAL IMPACTS ON THE OHER PROGRAMS ...... 43

5.1. Introduction ...... 43

5.2. Laser-Based Methods for Elemental Analyses ...... 43

5.3. Environmental Research ...... 45

5.3*1. Global Effects and Active Remote Sensing ...... 45

5.3.2. Transport Studies Involving Actinides and Heavy Elements ...... 48

5.3.3. Fossil Fuel Pollutants ...... 49

5.3.4. Chemical Sensors (Optrodes) ...... 50

5.4. Biological Research ...... 51

5.4.1. Imag-ng ...... 51

5.4.2. Fast Chemical Kinetics ...... 52

5.4.3. F1OW Cytometry ...... 54

5.4.4. Biosensors ...... 56

5.4.5. UltrasensitiveDetection and Analysis for Biological Research . . . . 57

5.4.6. Structural Analysis and Sequencing I ofBiomolecules ...... 58

5.4.7. Genome Sequencing ...... 59 I

5.5. Nuclear Medicine Using Stable Isotopes ...... 60

5.6. Radiation llosimetry ...... 61

APPENDIX I LASER ASSESSMENT STUDY ACRONYMS . . . . . 66

APPENDIX II INTRODUCTIONTO TERMINOLOGY ...... 68

APPENDIX 11I PEER REVIEW GROUP ...... 73

vii EXECUTIVE SUMMARY

The Department of Energy (DOE)* development and use of laser technology recognizes that new developments in laser for spectroscopic purposes. During the technology and laser spectroscopy can course of its study, this expert panel, substantially improve the ability to took a global view of laser science to carry out the mission of its Office of identify recent developments which could Health and EnvironmentalResearch (OHER). substantially improve the ability ofOHER In brief, the mission of OHER is to to perform its increasingly important support programs of research which allow mission. DOE to understand and anticipate long The panel made six specific term effects upon human health and the recommendations which dealt with three environment from the production and important areas. First the panel utilizationof alternate forms of energy, recommends that OHER closely monitor and and to apply the department’s unique be prepared to use the advances now being capabilities to solve numerous problems made in solid-state laser technology. in biology and medicine. These advances, comparable in nature to For some years, the OHER program has the revolution which began during the included some support for the development 1950’s in solid-state electronics, will of laser technology, and especially laser radically improve present-day laser spectroscopy. This support has provided technology. Secondly, the panel the basis for the demonstration of laser addressed the use of this advanced systems of remarkable sensitivityfor the technology to maintain the preeminent detection of chemical substances at the position which OHER has already created atomic and the molecular levels. At the for itself in the development of present time, both laser technology in selective and sensitive instruments for general and laser spectroscopy in the ani~lysis of atomic and molecular particular are surging ahead at an substances and to extend the use of these unprecedented rate. Thus, DOE requested to measure chemical pollutants in air, that its Office of Program Analysis study soil, and water. Finally, another area the opportunities in this 1aser of the recommendationsdealt with the use revolution that would further enhance the of lasers to determine structural and role of laser spectroscopy in the OHER dynamical features of macromolecules and mission. The purpose for this study is especially to develop X-ray lasers and to provide an independent assessment of other imaging techniques, including the long-term research needs for laser holographic ones, for sequencing DNA and technologies applied to advanced the human genome. spectroscopic methods and to address the Improvedanalyticaltechniques based content and priority of research needs on laser spectroscopy can help DOE to for the next 5 to 10 years. understand and anticipate the effects This DOE study was managed by upon human health and the environment Consultec Scientific, Inc. who furnished from by-products of the industrial from its staff the Principal Investigator society and can produce numerous other who, in turn, coordinated the benefits, such as a more complete enthusiastic efforts of a group of understanding of biological systems for consultants consisting of some of the diagnosis and cure of diseases and for world’s best scientists in the the biotechnology industry.

...... 0...... 4! ...... * See Appendix I for a list of Acronyms

viii RECOMMENDATIONS

The recommendations of the panel are We recommend that OHER support the presented in a prioritized list which we application of solid-state 1aser believe will produce the maximum technology to its programs and carefully beneficial impact of new developments in monitor the development of these new lasers upon the research programs of 1asers. Based upon developments by OHER. others, it may be cost effective by 1992 The rationale behind each to generate visible radiation for high recommendation is based on two factors. definition television projection, using First, each recommendation is considered solid-state lasers. By 1995 it should be to be a timely opportunity to take less expensive to generate ultraviolet advantage of the recent surge of radiation using (instead of excimer knowledge in laser science and technology lasers) diode-pumped, solid-state lasers to further the mission of OHER. The followed by nonlinear frequency second factor is based on feasibility; conversion. By that year, diode-pumped, we believe that each recommendation is solid-state laser sources up to the scientifically sound and technically kilowatt average-power level will be feasible. This list of recommendations available for application to chemical is the result of considerable study and processing, materials processing, reflection by the expert panel in which cutting, welding, and spectroscopic there was near unanimity of opinion on applications. We can expect dramatic the ranking of the recommendations. reductions in cost and volume and, at the These recommendations are based on the same time, improvements in reliability in-depth discussions of Sections 3, 4, and convenience of use. and 5. Section 5, in particular, Clear cut needs for OHER’S various contains a number of suggestions which missions include tunable lasers in the may be very useful to those doing infrared (IR), visible, and ultraviolet research for application of lasers to (UV) regions a:f:agnostic tools used in programs within the OHER mission. These monitoring environment for “embedded suggestions” represent a contaminants associated with energy broader base from which the prioritized production. Methods whichwould benefit recommendations were drawn. The from the timely application of solid- establishment of a scale for the state lasers include resonance ionization prioritization is difficult. For spectroscopy, other methods of sensitive instance, the first recommendation is detection, and techniques such as X-ray dominant to the extent that it would, in holography. See Section 5 for more fact, enhance each of the other technical details. recommendatiens. The last part of this section provides fundamental research 2. Deve1OP improved methods of recommendations dealing with more basic mo7ecu7ar structural analyses and research that supports the recommended seuuencinq, based on capabilities in technologies. These fundamentalresearch laser technology and sDectrosco~y. recommendations are not prioritized. We recommend that OHER further develop improved laser methodologies for 1. Accommodate the rapid molecular structural analysis. develo~ments now occurrinq in diode and Development of these methodologies would solid-state laser technology to fit the haveat least two important impacts: (a) needs of laser sDectroscoDy. sequencing information for DNA and proteins and (b) examination of Revolutionary developments are macromolecular structures in general. occurring in diode and solid-state laser There are several importantresearch technology (see Sections 3.1 and 3.2). priorities for sequencing. Laser-based

ix mass spectrometric techniques.couldplay discovered, and further research an increasingly important role in initiatives in this area should be sequencing because they provide exact pursued. Indicationsthat it impossible mass analysis of the sequence components. to volatilize solid-state atoms or Rapid.sequencing techniques using laser molecules directly from their mass spectrometry will be important for environmental matrix by laser- or ion- minimizing errors inherent in other beam-induced resorption are encouraging. methods. The use.of flow cytometry and The pieces for making major advances other rapid flow methods in combination in the selective detection of a wide with selective and sensitive detection variety of molecules are in place. By using lasers will enhance the speed of controllingthe wavelength and the number sequencing for such formidable tasks as of absorbledphotons, it is possible to DNA structural analysis. Direct imaging optimize sensitivity, selectivity, and techniques such as X-ray microholography structural informationfor rather complex for rapid identification,artdresolution molecules. The development of new of the base units are expected to play an methods clf analyses of environmentally important role in genome sequencing. important mixtures, the selective Understanding the arrangement of detection of DNA-bound nucleic acids, and 1arge assemblies such as membranes, the detection of laser-ionized molecules viruses, and intact DNA is an exciting at atmospheric pressure are three diverse area in the realm of macromolecular examples of possible payoffs. structures. The relationship between structure and activity of complex biological molecules such as DNA couldbe 4. !9evelo0 coherent detection explored using laser techniques. These methods. imaqinq techniques, and techniques include light scattering and holoqraohic methods for biological and X-ray holography (see Section 5.4.1). environmental applications.

We recommend supporting development 3. Improve the ca~ability for atomic of coherelntdetection techniques because and molecular detection, includinq of their applicationsto widely divergent isotope selectivity throuqh laser fields such as remote sensing and new methods. forms of microscopy. The following topics are examples of important The applications stemming from the applications of coherent detection detection of atoms and molecules ionized techniques. by lasers are growing rapidly (see High intensity 1aser sources Sections 4 and 5), and we recommend that (Sectio~e~~2), including p:]:~d lasers, this successful on-going OHER.effort be have developed almost continued with,a high priority. With the arbitrarily narrow linewidths in the IR, new breed of continuous wave solid-state visible, and UV spectral regions. This lasers, for example, it should be makes possible high precision Doppler possible to selectively ionize stable measurements of wind and cloud velocities nuclear isotopes for detection in medical over long distances. It also permits and environmental tracing., New, highly remote sensing of contamination in the reliable, pulsed vacuum-ultraviolet(VUV) atmosphere. Since these techniques are lasers could be used for single-photon also of direct importance to the (nonselective) ionization of molecules military, the National Aeronautics.and and for .a resonance excitation step in Space Administration (NASA), and other the selective ionization of inert gases. government agencies, it is recommended Isotopes of the i:;;;cgases are key to that OHER monitor these developments and understanding environmental fund particularmeasurements needed.which questions such as . the transport of are not supported by other agencies. groundwater. New developments in these coherent Novel methods for introducingsamples sources and detection methods (Sections into the gas- phase continue to be 3.2.4 andl5.4.1) offer great potential

A for revealinclinformation on biological discover and implement new methods for systems of direct interest to OHER; In the detection of chemicals in the particular, this capability shouldreveal environment, (Sections 4 and 5.3). the exact way nuclear radiation affects Examplesincludeabsorptionspectroscopy, different tissue and will thus improve fluorescence spectroscopy, resonance the understanding of the effects of low- ionization spectroscopy, ionization mass level and high-level exposures. An spectroscopy, and Raman spectroscopy. atomic tagging technique based on X-ray Further, with the advances in tunable holography may make it possible to read laser sources, coherent forms of these rapidly the sequence of base pairs in DNA spectroscopes have been developed that and to study its higher organizational allow long range or remote sensing of structure. The X-ray holographyprograms chemical species. As noted in the are so new that they are not heavily Brinkman report on Physics in the 1990’s, funded at this time, and it appears that in the past decade new forms of OHER is the appropriate agency to spectroscopyhave been discovered at such capitalize on this technology and an astonishing rate that the terms encourage its more rapid development. describing the spectroscopes were not There is currently an important even known one decade earlier. Continued window of opportunity to develop new support of research by the Department of approaches to imaging biological samples Energy provides an opportunity to with increased spatial resolution. discover and develop new forms of laser Coherent confocal microscopyhas recently spectroscopy that are applicable to the demonstrated edge resolutions that are a detection of chemical species in the air, factor-of-two improvementover incoherent soil, and water. The continued microscopy. Similarly, sub-wavelength development of the laser sources in microscopy methods are being proposed wavelength, power, coherence, and that depend upon near-field imaging. efficiency makes this area of research With the appearance ofX-ray lasers, the very promising. development of an imaging X-ray microscope will soon be possible. Finally, the gradient imaging approach 6. ImDrove detectors and digital making use of gradient fields such as the data jx-ocessinc7 as apo?ied to 7aser successfully demonstrated magnetic techno70qy andto imauinq techniques such resonance imaging (MRI) could be extended as tomoclraDhY. into the optical frequency region. Support by the Department o~orEne~;~ We recommend investing resources in provides an opportunity improvingopticaldetectors,particularly application of these new microscopic small solid-statedevices for wavelengths tools for biological research. ranging from the UV to the IR. Techniques such as capillary zone electrophoresis(CZE) make it possibleto 5. Improve laser methods for resolve attomole quantities of proteins. detection of chemical oo77utants in air% Flow cytometers can be used to examine soi7. and water. picoliter quantities of material. Better detectors (Section 3.3 and 5.4) are We recommend emphasizing the needed to support these state-of-the-art improvement of methods for global techniques. measurements of atmospheric parameters There are opportunities for the since they allow the study of the impact coupling of advances in computational of energy consumption on the global techniques, such as parallel processing, ecology. Laser radiation in the UV, to enhance the capability of optical visible, and IR spectral regions allows detectors. In particular, the extensive selective excitation and detection of development of signal and image molecular, atomic, and ionic species. processing for the field of high-energy During the past 20 years, advantage has physics and in the space program may be been taken of improved laser sources to very useful for OHER’S various missions.

xi Fundamental Research Recommendations. on OHER interestswhere our understanding of the fundamental processes is incomplete or even totally absent. Some In addition to the above suggestions of these which the panel identified are that deal with specific instrumentation, given below: the panel has several general recommendations for programs that are more fundamental in nature and tend to Photo-fragmentation of molecules. support the technologies recommended above. These general recommendationsare When high intensity lasers interact given below. with a gaseous sample of molecules, many Laser science is still young; of the Imoleculesmay break apart into current understanding is by no means smaller fragments, either neutral or complete. Under these circumstances,the charged. Unlike conventional light committee strongly endorses OHER’s policy sources, the exciting laser can cause of supporting basic research in selected sufficient population of excited levels areas of potentially great impact on its of the molecule so that the fragmentation mission. The continuation,or expansion, process may proceed in new and unexpected of this policy is seen as quite likely to ways. If this process could be pay substantial dividends. understood in detail so that it could be The rapid growth and wide use of controlled and utilized, it could laser technology over the past 25 years contribute in many ways to questions of has led to a great deal of new interest to OHER. fundamental knowledge about 1ight and matter and their interactions. The characteristics of laser light are so Laser ablation of solids. dramatically different from conventional light sources that scientists have A laser beam incident on a solid regularly succeeded in accomplishments sample can readily vaporize a portion of which were inconceivable before lasers. the sample. This process has the In retrospect, many of these potential to be very useful, for accomplishments required innovative instance, in analyzing the composition of changes in our thinking about the basic the sample. Currently, however, the physical processes involved. physical processes involved in such laser Laser technology continuesto advance ablation are not sufficiently understood rapidly, making accessible new phenomena to make this possible. In what form are which could never before be studied in the fragments emitted? How is the answer the laboratory. This explosion of new affected by the laser characteristics? knowledge is very likely to continue or accelerate in the future. Since many of the processes which are being seen in Ultrahiqh intensity laser fields. this “new light” have very strong potential for applications which are The achievable intensity from laser relevant to the OHER mission, OHER has sources (power per unit area) has grown recognized this aspect of laser science steadily since the laser’s invention, and by supporting a selected number of is likely to continue to do so. Current research programs which study problems intensity levels (-10’5W/cmz) are high related to important applications at a enough that the optical electric fields very fundamental level. The small number in the laser beam are already comparable of projects supported on this basis have to the ordinary electric fields which produced scientific results of extremely hold atoms and molecules together. It is high quality, and contributed in a very already clear that completely new noticeable way to the generation of this physical ideas and models wi11 be new knowledge. There is, however, still required to understand the interactions a great deal to be learned. There are of atoms and molecules with lasers of many questions of potentially high impact this ntensity, and that much effort will

xii be required to develop and test these Laser coolina, trarmina. andmaniwlation ideas. The potential for important of Darticles. applications is so high that these questions should be answered. Uithin the last few years, it has been demonstrated experimentally that Fundamental research for laser source laser light can be used to suspend atoms development. in vacuum and cool them to very low temperatures (103 K). Many other forms Development of new lasers often of mechanical manipulation of individual requires answering basic scientific atoms with lasers also appear to be questions “ diverse scientific feasible. The basic physical principles disciplines, ;;chasatomic and molecular involved in these new observations are physics, solid-state physics, non-linear not yet fully understood, although they optics, etc. Support of basic research have potential for application in areas related to laser source development, of sensitive detection which are of therefore, advances basic knowledge in interest to OHER. Laser tweezers are all these areas and, in addition, leadsto already being used to manipulate small improved lasers which are themselves objects such as cells or bacteria. valuable research tools.

xiii ~. INTRODUCTION

The Atomic Energy Commission charter In the last decade there have been [The Atomic Energy Act of 1946 (PL 79- spectaculardevelopments in laser science 585)] included a comprehensiveprogram of and technoloq~ and their all~licationsto applied and basic biological research a host of im~-ortantproblems. In laser relating to the utilization of spectroscopy alone, there has been a fissionable and radioactivematerialsfor renaissance in the abilityto selectively medical, biological,and health purposes. excite and detect atoms and molecules in As the DOE was created through various the gaseous, liquid, and condensed states legislative acts to broaden the charter of matter (Hieftje et al. 1981; Hurst and to other forms of energy production, the Payne 1988; Letokhov 1985, 1987; Lubman responsibilities in the areas of 1989; Omenetto 1979; Piepmeier 1986; biomedical research, worker safety, and Radziemski et al. 1987; U.S. DOE 1988). environmental protection were Support for the initial development of correspondingly broadened. one of these techniques--resonance Responsibility for conducting a ionization spectroscopy (RIS)--came Biological and Environmental Research almost entirely from the Physical and (BER) program rests in the OHER of the Technology Research Divisionof OHER. In DOE. Report DOE/ER-0185/4, “Research in addition, in carrying out the measurement Progress: FY 1988” (U. S. Department of sciences portion of that program, partial Energy 1988) can be consulted for support has been provided for some of the information on legislative history, other laser spectroscopes. The purpose program objectives, budget levels, and for this present study is to provide an technical content. Central to the independent assessment of the long-term present assessment are the broad goalsof research needs for laser technologies the BER program from the above report: applied to advanced spectroscopic methods. We need to examine the impact that laser spectroscopy could have on the The BER program has two main base programs and the special initiatives objectives: (1) to develop the of OHER. In doing this, we recognize knowledge base necessaryto identify, that while the OHER mission has become understand, and anticipate the long- more complex with the inclusion of term health and environmental chemical by-products of energy consequences of energy use and production, laser spectroscopes have development; and (2) to utilize the emerged as possible solutions to the Department’s unique scientific and associated measurement problems which technological capabilities to solve have arisen. major scientificproblems in medicine This report is organized as follows. and biology. Meeting the first Section 2 summarizes the current program objective requires an ability to of DOE’s Office of Health and rapidly assess the potential health Environmental Research and provides some and environmentalconsequencesof any remarks on how laser science and proposed energy option. This means technologycould beneficially impactmost developing broadly generalizable of the research programs. Section 3 knowledge and predictive principles, provides a brief global perspective on rather than informationrelevantonly laser technology and attempts to define to current energy strategies. important trends in the field. Similarly,

1 Section 4 provides a global perspective Hurst, G. S. and M. G. Payne (1988) on laser spectroscopy and addresses PRINCIPLE!SAND APPLICATI-ONSOF RESONANCE important trends. Thus, Section 5 IONIZATION SPECTROSCOPY, Adam Hilger, focuses on the trends in laser technology Ltd., Bristol, U.K. and Philadelphia). and spectroscopy which could impact the OHER mission in significant ways and Letokhov, V. S., Ed. (1985) LASER contains the basis for recommendations ANALYTICAL SPECTROCHEMISTRY (The Adam made in the executive summary. Hilger Series Optics and For those with limited familiarity Optoelectronics) (A~~m Hilger, Ltd., with 1aser technology and 1aser Bristol, IJ.K.). spectroscopy, reference is made to Appendix I for a list of abbreviations Letokhov, Vladilen S. (1987) LASER and acronyms. PHOTOIONIZATION SPECTROSCOPY (Academic Appendix II can serve a useful review Press, Inc., Orlando). or tutorial for those who are not deeply involved with laser spectroscopy. Even Lubman, D,,M., Ed. (1989) LASERS AND MASS those familiar with laser spectroscopy SPECTROMETRY (Oxford University Press, and laser technology may find it useful Oxford), in press. to know precisely what the authors of this document mean by certain specialized Omenetto, Nicolo, Ed. (1979) ANALYTICAL terms and expressions. Finally, a note LASER SPECTROSCOPY, Vol. 50 of Chemical on the style of referencing may be Analysis series (John Wiley & Sons, New appropriate. Whenever possible a bookor York, NY). a review article is referenced as the preferred citation. However, we Piepmeier, Edward H., Ed. (1986) frequently found it useful to reference ANALYTICAl.APPLICATIONS OF LASERS, vol. a number of individual papers of recent 87 of Chemical Analysis series (John origin or those which were not Wiley& Sons, New York, NY). conveniently found in the review articles. Radziemski, Leon J., Richard W. Solarz, and Jeffrey A. Paisner, lids. (1987) LASER SPECTROSCOPY AND ITS APPLICATIONS (Marcel Dekker, Inc., New York and REFERENCES Basel).

Hieftje, Gary M., John C. Travis, and U. S. DepartmentofEnergyReportDOE/ER- Fred E. Lytle, Eds. (1981) LASERS IN 0185/4 (1988) “Research in Progress: FY CHEMICAL ANALYSIS (The Humana Press, 1988,“ (Office of Scientific and Clifton, NJ). Technical Information).

2 ~. SUMMARY OF THE OFFICE OF HEALTH AND ENVIRONMENTAL RESEARCH (OHER) PROGRAMS: A PERSPECTIVE ON THE USE OF LASER SPECTROSCOPY

2.1. INTRODUCTION this program area, whose goal is to perform research and advanced The objective of the OHER research developments on analytical systems, prograinis to obtain knowledge that can utilize laser methods. be used to minimize the harmful effects of energy production on human health and the environment. Since the research is 2.3. ENVIRONMENTALPROCESSESAND EFFECTS supported at a fundamental level, numerous benefits follow from Comprehensive programs are carried applications to other programs. out in the atmospheric, oceanographic, The research program is categorized and terrestrial sciences, and integrated by financial allocation in Table 2-1 and into an ecological context. The focus of program organization in Table 2-2. In the Atmospheric Chemistry and Fluid order to place future uses of laser Dynamics program is the physical and spectroscopy in perspective, we follow chemical fate of pollutants in the the outline of the FY 88 Research in atmosphere. Particular attention is Proqress report (DOE/ER-01854) in the given to modeling transport and chemical remainder of this section, addressing changes for both flat and mountainous areas of need and opportunity without terrains. Wet and dry acid deposits from budgetary or organizationalrestrictions. fossil fuel energy conversion are special concerns. Laser-based instrumentation has been used in a few of these projects. 2.2. SOURCE AND DOSE DETERMINATION These include laser anemometers to ascertain transport properties of the A central focus of OHER Dosimetrv lower atmosphere; tunable, diode lasers Research is the detection and transport in systems to measure absorption due to of radiation and chemical sources in the the atmospheric deposition of ozone, environment where human health is at nitric oxide, and nitric acid; and an risk. This includes radioactive fallout, airborne IR laser system to measure SOZ toxic or carcinogenic chemicals, and by the photoacoustic method. natural environmental sources such as The Marine TransRort and radon. Laser detection methods have seen Transformation program emphasizes limited use to date in these areas. physical, chemical, and biological However, the need for detectors with much factors which affect the transport of greater selectivity and sensitivity materials in the oceans and includes should result in considerable increase in study of the ocean for waste disposal. the use of laser spectroscopy. Natural and artificial radioactive The OHER Measurement Sciences program sources serve as primary tools for these supports a number of studies relating to studies. These include natural laser spectroscopy for the measurement of fluorescence measurements, and some the chemical pollutants as atomic or limited use of remote sensing with molecular species. The goal of this lasers. Laser spectroscopy could play a program is to perform research and larger role in studying composition, advanced developments on analytical transport, and oceanic circulation in the systems. Nearly half of the projects in future.

3 Table 2-l.* Health and Environmental Research Budget for FY 1988

$(Millions) Research Programs

Total Laser Related

Physical and technological research to 38.0 3.3 characterize energy-related emissions humans may be exposed to and improve measurement and dosimetry instrumentation, improve under standing and prediction of atmospheric transport and diffusion, their chemical processes and deposition, and to define the fundamental processes that govern radio- biological action and bimolecular energetic.

Environmental research to determine the 21.2 0.9 mechanisms which control and influence total ecosystems and the cycling of energy by-products through them.

Human health research to quantify risks of 25.1 nil late effects of acute and chronic exposures via epidemiologic studies of workers and the general population.

Health effects research in biological 69.0 0.7 systems to experimentally define and under- stand dose-response relationships and factors influencing carcinogenic, mutagenic, and toxi- cological risks of energy-related exposures.

Nuclear medicine research to develop new 35.1 nil radioisotopes, labeled compounds, clinical procedures, and visualization devices for improved diagnoses and treatment of human diseases.

Capital equipment and construction. 11.5

Program direction. 4.0

Total $203.9 —$4.9

*Table taken from U. S. Department of Energy Report DOE/ER-0185/4, “Research in Progress: FY 1988,” (Office of Scientific and Technical Information, March 1988), p. XV.

4 Table 2-2.* Program Organization

Human Health Health Effects Ecological Physical and & Assessments Research Research Technological Research

Epidemiology Radiation Biology Marine Atmospheric Research Physics and Chemistry

Nuclear Chemical Toxicology Terrestrial Measurement Medicine Transport & Dosimetry

General Life Ecosystem Radiological Sciences Response and Chemical Physics

*Table taken from U. S. Department of Energy Report DOE/ER-0185/4, “Research in Progress: FY 1988,” (Office of Scientific and Technical Information, March 1988),-p. xvi. “ ‘

The Terrestrial Transport and Fate conventional: however, there are program deals with the movement and opportunities for laser spectroscopy in possible chemical change.of pollutants improving the instrumentation used to such as radon in the earth. Studies study the ecosystem. A recently using remote fiber spectroscopy (RFS) are publishedreport entitled, “Evaluationof of special relevance to this assessment Mid-to-Long Term Basic Research for study. The RFS technique can be used for EnvironmentalRestoration,” DOE/ER-0419, remote chemical detection by direct September 1985, addresses research needs spectroscopy or by using optrodes -- for dealing with radioactive, inorganic detectors puton the ends of fibers which and organic chemical wastes from the interact with the chemical to be detected operation, decommissioning, and cleanup to produce optical signals. This at DOE facilities after some 40 years of technology could greatly advance the operation. The plan identifies several measurement of chemicals in situ and high priority needs for improved WOU1 d obviate the need for most instrumentationassociated with sampling conventional drilling techniques. and measurement of containment sources The Ecosystem Functioning and and their transport through the ResDonse program integrates knowledge environment. Basic research on methods from the atmospheric, marine, and earth for remote monitoring and sampling and sciences for a better assessment of the for more sensitive laboratory assays to total impact of energy use. Most of the reduce sampling costs and health risk to methods used to study ecosystems are workers was recommended.

5 2.4. HEALTH EFFECTS The fourth subdivi sion, Radiological Physics and Chemistry, seeks to The Radiation ExI)osure Effects understand mechanisms for biological program has four subdivisions and seeks damage. One focus is the interaction of to understand human health risks radiatiorl with matter and following the resulting from exposure to ionizing chain of events through the physical, radiation or energy-related chemicals. chemical, and biological stages. The first subdivision, Human Studies, Microdosiimetry is used, and theories of focuses on populations exposed to the relative biological effectiveness ionizing radiation from occupational and (RBE) of radiation having different environmental sources. These include linear energy transfer (LET) values are atomic bomb survivors, radium dial developed. painters, uranium miners, and others The Lonq-Term Effects of Enerw- exposed to radiation. It is conceivable Related Chemical Aqents program supports that sensitive methods of 1aser the exploration of chemical toxicity spectroscopy could be used to measure problems associated with various energy long-lived isotopes induced by weapons technologies, including genetic effects radiation. However, many risks are due and carc:inogenesis. Since some chemical to the inhalation or ingestion of pollutants are synergistic in their isotopes which can emit alpha particles health effects, their long-term effects and cause localized damage from within are more complex than is their radiation the body. Thus, there is a strong counterp~art. Laser spectroscopy could emphasis on radon which is inhaled as a play an increasing role in the near chemically inert gas, but whose daughter future in studying the interaction of products are alpha particle emitters. chemicals with genetic material, Other problems of a similar nature especially in research that involves involve uranium mining or plutonium studies of llNA. production. In some of the measurement UnderGeneral Life Sciences program, approaches, specific exposure risks there arle five subdivisions. The first, through internal radiation dosimetry are Chemical Phvsics. Instrumentation, and assessed. In these, methods of sensitive Traininq, overlaps considerably with detection based on laser spectroscopy Radiological Physics and Chemistry and could help to determine pathways, with the Measurement Sciences program. transport rates, and localized dose. Some of the instrumentation activities The second subdivision, Mammalian include ”laser methods for quantitationof Radiobiolocw , involves studies of the electralphoresis, the scanning effects on laboratory animals of energy transmission electron m~nxcope (STEM), by-products most likely to be encountered neutron scattering, synchrotrons by humans. The analytical methods used radiation sources. in these experiments are traditional. A second subdivision, Structural However, laser spectroscopy could help to Biol oqy emphasizes innovative methods determine the distribution, retention, for biological structure determination. and dosimetry of internally deposited A major program deals with protein radionuclides in the laboratory animals. structure and conformation, using X-ray The third subdivision, Cellular and diffraction for crystalline material and Molecular Radiobioloqy , studies efforts gel filtration for molecules insolation. to enhance the understanding of late Studies in the areas of structure, somatic and genetic effects of radiation. function, and dynamical processes using Research in radiation genetics and such advanced techniques as synchrotrons cytogenetics provides essential radiatiam, neutron scattering, and STEM information on gene and chromosome are already impressive. mutations. Research is aimed also at A third subdivision, Molecular characterizing the role of molecular Genetics and Gene Regulation, contributes damage and its repair in the development to the knowledge of gene expression, gene of late radiation and chromosomal function, and cell growth in humans to effects. understand the consequences of

6 environmental stresses on man and for study of mechanisms from molecules to developing methods to diagnose and treat man. This, as well as the preceding diseases. This program contains many new subdivision, would receive a tremendous thrusts which rely on advanced boost from methods involving 1aser technology, such as several projects spectroscopy which would speed up genome involving genome sequencing, new methods sequencing (see Section 3.4). for rapid preparation ofDNA samples for automated sequencing, high resolution 2.5. NUCLEAR MEDICINE APPLICATIONS STEM used to detect heavy atom clusters, use of resonance absorption, mass DOE traditionally seeks to derive spectroscopic identification, and flow nuclear medicine benefits from by- cytometry used for genetic analysis and products of nuclear energy production. chromosome classification. Human genome A major part of this effort is the mapping is planned, using pulsed field production of radioactive atoms for use electrophoresis flow-sorted in radiopharmaceuticals. Other work chromosomes. Los OnAlamos National involves the preparation of chemical Laboratory is evaluating new base compounds optimized to seek specific sequencing approaches such as detection human organs, stable compounds for MRI, of fluorescing bases and the use of laser and special boron compounds for neutron resorption followed with mass capture therapy. spectroscopy. A direct imaging method of National laboratories, off-site base sequencing using X-ray holography laboratories, and some small companies with tagged bases is being investigated have been funded to develop by a small business firm (l19CRTechnology) radiopharmaceutical technology. Support- associated with the University of for the positron emission tomography Illinois at Chicago under an OHER (PET) scanner has provided a major research grant. diagnostic tool for the study of brain The fourth subdivision, DNA Damaqe disorders and heart functioning. With and Re~air Processes, involves efforts to support from private industry, three- isolate, clone, and sequence genes that dimensional detectors have been improved code for enzymes involved in the repair to a resolution of about 3 mm. of various types of damage from Separation andenri chment of nuclear ionizating radiation. Other projects medicine isotopes depend on large-scale identifybiol ogically important radiation enrichment facilities such as those at produced lesions in MA. Oak Ridge. Problems arise with the use The fifth subdivision, Cell Bioloqy of these facilities (some of which are under the General Life Sciences progra; obsolete) for producing small quantities supports research in basic cell biology of medical isotopes. Enrichment of to provide basic knowledge of cell stable and radioactive isotopes for function and structure necessary to medical applications may well be understand responses to radiation and economically feasible by the use of laser chemical exposures. Included are studi es spectroscopy. Separation of small on chemical carcinogenesis in human quantities of isotopes suchas sulfur and cells, cell biology and control of gene calcium by use of laser spectroscopy expression, regulation of cell membrane would alleviate the problems encountered support, and an integrated, multilevel with use of large scale facilities.

7 ~. STATUS OF RELEVANT LASER TECHNOLOGIES

3.1. INTRODUCTION 3.2.1. Qas and Liauid Lasers

In this section we examine the status of those aspects of laser technology &ilJum-Neon. Progress continues to which are relevant to laser spectrosco~y be made in gas laser technology. The or to the sensitive measurements of helium-neon laser is firmly embedded as chemical species. Here, we intend to a too’1 for optical alignment, have a broad perspective, leaving for interferometry,barcode reading, siting, Section 5 the discussion of possible and laser velocimetry. Even though impacts that laser technology could have diode-laser sources are now available in on the OHER program. Because we identify the red spectral region, helium-neon opportunities for programs in the future, lasers will continue to serve an it is necessary in the study of the importantneed because of their very long status of laser technology to identify operating lifetime and compact design. important trends. Those aspects of laser Arqon-Ion. Argon-ion 1aser technology torecei ve attention are light technology has matured. Today, large sources,detectors of laser interactions, frame argon-ion lasers operate at up to imaging techniques, reservoirs of atoms 20 Wofcw output power at approximately and molecules, and some examples of 40 kW oi? input power. The plasma-tube systems using lasers. lifetime has been extended to a few thousand hours by the use of segmented 3.2. SOURCES ceramic-metal construction methods. Wavelen@hoperationnowex tends into the The success ofDOE’s future programs UV region with the use of special mirrors to apply laser sources to remote sensing, for the! laser resonator. Argon-ion chemical analysis and detection, RIS, lasers find wide application in pumping isotopic detection and separation, and dye lasers for tunable laser output. other important matters depends on an Recently, argon-ion lasers have been used understanding of the present and future to pump the tunable titanium-sapphire development of these sources. This laser, which tunes over a780to 1000nm section addresses the current state of wavelength region and operates much like the art of lasers and nonlinear a cw dye laser. The operational cost of techniques for extending the frequency an argon-ion laser remains high, both in range of lasers to the UV and IR spectral terms of electrical power cost and regions. Table 3-1 provides an overview plasma-tube replacement. For this of laser sources, showing typical reason, large-frame argon-ion lasers are continuous wave (cw) power, typical pulse used in research and not in production energy, typical wavelengths and the applications. However, air-cooled, primary applications of representative argon-ion lasers at the 10 mW output types of lasers. Recent Iaser technology power level are used for production, for has brought rapid advances in the areas example, in color separation processes. of efficiency, power, wavelength, tuning Theargcm-ion laser technology is already range, pulse widths, and peak power mature and is not expected to change levels, which are discussed below. appreciably in the near term. Table 3-1. Summary of Laser Sources

TYPE TYPICAL TYPICAL TYPICAL PRIMARY CW POWER PULSE WAVELENGTH APPLICATION ENERGY nm

Gas He-Ne 10 mW 633 barcode reader surveying, interferometry

Gas Argon Ion 20 w 500 pump for ringdye laser and titanium sapphire, high resolution spectroscopy

Gas Kr Ion 10 w 600 Dye laser pump

Gas Carbon 20 w 9ooo- welding, cutting, Dioxide -25 kW 10 J 11000 heterdyne spectroscopy

Gas Eximer lJ ArF 193 photolysis, surgery, laser KrF 248 material treatment, pumps XeCl 308 for dye lasers XeF 531

Liquid 3oo- spectroscopy and other Dye Lasers 1 W 50 mJ 1100 research applications

Solid State, Diode Lasers 1 mW 3ooo- High Res. IR Absorption Various 30000 Spectroscopy and sensitive Lead atmospheric detectors Salts (bandwidth of 3 x 104cm-’

Diode Arrays 75 W 800 telecommunications, pump for Nd:Yag lasers

Nd:YAG 20 w lJ 1064 laser ranging, welding, surgery and pumps for dye lasers

Alexandrine 300 W 300 mJ Tunable materials processing, 7oo- spectroscopy, 800 remote sensing

Titanium 3 W 200 mJ Tunable potential dye laser Sapphire 780- replacement 1000

9 Carbon-Dioxide. The lOymwavelength pulsed discharge for heat removal and carbon-dioxide laser technology has excitation. The lasers operate at up to continued to advance in power, wavelength 3% efficiency for lifetimes oflOOO hours control, coherence, efficiency, and pulse before discharge electrodes need width. The high-power, carbon-dioxide exchanging. Special handling of fluoride 1asers now operate at greater than 25 kW gases is necessary for prolonged for industrial applications. The overall operation. However, excimer lasers have operating efficiency is approximately5%. become the standard source for UV The lasers are now fully developed radiation at 193 nm, 248 nm, and 308 nm. commercial sources that are useful tools Liquid Lasers. The dye laser on the factory floor. Lower-power, represents the most important cJass of carbon-dioxide lasers are used for liquid ‘lasers. Dye lasers offer medical applications and for research. wavelength tunability from the near IR Power levels of 20 W to 100 W are common near 1000 nm across the visible to theUV from low-pressure discharge tubes, 1 near 350 nm. Dye lasers can operate cw meter in length. These lasers offer when pumped by an argon-ion laser source wavelength selection in the 9 to 10 Lm or pulsed when pumped by an excimer laser region ‘with the use of an internal or a Q-switched, Nd:YAG laser source. grating. With feedback control, Further, dye lasers can be flashlamp linewidths of less than 1 kHz have been pumped tctproduce pulses with durations demonstrated. of about 1 microsecond at the energy Carbon-dioxide lasers making use of level of about 1 J. Lamp-pumped dye wave quides are now beinq manufactured by lasers are finding applications in a num-berof vendors. These polarized: medicine where the 1 AS pulse, low peak- single-mode lasers operate at up to 20 W power output is an advantage for optical of power for a length of less than 1 fiber delivery of the radiation. meter. Their compact size and guided The cwdye laser pumped by an argon- wave operation make these lasers useful ion laser is a very widely used tunable for laboratory scale applications aswell source of laser radiation in the research as for coherent-laser radar. The carbon- environment. Dye lasers operate at over dioxide, coherent-laserradar systems are 1 W of optical output power. Wavelength now well developed and are being used for control is achieved using intracavity applications from wind velocity sensing elements such that dye lasers can to coherent-laser radar for military continuously tune over a wide wavelength applications. range. The cw dye laser can also be Excimer. Following the discovery of tuned continuously over a 30 GHz region laser action on dimer and excimer with a Iinewidth ofl MHz. With feedback transitions of KrF, XeF, and ArF in the control, the dye laser linewidth has been early 1970’s, excimer-laser technology reduced to less than 1 Hz. has undergone a rapid development. These Dye lasers, pumped either with the laser sources operate in a pulsed mode excimer laser or a Q-switched, Nd:YAG with a large aperture that is typically laser, can be pulsed at repetition rates centimeters in dimension. They operate from 10 t{zto greater than 100 Hz. These with pulse durations of tens of dye lasers operate with 10 to 100 ns nanoseconds”with pulse energies up to pulse widths at peak powers greater than 1 J/pulse for commercial laser sources. 10 mW. The tunable, high peak-power, Pulse repetition rates exceeding 200 Hz radiation can be frequency extended by with average power at the 1 kW level have nonlinear techniques into the UV and the been reported. near IR. The mode-locked, cw dye laser Excimer lasers are used in research with proper group velocity control has to pump tunable dye lasers and are used operated at less than 5 fs pulse widths. directly for excitation of atoms and The dye “laserthus provides the shortest molecules. Recent applications of optical pulses yet generated. It is excimer lasers include UV lithography and expected that the dye laser will remain UV circuit repair. The laser sources the laser source of choice for research require flowing gas and high-voltage, applications. Dye lasers have not been

10 chosen for man,‘ production applicat ons construct a II-VI compound laser for because the dye must be changed direct blue generation. periodically. The recently introduced In an alternative approach, diode titanium-sapph re laser will eventually lasers have been efficiently frequency- impact the use of dye lasers. doubled to generate 410 nm radiation. Two approaches to the doubling have been demonstrated: (a) resonant external 3.2.2. Solid-State Lasers cavity doubling in potassium-niobate crystals and (b) direct doubling in a channel-guide, lithium-niobate doubler Diode Lasers. Diode lasers, other that used quasi-phasematchingto achieve solid-state lasers, and nonlinear optical the 410 nm output. The possibility now devices were first demonstrated at the exists for doubling diode lasers directly dawn of the laser age over 25 years ago. to efficientlygenerate blue radiation by Today, it is the convergence of these using inexpensive, 1ithographically technologies that “ rapidly defined waveguide doublers in lithium revolutionizing the l!ser source niobate. capability. For example, diode-laser The average output power of diode sources are now being shipped in lasers is increasing by a factor of two commercial products at over 12 million per device each year. This year a units per year. Diode lasers operate at commercial product was introduced that over 1 W of cw power per facet with more generates 10 W of cw output power. A than 75 W of cw power per diode-laser 50 W, cw-output power device has been array. The quantum well type of diode demonstrated in the laboratory. The 1asers operate with an internal output power is expected to exceed differential quantum-efficiency of 30 W/mm of diode-laser length. There has greater than 95% and with an overall also been progress on two-dimensional electrical efficiency of greater than diode-laser arrays. Recently, face- 50%. Laser lifetimes have been extended emitting diode 1asers have been to tens of thousands of hours and even up demonstratedthat are expected to achieve to 100,000 hours for diode lasers used in greater than 40% operating efficiency. fiber-optic communications. Two-dimensional arrays of diode lasers Finally, and of crucial importance may be expected to emit greater than 1000 for the future, diode-laser prices are W/cm* of optical power. now following the semiconductor device Applications of diode lasers include historical pricing trends and are falling pumping of solid-state lasers, reading a factor of four per year per watt of and writing compact disks for digital and output power. Indeed, the goal of for analog information,barcode readers, semiconductor manufacturers in Japan and optical radar, electro-optical sampling in the United States is to produce 1 W cw of high-speed circuits, and chip-to-chip diode 1asers for less than $10 each by communication. Diode lasers are the most 1992. efficient source of coherent or The diode-laser technology is also incoherent optical radiation. extending available wavelengths. For Applicationsof frequency converted diode example, diode lasers are now operating lasers include red, blue, and green at visible wavelengths of less than displays for projection of high 650 nm at 3 mW of output power. Pulse definition television images and for operation’ has been obtained into the color separation. yellow region. Blue emitters require II- Other Solid-State Lasers. During VI compounds with pnjunctioncapabil ity, the past five years, solid-state lasers thus the prospects for an electrically have undergone a rapid evolution in pumped diode laser in the blue appears to power, wavelength diversity, tunability, be at least 10 years away. However, and efficiency. The ruby laser, the electron-beam pumped and optically pumped Nd:YAG laser, and the Nd:Glass laser are diode lasers are being studied as are the all well developed technologies. The materials parameters necessary to flashlamp-pumpedlasers now generate very

11 high peak power in Q-switched pulses. Nd:Glass. Slab lasers of the YAG variety For example, the commercial unstable- have operated at greater than 400 W of resonator, Nd:YAG laser introduced in average pc)werwith 3% efficiency. Slab 1976 uses flashlamp pumping to produce lasers using glass have operated at 1 J/pulse at up to 30 Hz repetition rate 30 J/pulse!at 1 Hzor10 J/pulseat5 Hz. in a 10 ns, Q-switched pulse or 100 MW of Even higher average-power appears peak power. This laser can be possible, using the moving-slab laser efficiently frequency-doubled to the concept to dissipate the heat over a wide green and converted furtherto 355 nmand area. A moving-slab, glass laser with 266 nm in the UV. The frequency-doubled 390 W of average power has been Nd:YAG laser is useful for pumping dye demonstrated in Japan at Hoya, Inc. lasers as mentioned above. Recent Flashlamp-pumped,solid-statelasers progress in this technology has led to have efficiencies in the 3%to 5% range. injection seeding for single-axial mode Q-switchecl,or mode-locked, solid-state control. Single frequency operation by lasers for operation at high peak-power injection seeding leads to very stable levels are 1% efficient. A leap in peak-to-peak power output which is efficiency has been obtained by replacing important for frequency-doubling the flashlamp with diode lasers as pumps applications such as spectroscopy or in solid-state laser systems. materials processing. Diode-Pumi)ed, Solid-State Lasers. In the late 1970’s the tunable, 13ypufil solid-state lasers with diode alexandrine 1aser was introduced. lasers, the power of many diode lasers Alexandrine is a crystal doped with can be summed and extracted in a chromium ion that provides tunable diffraction-limited beam of a single radiation in the 700 to 800 nm region. frequency,, Further, the ability of ions The laser can be flashlamp pumped, cw doped into a solid to store optical arc-lamp pumped, or pumped with other energy for many milliseconds allows the lasers. To date, alexandrinelasers have cw diode--laser.power to be extracted in -beendeveloped for research applications, high peak-power, Q-switched or mode particularly for remote sensing. Recent locked pulses with pulse widths of work has led to al.exandrite-laserswith nanoseconds to picosecond. Diode- output power levels in excess of”300 W pumped, solid-state lasers were first which operate at 3% efficiency and are explored in 1963 shortly after the useful for industrial applications. invention of the diode laser. Today, Thetunab7e, alexandrinelaser ledto diode-pumped, solid-state lasers are further investigationsoftunabl e, solid- operating with Nd:YAG, Nd:YLF, Nd:Glass, state lasers. In particular, the and Ho:YAG at 2.1 microns, and in Er:YAG tunable, titanium-sapphire laser can be at 1.55 microns and 2.9 microns. Diode- Iaserpurnped like a dye Iaser and operate pumped, nonplanar-ring, Nd:YAG lasers over a wide tuning range, from 700 to have operated at a single frequency with 1000 nm. Titanium-sapphire lasers have linewidths that are less than 2.9 Hz and been introduced as commercial products subhertz linewidthsmay be possible soon. for tunable sources complementary dye Diode-pumped, solid-state lasers are Q- lasers in the near IR region and switched with kilowatt peak power levels, producing 1 W of cw radiation. are mode locked with less than 10 ps The importance of high average-power pulse wi~dths, and are operated CW. has led to the development of Nd:YAG Programs now underway promise to raise laser sources for industrial the average power level to greater than applications, Producing kilowatts of 100 W by 1992, and have been considered average power. To date, companies have for producing megawatts of average power. introduced multiple-rod lasers that Recently, and of critical importance generate more than 1 kWof average output to many applications, diode-pumped, power in a multimode, nondiffraction- solid-state lasers have now operated at limited beam. There is work in progress greater than 20% electrical efficiency. to generate high average-power by using The technology is advancing rapidly slab geometry lasers of both Nd:YAG and in Europet Japan, and in the United

12 States. The recomition of the given by 1s/7’, where As is the wiggler importance of this new-solid-state laser period, (i.e., the distance to complete technology is illustrated by the fact one cycle of the spatially varying that more than seven startup companies magnetic field), and 7 is the ratio of have been formed in the past two years to the electron energy to the electron rest exploit diode-pumped, solid-state-laser energy (0.511 MeV). Thus, the wavelength technology. However, the United States of the free-electron laser is controlled is in danger of losing the lead in a by the 1ength As, determined by the technology that was pioneered in this magnetic field, and the energy of the country because of the lackof investment electron accelerator. With electron in the low-cost production and energies of 100 MeV it is practical to manufacture of diode lasers and diode- produce wavelengths in the visible laser arrays for potential applications region. For a fixed wiggler period, a to commercial markets. tuning range of a factor of 2 or 3 is The high efficiency of solid-state achieved by changing the electron energy. lasers when pumped by diode-laser arrays Although the laser gain decreases opens many application areas that were with increasing frequency, this can be heretofore impractical to consider. 1argely offset by producing higher Among them are laser-driven chemistry, intensity electron beams of very low isotope separation, remote sensing, spread in emission, as demonstrated by coherent laser-radar, high-definition the development of a photoelectric television (HDTV) image projection, UV injector at Los Alamos National and X-ray lithography, and optically Laboratory. New techniques are also driven particle accelerators. Some being studied for producing much shorter industrial applications that require wiggler periods permitted by the higher compact, highly efficient, high average- quality electron beams. Acceleration power laser sources have also been techniques based on lasers could permit considered. much shorter accelerators than does the current microwave technology. These developments, together with the new 3.2.3. Free-Electron Lasers mirror technology, may permit widely tunable, compact, and efficient free- Free-electron lasers make use of electron lasers for wavelengths as short electron accelerators and other as 10 nm and as long as millimeters, with specialized components such as magnets very high peak and average power. (cal1ed wiggler magnets) to produce Free-electron 1asers producing high magnetic fields with spatial periodicity. average-power at short wavelengths would The free-electron laser is usually viewed be useful in computer chip fabrication, as a user-group facility, since it is too in solid-state research, in biological expensive to be widely duplicated. The and medical applications, and in chemical first free-electron laser, at Stanford processing. Their intensity far exceeds University, has been replaced with a new that of synchrotrons within their device. Since 1985 about 10 devices have frequency operating range. been operated at wavelengths from the millimeter region to the visible (Brau 1987). 3.2.4. X-Ray Lasers

In these lasers, electron beams are The promise of coherent sources of passed through wiggler magnets at high intensity radiation in the X-ray velocities close to the speed of light. region as a tool for condensed matter Thus, the electrons execute a wiggly research, (particularly in the field of motion along the path through the biology and medicine), has led to magnet”ic field and radiate at the continuing efforts to produce X-ray frequency of the oscillation. The 1asers. These lasers require extremely wavelength of the radiation is Doppler high pumping rates to generate the shifted to shorter wavelengths and is necessary population inversion. For

13 instance, it has been shown that a 1 kV which pred’ict that the present laser pump 1aser requires a deposition rate should produce wavelengths as short as >10’4 U/cm3, and it is only recently that 1 nm. A self-channelling mechanism is the appropriate power densities have predicted which should produce X-ray become available, using high power lasers laser beams a few micrometers in as the pump. Two general techniques have diameter. been proposed, and both have been It is important to note that the successfully demonstrated. X-ray laser mechanism postulated to The first of these, originally explain thlese experimental results would demonstrated at Lawrence Livermore result in pulses of 100 fs duration with Nat~onal Laboratory, involves the laser sufficient intensity to produce X-ray heating of a cylindrical volume with a holograms of living material in its cross section of several tens of hydrated form without distortion. The micrometers and several centimeters in target would be completely destroyed by 1ength. This requires a sufficiently both thermal and radiation damage high temperature to produce neon-like effects, blut only after the information ions of moderate atomic weight, such as had been obtained. Other laser processes selenium. Electrons in the plasma are with longer pulses could not avoid these allowed to cool and recombine to fill the destructive effects. inner shell vacancies at a sufficient rate to produce gain and subsequent lasing. Since this process competes 3.2.5. ~nlinear Freouencv Extension against black-body radiation, as the plasma is optically thick, it is quite The potential applications of inefficient (-10-’0), but the very high lasers, even with average power in the power and energy used produces an X-ray kilowatts region, is limited unless the beam of high intensity. Because of the radiation can be shifted by nonlinear increasingly strong competition with techniques to a frequency region of other modes of energy loss, it appears interest. Some recent demonstrations doubtful that wavelengths <3 nm (400 eVj have been made of nonlinear optical can be produced by this technique, techniques which al 1ow wavelength although this wavelength would work in extension of the diode-pumped, solid- the water window. However, radiation state lasers which are efficient at the damage to biological material appears to milliwatt-power levels. It has long been be a severe problem. recognized that lasers with high peak A second approach for producing an power can be efficiently converted to X-ray laser has been demonstrated by a other wavelengths by second harmonic group at the University of Illinois at generation, su:n~d~~cnce frequency Chicago, in which the pump energy is generation, parametric coupled only to the electrons, leaving oscillation. Pulsed lasers with high the ions cold. An extremely high- peak powe!r operating in the near-IR intensity UV beam pulsed in femtoseconds region can be frequency-doubled, produces inner shell excitation of -tripled, and -quadrupled with greater multiply excited states by an efficient than 50% conversion efficiency in multiphoton coherent process. This potassium di -hydrogen phosphate (KDP). results in highly selective transitions what is new is the demonstration in to either low lying or ground state the laboratory of efficient harmonic levels of the ions, ensuring inverted generation at the milliwatt-level withcw populations. The resulting high gain of laser sources. By taking advantage of the stimulated emission produces a the frequency stability and single traveling-wave laser which fo?lows the frequency operation of the diode-pumped, pumping pulse due to the short emission solid-state laser, cw doubling using time of the transitions. A number of external resonance -techniques has been lasing transitions between 50 and 100 nm achieved i~t greater than 56% conversion have been observed in argon, krypton, and efficiency. For example, 52 m~ofcw at xenon; and scaling laws have been derived a 1064 nm output from a diode-pumped,

14 Nd:YAG, ring laser has been converted to efficiency with less than a 20 I(Hz 30 mW of cw green radiation with an linewidth. overall conversion efficiency of 56%. Parametric oscillators have also The internal conversionefficiencywithin been operated in a sync-pumped, mode- the 12 mm long, lithium-niobate crystal locked manner with tunable output at was 72%. Improvements in the coatings 300 fs pulse lengths. Ultraviolet applied to the crystals will likely raise radiation from the third and fourth the conversion efficiencyto greater than harmonics of a Q-switched Nd:YAG laser 85% in the near future. has been used to pump barium borate as For efficient nonlinear conversion, OPO’S. Using the crystal beta-barium- the phase velocity of the fundamental and borate, (first grown in Germany but harmonic waves must be matched. developed in China), a 355 nm pumped Traditionally, this was accomplished by parametric oscillator has been using the birefringence of the nonlinear continuously tuned over a wide range of crystal to offset dispersion. Another wavelengths. This widely tunable OPO approach is to reverse the sign of the operated at 30 Hz repetition rate at over nonlinear coefficient each time the phase 35% optical conversion efficiency at of the fundamental and second harmonic greater than 140 mW of average output wave is reversed; accomplished by power. The tuning range was achieved by reversing the sign of the polarization in rotating the crystal angle by just a few a ferroelectric crystal such as lithium degrees. Upon injection seeding, the niobate. Recent results have led to the parametric oscillator operated at a first demonstration of this periodic single frequency. poled phasematching for harmonic In summary, OPO’S offer the generation of blue light at 420 nm in potential for all solid-state, tunable lithium niobate. This technology allows sources with wavelength tuning from the nonlinear processes to be optimized UV to the IR. The improvement in across the entire transparency range of nonlinear materials, coupled with the lithium niobate from 400 nm to beyond improvement in laser-pump sources, has 4000 nm in the IR. For example, a diode led to increased interest in parametric laser can be frequency doubled in a oscillators as solid-state alternatives lithiurn-niobate waveguide by using to the dye laser. periodic poling to phasematch the Hiuh AveraQe-Power, Nonlinear interaction. This creates the Processes. High average-power,nonlinear possibility of using lithographic, optical conversion is now being planar-processing techniques to build investigated. Recent results show that frequency doubling crystals on planar the expected doubling efficiency to green substrates in large quantities and at low and to the UV is greater than 85% in each cost. step of the process. For example, Tunable Parametric Oscillators. The second-harmonicgeneration of a 10M, CW, optical parametric oscillator (OPO) Nd:YAG laser using external resonant converts high frequency radiation into doubling in barium borate should give8 W tunable low frequency radiation. For of green output. A second barium-borate example, green radiation from a doubled crystal should allow efficient generation Nd:YAG laser source can be used to pump of 266 nm. Thus, based on the a parametric oscillator in lithium theoretical findings, the harmonic niobate to generate tunable output from generation processes are no longer the the 680 to 4000 nm spectral range. limiting factor in producing efficient Previous work in parametric oscillators radiation at wavelengths extending into has used pulsed-laser sources for the UV. pumping. Recent work has shown that the Earlier work has shown that the parametric oscillators can operate cw barium-borate crystal can be used to with a 910 mW threshold. When pumped at frequency double even when the average 20 mW, the cw Iithium-niobateparametric power levels exceed 100 W without special oscillator operated at 80% conversion precautions. There is an opportunity,

15 with future research, to extend harmonic damage arising from nonlinear self- generation to the average-power level of focusing due to self-phase modulation. kilowatts using barium borate. This, in turn, opens the possibility for development of an all solid-state source of tunable, CW, coherent radiation with 3.3. DETECTORS FOR LASER SPECTROSCOPY greater than 20% electrical efficiency. Such a laser source could be used to improve the separation rate of isotopes 3.3.1. ~on Detectors and at lowermost. Further, such a laser source could offer the potential of There is a growing need in many extending laser processing to other fields of research for photon detectors chemical applications since the cost per that are faster, have a broader spectral photon is less than the most efficient range, and are more sensitive than incoherent source of radiation--the currently available detectors. Research mercury discharge. applications include monitoring fast- Picosecond and Femtosecond Laser pulsed lasers and characterization of Sources. The 1970’s were characterized high-speed optical components such as by the picosecond-time scale. The 1980’s modulators, detectors for atomic and ushered in the femtosecond-time scale. molecular spectroscopy, and integrated To date, the shortest optical pulse optical detectors for the new generation generated is approximately 5 optical of compact flow cytometers. cycles in 1ength. The pulse was Solid-state photon detectors offer generated in adyelaser by using methods fast response times and small size. At to cancel group velocity dispersion. lower frequencies, the most sensitive In recent work, short pulses have photon d[~tectors are avalanche been generated in Nd:Glass, Nd:YAG, and photodiodes because of the internal gain Nd:YLF when these were pumped with diode of these devices. (Bowers and Burrus lasers. The pulse lengths for cw 1987). At higher frequencies, a p-i-n operation under diode-laser pumping are photodiode followed by a field effect now 6 ps. However, by using pulse transistor (FET) amplifier is a better, chirping in an optical fiber followed by choice. Photoconductive detectors have pulse compression using a grating pair, the edge when response times of less than the pulse length can be reduced to the 1 ps are recluired,although the detector 10 to 100 fs region. This source of sensitivity is greatly reduced. short optical pulses requires no water Solid-state photon detectors can be cooling and can be operated as a simple incorporated directly into integrated plug-in to existing sampling scopes. optical assemblies. Monolithically These picosecond and femtosecond, all integrated amplifiers and detectors have solid-state, lasers will find been fabricated using standard ion- applications in electro-optic sampling, implanted technology (Wojtczuk et al. chip-to-chip communications,and perhaps 1987). A photoconductor version of the in biology for measurements of lifetimes device has a relatively high gain of 27 of excited states. and a bandwidth of 25 MHz. A photodiode Recent work has also shown that it version has a maximum sensitivity at is possible to generate pulses with 800 nm, making it potentially useful in terrawatt peak power by using pulse flOw cytometry for detecting expansion and amplification,followed by fluorescence. Shottky-barrier photo- pulse compression. A group at the detectors with an interdigitalelectrode University of Rochester has obtained configuration have been fabricated on a pulses with terrawatt peak power, i.e., silicon-on-sapphire (s0s) substrate 1 J in a picosecond pulse, by the use of (Bruce et al. 1987). The impulse pulse compression from 1000 ps to 1 ps. response time is less than 30 ps. The This technique is important because it wavelength range is from the IR to the allows efficient extraction of stored Uv. This design provides efficient energy from a solid-state laser without coupling of light from an optical

16 waveguide into the photodetector into photons with an average wavelength fabricated on the same substrate. of 443 nm. This longer wavelength Measurements of the fluorescence radiation is easily detected with a blue- lifetimes of biologicalmolecules require sensitive photocathode material. photon detectors with high quantum During the very early years ofRaman efficiency, high gain, fast time spectroscopy development, most spectral response, and the ability to do photon data were detected by photographic counting. Microchannel plate emulsions. With the advent of scanning photomultipliers (MCP-PMT) have been spectrometers, photoelectric detection, developed recently with these based upon a photomultiplier fabricated characteristics (Kume et al. 1988). A from a photoemissive cathode and a microchannel plate (MCP) is a secondary current amplifier within a glass electron multiplier constructed from envelope, became the usual basis for millions of glass capillaries fused recording Raman data. The most common together and cut to form a thin disk. photocathodes are the bialkali systems The MCP is placed between the as, for example, (Cs)Na,KSb. Raman- photocathode and the anode of the PMTand scattered radiation generally involves has a potential across it of perhaps low light 1evels, producing weak 3000 V. An electron striking the wal1 of electrical currents from these surfaces. one of the microchannels produces When the current is of the order of 10” secondary electrons which give rise to a to 10’3 amps, photomultipliers are cascade of further secondary electrons. conventionally used in pulse or photon The gain ofa single stage MCP-PMT can be counting modes. For photocurrents in the as high as 104. With an MCP-PMT having 10-9to 10-” amp range, direct current 6ym diameter capillaries, it is possible detection methods may be applied. For to measure fluorescence decay time systems with high background radiation, constants as small as 6 ps with a phase-sensitivedetection techniqueshave precision of 3 ps. Further development proven useful. The low signal levels of these devices can be expected to encountered in Raman studies can also be result in faster response times and recorded with a variety of intensified higher gains. optical multichannel array detectors. Silicon-intensifieddetectorta~g::e ~:;;~ These detectors can be cooled to about vidicon photon -40°Cto reduce noise levels. The charge- numerous applications ‘ coupled device (CCD) arrays are being spectroscopy (Olesik and Wal’~ers~~~~~ applied to Raman spectroscopic studies. An image intensifier multiplies and Since a CCD may contain an array of 400 focuses photoelectrons onto a silicon x 600 pixels, large amounts of data can target. The target is an array of be generated with a single exposure of 500 x 500 capacitors that store signals the chip to the Raman radiation. The related to the number of photoelectrons two-dimensionalityofthese low-noiseCCD striking each picture element (pixel). detectors will be useful for obtaining The pixels are read out by a scanning both spectral and spatial information. electron beam. Time-resolved emission spectra have been obtained by suitably programming the scanning readout. The 3.3.2. Ionization Detectors two-dimensionalcapabilityofthevidicon has been used for fluorescence Sensitive detectors measurements by dispersing the excitation (electrical fragments of mat;~r) ;~l~ 1ight along one dimension and the been available since the turn of the 20th emission light along the other. century. The first electrical counter Sodium salicylate is commonly used (the precursor to the proportional as a photon detector in the wavelength counter) was developed by Rutherford and range from 20 nm to 360 nm. The sodium Geiger in 1908. In the succeeding 40 salicylate, which is usually sprayed years, this detector was refined by directly onto the outside of the Curran et al. (1949) to such a degree photocathode, converts the UV photons that single electrons could be detected.

.- A closely related detector, the Geiger- graphics and in high-speed processing of Mueller counter, can also count vast amounts of data with sophisticated individual electrons, but, unlike the software, together with the steady proportional counter, it cannot decrease in computing cost, makes distinguish one electron from several or possible thleprocessing and display of a a million created inside its gaseous wide variety of sensor data as video medium at the same instant in time. images. Internal relationships, The proportionalcounter was combined structure, and meaning can be recognized with lasers to count single atoms. This and analyzed more easily in the video was made possible by the development of format. Excellencexamplesa recomputer- RIS by Hurst et al. (1975). RIS is a aided, X-ray tomography and nuclear- process (see Section 4.5) in which a magnetic-resonance scans, both of which laser can be “tuned to an atom” so that can reveal details of the structure of each atom of the selected type will give the soft tissues and organs of the human up one of its orbiting electrons when body. New detectors such as the CCD or subjected to a single pulse of a laser. charge injecteddevices (CID) record two- This illustrates the enormous analytical dimensional, digital images with much sensitivity for counting atoms when laser higher sensitivity (-1 photon) and spectroscopy is combined with ionization dynamic range or contrast (-105)than does detectors. film but with somewhat lower resolution In addition to the proportional and (-200 lines/mm). New video-imaging the Geiger-Mueller counters, there are devices such as flat-screen, full-color, other methods for counting single liquid-crystal displays are also electrons. In an evacuated enclosure, appearing. These promise much lower single electrons can be accelerated to cost, improved convenience, and greater strike the first surface of an “electron reliability than do cathode-ray tubes. multiplier” tube where an electron Sophisticated computer codes permit cascade is initiated. The “channeltron” the two- and three-dimensional and time is another detector in which electrons development of complex data fields such are amplified sufficiently to record one as temperature,density, composition, and initial electron. structure in real time. Image Often it is desirable to do a mass enhancement can filter, increase analysis of the atom (or a molecule) contrast, or reduce noise to bring which has been ionized with a laser. In details out of an apparently featureless this case, it is necessary to detect the background in many situations. Computer positive ion. Fortunately, the positive codes also permit the construction of ions can be accelerated into a vacuum to colored video images of models or a modest energy so that on impact with a simulations in two and three dimensions converter plate, several electrons are which can be rotated, stretched, or emitted. Thus, single electron detectors modified as directed or under the can be used to detect single ions. By influence of a superimposed environment. using a combination of lasers with mass spectrometers (see Sections 3.6.1 and 4.5), it is possible to achieve nearly 3.5. RESERVOIRS single-atom sensitivity, even when both the type of atom and its mass are selected. 3.5.1. @production

A key development for trace 3.4. IMAGING detection ofatoms and molecules by laser radiation is the design of new sources A number of recent developments are for vaporization into the gas phase. The providing a basis for important problems are very different, depending contributions of video imaging in areas upon whether the sample is atomic or of interest to OHER. Computer molecular. In the case of atomic developmentwith increasingcapabilityin reservoirs, the main problem is to

18 vaporize a refractory sample (either as atomic reservoirs include electrothermal a solid or dissolved in a liquid) into atomizers such as graphite furnaces, free atoms in the gaseous state. The boats, crucibles, and metal filaments. sample must then be transferred into the Graphite furnaces provide an extended probing volume of the light source and probe time for enhanced sensitivity over remain in that volume for a period of flame methods. In addition, pyrolytic time long enough to assure efficient graphite furnaces, with an argon buffer, detection by laser-based spectroscopic provide rapid atomization in a uniform methods. In the case of molecular temperature enclosure with almost reservoirs, the main challenge is the negligible matrix interference. This development of methods for volatilizing lack of matrix interference is due to a thermally labile molecules without temperature profile in which the breaking them apart. This will be background organic material is ashed out critical for the detection of before the measurement is performed. biomolecules, especially in combination Graphite furnaces combined with laser with 1aser ionization for mass methods (fluorescence and ionization) spectrometry. have reached sensitivities in the attogram range.

3.5.2. Classical Atomic SDectroscooic Methods 3.5.3. Glow Discharges

With existing atomizationtechniques, A more recent development in atomic attention must be given to two main reservoirs is the glow discharge source. areas: (a) the probing volume and In this method, a discharge is formed in temporal duty factor of the laser source a pressure range of <10 torr. Cathodic in the atomizer and (b) the residence or sputtering occurs in the discharge with transit time of the atoms in the probe the result that its negative glow region volume. To achieve high sensitivity and becomes a reservoir of neutral sputtered the minimum sample size, atomic sources atoms plus excited atoms and ions arising must be designed to match the shape and from the sputtered flux. In the absence extent of the probe volume of the laser. of thermal evaporation, bulk diffusion, Residence time depends upon the type of or surface diffusion, the composition of atomizer and the specific conditions the material sputtered from a present, such as gas flow velocity and multielement solid under steady-state atomizer probe volume. There are several conditions reflects the composition of types of classical atomic sources that the bulk solid. The work of Harrison and have been used with laser-based methods co-workers (Harrison et al. 1986) has and for which further development would shown this source to be a useful method be increasingly useful for trace for trace analysis of materials on analyses. Combustion flames and plasmas surfaces in combination with specific are widely used for atomic analyses with 1aser detection of the sputtered classical light sources and are known for neutrals. This method has outstanding their reliability, speed of analysis, potential for trace analysis of metals multielement determination capability, and dielectrics which is of considerable and detection limits. These limits may interest to the semiconductor industry. lie in the pg/ml range for some elements Several interesting developments include when laser-based methods are used (Skoog the pulsed glow discharge which can be 1985; Christian and O’Reilly 1986). One used effectively with pulsed 1aser of the most widely used of the plasma sources. The second important sources is the inductivelycoupled plasma development is the use of magnetically (ICP) which, because of its high plasma confined plasmas (McCaiget al. 1989) temperature (7000-10,000 K) provides which can operate at relatively low excellent atomization and thus remarkably pressure (10”4torr) and at sputtering high sensitivity (pg/ml) even for the currents equivalent to or greater than most refractory elements. Classical that of the glow discharge described

19 above. These devices, called magnetrons, In the resorption process, neutral have great potential for rapid sputtering molecules, rather than ions, generally in the semiconductor industry. form the bulk of the desorbed material. Several groups have thus used a second UV laser to induce ionization with the 3.5.4. Laser-Induced Evaporation distinct a~dvantages of (a) increased sensitivity due to the efficient A very promising method for multiphoton ionization (MPI)process, (b) vaporizing both atomic solid samples and enhanced selectivity based upon the large biomolecules uses laser-induced spectral i~bsorption features of the evaporation (Lubman 1988a, 1988b; molecule in the MPI process, and (c) Grotemeyer and Schlag 1988; Hayes 1987; controlled fragmentation for structural Coates and Wilkins 1987; Beavis et al. analysis usingMPI. Although this method 1988). In the case of atoms, the process has only been applied to molecules requires focussing a high-powered pulsed <1,500 amu,,it has the potential to have laser to ablate the atoms from the solid, significant impact the detection of large creating a plume that may consist of biomolecules. There is considerable neutrals and ions. In laser methods of potential here for the development of atomic analyses, this type of atomization this technique for application with has several advantages: (a) an classical enzymatic methods, to the improvement in the duty cycle since a sequencing of proteins and DNA fragments pulsed laser is used for vaporization and (see Section 5). for subsequent laser excitation, (b) an improvement in the background since bulk heating of the sample is minimal, (c) 3.5.5. ~ticle-Induced Va~orization achievement of high spatial resolution for microprobe experiments, (d) probing Ion sputtering has been shown to be of both metals and dielectrics with a very promising atomizationmethod with minimal sample preparation, and (e) the capability for high spatial absolute sensitivity in the femtogram resolution<) thus making it suitable for range. the local analyses of solids and thin In the case of molecules, ,a pulsed films with sensitivitiesreachingsub-ppb laser is used to desorb rapidly a sample 1evels. The neutral atom yield, as a from a surface before the molecules have rule, is 2 to 4 orders of magnitude time to kinetically decompose (Lubman higher than the ion yield and is only 1988b). This is a much more gentle weakly dependent on the matrix. The process than is atomic ablation since the sputteringyield reaches its maximum when sample is simply vaporized off the the ion energy is typically in the range substrate surface and no apparent damage of 5 to 50 keV. When the sputtering is to the surface occurs. This process is employed to initiate the resonance generally thought to depend upon a rapid ionization process, it is referred to as temperature increase on the surface in sputter-initiated resonance ionization which ions and neutrals are produced in spectroscopy (SIRIS) (Parks et al. 1983; a ratio that depends on the temperature for additional detail, see Hurst and reached on the surface. In much of the Payne 1988). This technique has been work, a pulsed C02 laser produces ions used for sensitive depth profiling with froma surface that are directly detected important applications in the in a mass spectrometer. This laser has semiconductor industry. Winograd and his been used to detect compounds such as colleagues (Baxter et al. 1986) have made oligonucleotides and peptides in a mass important contributions to the spectrometer. More recently, UV sources understanding of ion sputtering and have been used for direct resorption and associated surface interactions using ionization of bovine albumin at M/z of RIS. 66,750 amu, other high molecular weight In the molecular domain, Beavis et compounds (>10,000 amu) have been al. (198f3) have used fast atom detected (Karas and Hillenkamp 1988). bombardment (FAB), where a beam of

20 neutral argon atoms is used as a means to opportunities here for further induce resorption of small peptides from investigation of polar SF solvents for a substrate for detection by MPI. Ion extending this promising method to resorption and electron-stimulated biologicalanalyses. Other opportunities resorption (ESD) have also been used (by for volatilizing biological materials the Becker group at the Stanford Research will arise from the use of highly polar Institute) to vaporize polymers and liquid solvents for jet introduction. biomolecules for detection byMPI andVUV These methods appear particularly ionization (Schuhle et al. 1988) ESD has promising for interfacing with liquid been found to be a very effective means separation techniques for the study of of vaporizing fragile molecules with biological molecules in real matrices minimum decomposition. These techniques (see Section 5). generally provide a much lower flux of material per unit time than does laser resorption, and thus it is necessary to 3.5.7. Other Methods use a high repetition rate for the detection laser in order to obtain Several special techniques may be reasonable sensitivity. Nevertheless, applicable where extreme trace detection these methods will provide exciting is required. These include the atom opportunities for vaporizing 1arge buncher or multistage atomization method biomolecules and polymers for detection where the analyte is preselected by a with mass spectrometers (see Section 5). laser-assisted separation technique and/or mass separator and is accumulated in a foil (ion implantation) before final 3.5.6. Su~ercritical and Liquid atomization (by ion sputtering or laser In.iectionin Jets ablation) takes place. These methods may also be applied to more stable molecular Liquid injection and supercritical systems. Finally, the development of ion fluid (SF) injection have served as a traps and laser cooling methods, where means of introducing nonvolatile atomic or molecular ions or even neutral molecules and chelated metals into mass atoms can be held for long periods of spectrometers for some time (see, for time, would allow spectroscopicdetection example, Pang and Lubman 1988). The studies to be performed with impressive analyte of interest is dissolved in a levels of accuracy and resolution. solvent and injected as a gaseous jet into vacuum. Supercritical fluids present important opportunities since 3.6. SYSTEMS they can dissolve large molecules like a liquid but then expand as a gaseous jet in a vacuum. This presents excellent 3.6.1. Laser Mass SDectroscoDY Systems opportunities for selective detection by MPI or laser-induced fluorescence (LIF) Mass spectrometry has proved to be based upon supersonic jet spectroscopy. a powerful tool for chemical analysis The combination of SF injection and jet based upon exact mass/charge (M/q) spectroscopy is being studied by several identification of molecules. The mating groups. The groups of Lee and Goates at of lasers to mass spectrometers (MS) is Brigham Young University are,developing a natural combination since direct laser methodology for C02 and N20 capillary resorption and MPI methods are often able separation of polynuclear aromatic to produce molecular ions or hydrocarbons (PNAH) in coal liquids with quasimolecular ions for identification; selective laser (LIF) spectroscopic these methods have been extended for detection. In recent work, Lubmanand co- structural analysis of polymers, workers have used highly polar SF ammonia peptides, and oligonucleotides (Lubman solvent to solubilize polar biological 1989; Grotemeyer and Schlag 1988; Campana molecules such as nucleosides and small 1987). In addition, these methods have peptides. There will be important been used to study PNAH’s in coal fluids

21 (Dobson et al. 1986) and coal macerals. techniques. The most recent developments The main limitation in this methodology in resolution improvement for TOF have is the relatively low repetition rate of been achieved using velocity compaction. most pulsed UV laser systems. In order There are real opportunities here for to use scanning mass spectrometerswith developing sophisticated scanning laser-induced MPI, continuous jet electric fields for achieving very high expansions with high repetition rate resolution as shown by Enke’s group at lasers (>104 Hz) must be developed. Michigan State (Yefchaket al. 1989). However, the alternative has been to use A second methodology that has been pulsed injection sources to produce high used with li~serionization anddesorption density pulsed molecular beams coupled is ion trilpping, using ion cyclotron with pulsed lasers. This requires the resonance devices (ICR) (Gross and Rempel use of mass spectrometers such as time- 1984) and other forms of ion traps of-flight (TOF) and fourier-transform (Allison and Stepnowski 1987). The ICR- mass spectrometry (FTMS) devices which MS utilizes a high magnetic field to can detect the whole mass spectrum on cause ions to undergo a cyclotron motion every laser pulse. in a small ion storage cell under vacuum Early TOF devices were basically (10+ torr). According to the cyclotron flight tubes with a diode source equation, ions are mass sorted according acceleration region. These early devices to their frequency of revolution. were capable of a resolution of several Very high resolution (in excess of hundred at best. This may be the MS 10’)has been obtained in the modern FTMS technology of the future for biological version of this device (Shomo et al. applications and analyses, especially in 1985), and thus it has tremendous conjunction with laser resorption, laser capabilities for 1arge biomolecule microprobe, and laser MPI ionization analysis. This method can be used in a sources. The most promising TOF-MS tandem mode to elucidate MPI technology appears to be the reflection. fragmentat~on and photodissociation In this device, an ion reflector is used mechanisms and for structural analysisof to reverse the flight direction of the biomolecules. MPI or laser resorption ion packet. In the process, the initial may initially create several fragment energy spread of the ion packet is ions, and the resulting fragments can be minimized and the resolution increases studied in the tandem mode to understand dramatically. Since its introductionby fragmentation patterns or the structure Mamyrin and co-workers (1973), of a specific fragment of a biomolecule modifications have increased mass such as a DNA or protein. DOE is resolving powerto 10,000 (Grotemeyerand presently funding some work using this Schlag 1988; Campana 1988). Recently, technique iForthe study of inorganic gas one group has introduced a multipass phase reactions mechanisms in the lab of reflection device (Sakurai et al. 1985). Ben Freiser at Purdue. Some work on In principle, these devices could provide structural analyses using FTMS has been ultrahigh resolution (>100,000). performed by several groups (see, for Supersonic jet expansions used in example, Lubman 1989). conjunction with reflection devices, The drawbacks of this method, provide enhanced resolution by further relative to TOF devices, are the expense minimizing the energy spread in the of the FTMS (>$300,000) and the low- initial sample of molecules. pressure requirement. The latter has Other methods having enhanced TOF been partially solved using dual cell resolution include the use of energy devices. Simple ion trap devices using selectors or electric sectors in tandem combinations of rf and dc fields are now with the TOF device (Campana 1988). being used to trap ions in a storage cel1 Significant enhancementof resolutionhas at low pressures for hours, in some been obtained using this method, cases. A number of excellent reviews especially in conjunction with gas exist on the available modes of chromatography (GC) and high performance operations of these different traps liquid chromatography (HPLC) injection (Gross and Rempel 1984). They can be

22 used in a tandem mode very similar to refractive index detector can be used FTMS. The mass resolution of these with very small volumes. The deflection devices is not very high; however, they type of refractive index detector can are relatively simple and inexpensive to also benefit from the collimation build. They present excellent provided by a laser beam, as can opportunities for detection of relatively interferometricdetectors. AFabry-Perot small (c500 amu) labile biomolecules in interferometer has been used with a conjunction with direct laser resorption helium-neon laser to achieve a limit of and MPI methods. detection (LOD) of 20 ng. Absorption detectors are fundamentallylimited by shot noise. The 3.6.2. Laser Chromatoqraohic Systems high power available from lasers makes it possible to approach saturation of the The development ofmicrocolumnl iquid absorption transition. Intracavity chromatography during the present decade absorption schemes have been used to has led to significant improvements and obtain a LOD of 5 x 10-5absorbance units new opportunities in HPLC measurements. (Yeung 1986). Thermal lens calorimetry Among the advantages provided by the has been used with laser beams for HPLC microcolumns are higher efficiencies, detection. The intensity across a improved detection performance, benefits single-modelaser beam follows a Gaussian derived from reduced flow rates, and the intensity distribution therefore more ability to work with smaller samples heating will occur in the center of the (Novotny 1988; Barth et al. 1988). beam than at the edges. Observation of Reduced flow rates have increased the changes in the refractive-indexgradient mass sensitivity of concentration resulting from this heating has been used sensitive detectors. The microcolumns to obtain a LOD of 1 x 104 absorbance have also made it possible to exploit units in an 8 pL flow cell. Absorbance certain laser technologies for improving measurements in liquid chromatography the detection sensitivity (Yeung 1986). would be significantly improved by the These laser properties include high development of compact, tunable UV average-power, collimated beams, 1asers. monochromatic beams, and time resolution Laser-induced fluorescence as an (with pulsed beams). Raman scattering HPLC detector is very attractive from the and fluorescence can be separated by standpointof sensitivityand versatility using the time resolution feature. (Novotny 1988). High excitation Refractive index detectors can be intensity favors laser illumination. improved significantly with the use of However, the major limitations are stray 1asers. The reflection type of light, fluorescence or Raman scattering refractive index detector makes use of from the solvent, and fluorescence from the light reflected at the glass/liquid cell walls and windows. The collimation interface when the incident beam is near of the laser beam helps to reduce the the critical angle. The amount of light stray light problem. The monochromatic transmitted and reflected depends light from a laser helps discriminate strongly on the refractive index of the between fluorescence and Raman light. A liquid relative to the glass at the sheath-flow cuvette can be used to move interface. As different componentselute the cuvette walls away from the column from the column, the liquid refractive effluent. With this geometry, a LOD of index changes, causing a change in the 53 pg has been reported for 8 mW of amount of reflected light. The excitation at 488 nm. (Herschbergeret collimation of the laser beam increases al. 1979). A flow cell using fiber this sensitivity. The size of the optics in a fluorescencedetection system interaction region can also be reduced by has been developed and used to obtain a focusing the laser beam. Since only the LODof 1 pg (Yeung 1986). A laser-based region within a few wavelengths of the detector for HPLChas been developed that optical interface takes part in the enables simultaneous absorbance, interaction, the reflection type of fluorescence, and refractive index

23 detection for microcolumn liquid appropriate detectors is the major chromatography (Wilson and Yeung 1985). challenge. The refractive-index detector can measure Detection schemes for CZE include 6 ng of benzene, the absorbance detector absorption, fluorescence, conductivity, can measure 48 pg of bromocresol green, electrochemical,mass spectroscopic, and and the fluorescence detector can measure radioisotopicmethods. Direct absorbance 0.8 pgofpropylamine-NBD derivative. An in the UV has a detection limit of 10+ M. argon laser at 488 nm is the light Yu and Dovichi (1989) have recently source. Another detection system uses a reduced tlhis limit by two orders of helium-cadmium laser at 325 nm and can magnitude, using photothermal absorbance measure the fluorescence from femtogram with a 130 mW argon-ion laser. amounts of the dye coumarin 440 (McGuffin Conductivity detection has a detection and Zare 1985). Fluorescence detection limit of 10*M and has been used in CZE would be greatly improved with the to detect lmetalions, organic acids, and development of compact, tunable lasers inorganic ions (Foret et al. 1986). with wavelengths ranging from the UV to Electrochemicaldetection 1imits with CZE the IR. have been pushed down to 200 to 400 Optical activity has been used as a attomoles for catecholamines in a 26 pm chromatography detector (Yeung 1986). inside-diameter capillary tube When an optically active eluent is used, (Wallingford and Ewing 1987, 1988). An all of the eluted components are detected electrospray-ionization interface has unless they have the same optical been developed between a CZE apparatus activity as the eluent. If the same and a mass spectrometer (Mack et al. sample is first eluted with an optically 1970; Whitehouse et al. 1985; Olivares active eluent and then eluted with a et al. 1987; Smith et al. 1988). A CZE racemic mixture of theeluent, the eluted separation of ‘P-1abeled guanosine material can be quantified without the triphosphate and adenosine triphosphate need for standards and without prior injected alt5 x 10-8M has been detected identification of the components. with an on-line radioisotope detector Electrophoreticseparationshavebeen (Pentoney et al. 1989). Cheng and revolutionized by the development of Dovichi (,1988) have coupled a flow capillary zone electrophoresis (Virtanen cytometer to a CZE apparatus and used a 1974; Mikkers et al. 1979; Jorgenson fluorescence detector to push the limits 1981, 1983). In their review of of detection for fluorescing derivatives capillary electrophoresis (Gordon et al. of amino a~cidsto 5 x 10”’2M of alanine. 1989), the authors state that the use of capillaries as the electrophoretic migration channel will put CZE on the same footing as HPLC. In electrophoresis, molecules are separated on the basis of their electrophoretic REFERENCES mobility. The molecules to be separated are loaded into a tube filled with an electrolyte buffer, and a potential Allison, J. and R. M. Stepnowski (1987) difference is applied across the ends of Anal. Chem. 59, 1072A-1086A. the tube. The capillaries in CZE have thin walls and inside diameters of Barth, Hc~ward G., William E. Baker, 5.75 pm. The high surface-to-volume Charles H. Lochmuller, Ronald E. Majors, ratio means that heat is easily and F. E. Regnier (1988) Anal. Chem. m, dissipated through the wal1s and 387R-435R. convection does not occur. Resolutions equivalent to 1 million theoretical Baxter, J. P., J. Singh, G. A. Schick, P. plates have been obtained (Lauer and H. Kobrin:,and N. Winograd (1986) Nucl. McManigill 1986). Because of the small Instrum. Methods in Phys. Res. ~, 345. quantities of material that can be Beavis, R,,C., J. Grotemeyer, and E. W. analyzed by CZE, the development of Schlag (1988) paper presented atthe 1988

24 ASMS Conference on Mass Spectrometry and Harrison, W. W., K. R. Hess, R. K. Allied Topics, San Francisco, CA. Marcus, and F. L. King (1986) Anal. Chem. 58, 341A-356A. Bowers, John E. and Charles A. Burrus (1987) J. Lightwave Tech. ~, 1339-1350. Hayes, J. M. (1987) Chem. Rev. 87, 745-760. Brau, C.A., RECENT DEVELOPMENTS IN FREE- ELECTRON LASERS, Laser Focus/Electro- Herschberger, L. W., J. B. Callis, and Optics, Feb. 1987. G. D. Christian (1979) Anal. Chem. 51, 1444.

Bruce, D. M., R. J. Seymour, D. Cheong, Hurst, G. S. and M. G. Payne (1988) P. E. Jessop, and B. K. Garside (1987) PRINCIPLES AND APPLICATIONS OF RESONANCE Can. J. Phys. 65, 924-928. IONIZATION SPECTROSCOPY (Adam Hilger, Bristol, UK), Chapter 8. Campana, J. E., Ed. (1987) ANALYTICAL INSTRUMENTATION, Vol. 16 (Special issue Hurst, G. S., M. G. Payne, M. H. Nayfeh, on Time-of-Flight Mass Spectrometry - J. P. Judish, and E. B. Wagner (1975) Marcel Dekker, Inc.). Phys. Rev. Lett. 35, 82.

Campana, J. E., Ed. (1988) ANALYTICAL Jorgenson, J. W. andK. K. Lukacs (1981) INSTRUMENTATION, Vol 17 (Special issue Anal Chem. ~, 1298. on Lasers and Mass Spectrometry - Marcel Dekker, Inc., New York). Jorgenson, J. W. and K. D. Lukacs (1983) Science u, 266.

Cheng, Y. F. and N. J. Dovichi (1988) Karas, M., and F. Hillenkamp (1988)Anal. Science ~, 562. Chem. ~, 2301-2303.

Christian, G. D. and J. E. O’Reilly Kume, H., K. Koyama, K. Nakatsugawa, S. (1986), INSTRUMENTAL ANALYSIS (Al1yn and Suzuki, and David Fatlowitz (1988). Bacon, Boston), 2nd edition. Lauer, H. H. and D. McManigill (1986) Coates, M. L. and C. L. Wilkins (1987) Anal. Chin.~, 166. Anal. Chem. 59, 197-200. Lubman, D. M. (1988a) Mass Spec. Reviews Curran, S. C., A. L. Cockroft, and J. ~, 535-554. Angus, (1949) Philos. Msg. 40, 929. Lubman, D. M. (1988b) Mass Spec. Reviews Dobson, R. L. M., A. P. D’Silva, S. J. ~, 559-592. Weeks, and V. A. Fassel (1986) Anal. Chem. 58, 2129-2137. Lubman, D. M., Ed. (1989) LASERS AND MASS SPECTROMETRY (Oxford University Press, Foret, F., M. Deml, V. Kahle, and P. Oxford), in press. Bocek (1986) Electrophoresis ~, 430. Mack, L. L., P. Kralik, A. Rhonde, and Gordon, M. J., X. Huiang, S. I. Pentoney, M. Dole (1970) J. Chem. Phys. ~, 4977. Jr., and R. N. Zare (1989) Science 242, 224-228. Mamyrin, B. A., V. I. Karataev, D. V. Shmikk, and V. A. Zagulin (1973) Sov. Gross, M. L. and D. L. Rempel (1984) Phys.-JETP (Engl. Transl.) x, 45-48. Science ~, 261. McCaig, L., R. Sacks, and D. M. Lubman Grotemeyer, J. and E. W. Schlag (1988) (1989) “Radiative and Electrical Angewandte Chemie ~, 447-592. Properties of a Planar Magnetron Glow

25 Discharge Device,” Applied Spectroscopy Shomoj R. E., A. G. Marshall, and C. R. (in press). Weisenberger (1985) Anal. Chem. 57, 2940- 2944. McGuffin, V. L. and R. N. Zare (1985) Appl. Spectroscopy ~, 847-853. Skoog, D. A., PRINCIPLES OF INSTRUMENTAL ANALYSIS (1985) (Saunders College Mikkers, F. E. P., F. M. Everaerts, and Publishing, Philadelphia), 3rd edition. Th. P. E. M. Verheggen (1979) J. Chromatogr. ~, 11. Smith, R. D., J. Olivares, N. T. Nguyen, and H. R. Lldseth(1988) Anal. Chem. 60, Novotny, Miles (1988) Anal. Chem. 60, 436. 501A-51OA. Virtanen, R. (1974) Acts Polytech. Scan. Olivares, J. A., N. T. Nguyen, C. R. Chem. Incl. Metall. Ser. ~, 1-67. Yonker, and R. D. Smith (1987) Anal. Chem. 59, 1230. Wallingford, R.A. and A. G. Ewing (1987) Anal. Chem. 59, 1762. Olesik, John W. and John P. Walters (1984) Appl . Spectros. 38, 578-585. Wallingford, R. A. and A. G. Ewing (1988) Anal. Chem. 6Q, 258. Pang, H. M., andD. M. Lubman (1988) Rev. Sci. Instrum. 59, 2460-2463. Whitehouse, C. M., R. N. Dreyer, M. Yamashita, and J. B. Fenn (1985) Anal. Chem. 57, 675. Parks, J. E., H. W. Schmitt, G. S. Hurst, and W. M. Fairbank, Jr. (1983) in Wilson, Steven A. and Edward S. Yeung PROCEEDINGS SPIE CONFERENCE ON LASER- (1985) Anal. Chem. U, 2611-2614. BASED ULTRA-SENSITIVE SPECTROSCOPY AND DETECTION V, ~, Richard A. Keller, Wojtczuk, !$. J., J. M. Ballantine, S. Chairman/Editor (SPIE -The International Wanuga, and Y. K. Chen (1987) J. Society for Optical Engineering, Lightwave Tech. LT-5, 1365-1370. Bellingham, WA), pp. 32-39. Yeung, Edward S. (1986) in ANALYTICAL Pentoney, S. L., Jr., J. F. Quint, and APPLICATIONISOF LASERS, edited by Edward R. N. Zare (1989) Anal. Chem. 61, 1642. H. Piepmeier (John Wiley, New York), pp. 557-586. $akurai, T., Y. Fujita, T. Matsuo, H. Matsuda, and 1. Katakuse (1985) Int. J. Yefchak, G, E., G. C. Enke, and J. F. Mass Spectrom. Ion Processes~, 283-290. Holland (1989) Int. J. Mass Spectrom. & Ion Processes (in press). Schuhle, U., J. B. Pallix, and C. H. Becker (1988) J. Vat. Sci. Technol. A Yu, M. and N. Dovichi (1989) Mikrochim. t5(3),936-940. Acts (in press).

26 ~. STATUS OF LASER SPECTROSCOPY

4.1. INTRODUCTION excitation of the samDle. There are a variety of ways in which the presence of “Spectroscopy” is the study of the this excitationmaybe detected, however, energy levels of atoms, molecules and and each defines a method of solids, as revealed in the characteristic spectroscopy. Generally, while each optical lines which are absorbed or method has advantages in some emitted in transitions between these circumstances, none is preferable in all levels. Before the invention of the circumstances. In “absorption laser in 1958, these spectral lines were spectroscopy” the presence of sample studied with one of the two methods of excitation is detected through the classical spectroscopy -- (a) “emission consequent loss in intensityof the laser spectroscopy” where the light emitted beam. In “fluorescence spectroscopy,” from an electrically or thermochemically the excitation is detected by collecting excited sample is spectrally analyzed to the photons which are spontaneously reveal the characteristicemissionlines, emitted as the excited sample decays. In and (b) “absorption spectroscopy” where RIS and its many variants, the laser- a continuum light source is passed induced excitation has the effect of through an absorbing sample and then ionizing the sample atom, thus allowing spectrally analyzed to show the reduced for very efficient detection of the intensity at the characteristic excited species in a mass spectrometer. wavelengths. With the invention of the “Photoacoustic spectroscopy” is a laser, light became available in a new technique where a modulated laser beam form -- a highly directional, spectrally generates periodic absorption of laser narrow and very intense beam -- and this energy which is converted by collisions has led to the development of many new to a sound wave at the modulation methods of spectroscopy which have frequency. “Photothermal spectroscopy” greatly increased our ability to study is somewhat similar except that the and to utilize atomic and molecular collisionally dissipated excitation spectra. Many of the applications of energy is detected by the temperature these new techniques, such as sensitive rise which it causes in the sample. This detection of chemical pollutants and can be sensitively detected by the radioisotopes, are directly relevant to thermally induced change in the sample’s the OHER mission. The rapid pace of index of refraction. There are many development of new laser sources and new variations on each of these schemes, and methods of spectroscopy continues today, recent developments affect all of them. and judging from past experience, it is These are discussed briefly below. quite likely that many techniques which In addition to the many new methods will be widely used in ten years time which have been developed to detect have not yet been invented. With this in laser-induced atomic transitions, the mind, this section gives a brief review high intensity of laser light has made it of the current status of 1aser possible to induce atomic transitions spectroscopic methods, with an emphasis which were unknown to classical upon recent developments and trends wh’ich spectroscopists, and which typically suggest future directions. involve simultaneous absorption or Almost al1 methods of 1aser emission of many photons. New light spectroscopy involve a narrow-band sources have spawned a whole area of tunable laser which interacts with a investigationknown as “nonlinearoptical sample of atoms or molecules and which spectroscopy.” Many of the new may, if its frequency matches one of the techniques developed in these characteristic atomic frequencies,cause investigations, such as “multiphoton

27 absorption” and “stimulated Raman 4.2.2. Atomic Fluorescence scattering,” have become important spectroscopic tools in their own right Atomic and ionic fluorescence with and are also discussed below. laser excitation is a very versatile technique (Omenetto 1989). Several excitation/detection schemes are available b:~which the large majority of 4.2. FLUORESCENCE elements, Including nonmetals, can be investigated. To accomplish this goal, the technique needs (a) one or two 4.2.1. Introduction lasers, tunable from the low UV to the visible, with power sufficient to optically saturate the excitation Lasers have had the most profound transition chosen; (b) low background, impact on the techniques of atomic and efficient atomizers characterizedby high molecular fluorescence, not only by values of quantum efficiency, i.e., low pushing the detection sensitivitiesdown quenching; and (c) a highly efficient to impressive levels -- close to single optical collection system since the atom and molecule -- but also by allowing fluorescence is isotropically emitted the exploitation of entirely new over 4X steradians. The most favorable experimental approaches in which photon analytical scheme usually involves the irradiance, narrow spectral bandwidth, use of nonresonance fluorescence and temporal resolution combine to transitions, i.e., different excitation provide a unique tool suitable for and emissicm wavelengths, so that any chemical analysis as wel1 as for nonselective scattering originating in fundamental spectroscopy studies. As a the atom reservoir and occurring at the result, the entire field of basic and laser excitation frequency is avoided. appl ied fluorescence has been Detection limits for aqueous revolutionized by the advent of lasers. solutions nebulized in flames and plasmas In combustion diacrnostics,laser- are in the low nanogram-highpicogram per induced fluorescence provides the milliliter range. Although this attractive possibility of achieving sensitivity is adequate for dedicated simultaneously spatial and temporal biological/’bioclinical applications resolution, addressing fundamental (e.g., the determination of some toxic physical parameters such as temperature elements lilkelead and cadmium in blood), . and number densities in the plasmas instances higher absolute investigated (Lucht 1988). In chemical iltec~il; powers are necessary -- down to analysis, one reaches both sensitivity the femtogram level. The use of the and selectivity which are essential in technique has, therefore, been the field of microtrace analysis, i.e., monopolized by atomizers which would the analysis of samples of limited sizes allow reaching such powers of detection. and of in situ microdistribution Thermal vaporization with furnaces at analysis. For example, it might be atmospheric pressure or under vacuum has desired to study the distribution of an practicallysupersededflames and plasmas e?ement in the various protein fractions in the analysis of biological samples at rather than simply determine its natural ultratrace levels. For a few elements ubiquitous concentration in the blood (e.g., silver, lead, thallium, and serum. The determination of an element cadmium), absolute detection limits are at a concentration of 1 rig/g in an in the 1Ow femtogram range and individual cell (ea. 1 #g) would then concentration detection limits are well need a method with a detection power of belowl pg/rnl(assuming a sampling volume 1 fg (Tolg 1988). of a few tens of microliters). Indeed, An overview of the status and current for a given experimentalsetup, including trends ‘ atomic and molecular pulsed laser excitation and atomization fluoresce~~e with laser excitation is in a furnace tube surrounded by an argon reported below. atmosphere, the experimentally obtained

28 detection limits are close to those which ultracold molecules with sharp spectral can be calculated theoretically, i.e., features for analysis (Hayes 1987, the technique is close to the ultimate Lubman, 1987, 1988a, 1988b). This detection sensitivities obtainable. involves seeding a small percentage of Two major trends can currently be the large polyatomic molecules in a large identified. First with furnace bath of a light carrier such as argon or atomizers, new methodologies are helium and expanding through a pinhole constantly being developed, aimed at orifice into vacuum. In the expansion, avoiding furnace background emission and the energy of the internal molecular scattering. For example, theuse of two- modes is relaxed through two-body step excited fluorescence with detection collisions with the carrier gas, in the low VUV has been successfully resulting in ultracold molecules. Since tested. Also, background correction a pulsed laser is used for excitation, techniques based on the Zeeman effect, the SSJ is also generally pulsed to the polarization of scattering, and reduce vacuum p~a~eng requirements. nonlinear intermodulated fluorescence These methods revolutionized have been reported. Secondly, many detection and measurement in the gas elements, e.g., the refractory metals, phase, allowing for incredibly high are still not detectable at ultratrace sensitivity while obtaining selectivity 1evels. Therefore, increasing attention and wavelength resolution. is devoted to atomizers such as pulsed To enhance fluorescence detection glow discharges or to atomizationmethods selectivity, a number of additional such as 1aser ablation and ion techniques have been interfacedto these sputtering. These atom reservoirs are measurements. These include dispersed coupled with the nonresonance, time- fluorescence where an emission spectrum resolved fluorescence technique, thus of the fluorescence is obtained. This avoiding scattering problems. increases the selectivity but lowers the sensitivity. Another method often used involves time-resolved measurements 4.2.3. Gas Phase Molecular Fluorescence (Hayes 1987) for enhanced molecular identification. The state of the art in gas phase Several trends can be noted in LIF- fluorescence measurements is LIF in SSJ. Thus far, most of the systems supersonic jets (SSJ) (Zare and Dadigian studied were PNAH’s which have already 1974, Hayes 1987, Smalley et al. 1977, been wel1 characterized (Imasaka 1988). Miller 1984, Levy 1980, 1984). The LIF However, more recent work has extended method involves using a high-powered this method tgf understand the basic pulsed laser to selectively excite a spectroscopy other classes of molecule from the ground electronic state molecules such as N- and S-heterocycles, (S.)to an excited electronic state (S,). quinones, 1inear hydrocarbons and The total integrated fluorescenceis then alcohols, and carboxylic acids. Several monitored as a function of wavelength. groups are interfacing chromatographic Although high sensitivity is obtained by methods to supersonic jets for detection monitoring the total fluorescence, and identification of molecules in real selectivity is based upon the absorption matrices. This method appears of the narrow band laser radiation. The particularly promising for identifying end result is that an excitation spectrum and quantifyingcomponents uncomplicated is obtained. Further, in the LIF method, matrices such as PNAH’s in coal (Hayes dispersed emission obtained line by line . and Small 1982) or even small metabolizes excitation provides a state in blood and urine (Lubman 1988b). A ~?stribution of the ground state number of methods have been developed for molecules obtained from, for example, a interfacing GC to LIF, including (a) reaction. Since most polyatomic direct capillary chromatography molecules have broad unresolvable injection, (b) a pulsed injectionmethod, spectral features at room temperature, and (c) a sheath flow method where the the SSJ method has been used to provide effluent of the GC is entrained within a

29 fast flow of argon. In the case of level of -5 bases in 10’. In more recent nonvolatilecompounds, limited work using work, this glrouphas used this technique supercritical fluids, liquids, or pulsed to detect a PNAH-nucleoside adduct. laser resorption has been used to entrain A second method widely used to labile molecules into jets for LIF obtain selectivity of fluorescent detection. However, much more extensive molecules in the condensed phase is work, including instrumentation synchronous fluorescence spectroscopy in development, needs to be performed to which the excitation and emission utilize the capabilities inherent for frequencies (AA) are scanned together to detection of small biomolecules using maintain aconstantwavelengthd inference these techniques. between them (Vo-Dinh 1978, Vo-Dinh and Martinez 1981). In this method, a signal is observed only when AA matches the 4.2.4. Condensed Phase Fluorescence interval between one absorption band and one emission band. If one selects the AA correctly to match one unique pair of The state of the art for obtaining absorption and emission bands, the selectivity “ condensed phase synchronous spectrum will show only a fluorescence me~~urements appears to be single peak in an otherwise complicated matrix isolation spectroscopy at 4 K spectrum. The resulting selectivity has combinedwith fluorescenceline narrowing been used to separate mixtures of PNAH’s (FLN) spectrometry (Wehry 1983, 1984; using fluorescence spectroscopy, which Perryet al. 1983; Personov 1983; Sanders otherwise could not be separated by this et al. 1986; Cooper et al. 1988). The method. Tlhe excellent sensitivity of matrix may involve dissolving the this method has been used to allow the material directly in liquid helium (or measurement of a single adduct in at neon) or in amorphous glasses. The cold least 107 DNA bases (Rahn et al. 1982). matrix is used to simplify the spectra of One of the present trends in solid- large molecules by cooling out the state fluorescence work appears to internalmodes of vibration and rotation. involve investigating microstructure in However, solute molecules in the matrix solid materials. Light microprobe still exhibit inhomogeneously broadened techniques are diffraction limited so vibronic absorption bandwidths since the that new methods will be needed to study molecules locate a large number of sites. microstructure below 200 nm. One new - Thus, if a narrow band laser is used, it method being developed to overcome this will select only a narrow subset of the limit utilizes exciton fluorescence and solute molecules that absorb the laser migration in solids. Exciton migration radiation (Personov 1983). The selected and fluorescence in the solid state has set of molecules will fluoresce, and been a very active field for many years relatively narrow lines characteristic now. Recent work has shown that a of the molecules are obtained. pipette filled with anthracene can This technique has been quite transfer energy to a target fluoroprobe effective as a selective tool for in a spot of <100 nm. This method identifying PNAH’s which have relatively apparently uses UV laser light to excite high fluorescent quantum yields and one end of an anthracene crystal in which relatively sharp spectral features using excitons are caused to migrate to the FLN in matrices. Recently, work at Ames other end of the crystal in the tip of a Laboratory has been performedon DNA-PNAH pipette <100 nm in diameter. The energy adducts using FLN in amorphous glass is confined to a very narrow region of matrices. Several adducts in laboratory space and if;transferred to a fluorescing mixtures were directly identified using molecule at the pipette tip. If energy the selectivity capabilities of this can be transferred in a region of space method. Using an intensified diode- <10 nm, then this may be significant for array, optical multi-channel analyzer, detecting small structures in biological the Ames group was able to detect an studies and even pinpointing the position adduct of benzo[a]pyrene for a damage of large fluorescence adducts in PNAH’s

30 ..-.,

*’

situ without excessive sample “FM spectroscopy” is that by frequency ~;eparation. modulating (FM) the laser source, the frequency derivative of the sample absorption is converted into an amplitude 4.3. ABSORPTION SPECTROSCOPY modulation of the transmitted laser intensity at the FM frequency. Since the While absorption spectroscopy is FM frequency can be chosen to lie well ,. certainly not the most sensitive method above the frequencywhere laser amplitude of laser spectroscopy, it has other fluctuations are important, this allows advantages which make it preferable in the observation of shot-noise-limited many applications. For one thing, it is absorption signals. This basic idea has extremely general in its applicability. now been well established by experiments The use of absorption spectroscopy is not using a wide variety of laser sources, limited, for instance, to optical and has made it possible to clearly transitions whose upper state decays observe absorption as small as 10-7, , primarily (and rapidly) by fluorescence, consistent with the shot-noise limit. ~ but is equally effective regardless of These relatively recent experimental the decay path of the excited levels. advances in absorption spectroscopy ,,Secondly, it is a relatively simple should stimulate a reexamination of the ,,, method, in which the signal information relative merits of absorption is conveniently concentrated in a spectroscopy in many applications. collimated laser beam. Finally, it is a A variation on simple absorption nonintrusive method which inconveniently spectroscopy is “intracavity absorption applicable to remote sensing spectroscopy,“wherethe absorbing sample applications. For all these reasons, is placed within the optical cavity of absorption spectroscopy continues to be the laser oscillator. Since the output widely used. Actually, the fundamental power of the laser is extremely sensitive sensitivity limits with absorption to small losses in the cavity, this spectroscopy are much better than those method is very sensitive to weak usually achieved in practice. Shot noise absorption. A further variation is in the laser intensity will determine the “laser magnetic resonance” where the minimum detectable absorption,even in an intracavity sample is placed in a strong otherwise ideal experiment, since these magnetic field. Through the Zeeman fluctuations will be mistaken for a real effect, tuning the magnetic field tunes absorption signal. This, however, the atomic transitions through resonance corresponds to very small absorption. with the laser, without changing the For instance, a 10 mW visible laser laser frequency. This technique has contains about 10’6 photons/second; so found wide application in regions of the with a one-second avera ing time, spectrum~usua:~~sthe far-infrared (FIR), ! absor~tions as small as 10- should be where 1aser sources are obser~able within the shot noise limit. unavailable or inconvenient but where ln practice, much larger absorption are many fixed-frequency lasers are known. often difficult to observe, leaving Since the FIR is a rich spectral region, considerable room for improvement in especially for molecules, laser magnetic experimental methods of absorption resonance has been widely applied since spectroscopy. its invention and has been used to study One important recent development in and characterize the spectrum of many absorption spectroscopy is the invention molecules and molecular ions (Evenson of techniques which circumvent the 1981). sensitivity limit due to the spectrum of amplitude noise of the laser source. Especially at low frequencies (<10 MHz) 4.4. RAMAN SPECTROSCOPY these intensity fluctuations can be many 4 f- orders of magnitude greater than the Spontaneous Raman spectroscopy, a shot-noise limit (Gehrtz et al. 1985). weak, two-photon, inelastic scattering The basic idea of what has been called phenomenon, represents a general

31

I molecular effect that is applicable to frequency of the scattered 1ight, the any physical state and characteristicof magnitude of the exciting radiation, and all materials. Although electronic, the quantum mechanical transition prob- vibrational, and rotational transitions ability for the vibrational levels under can be probed by Raman spectroscopic observation. As the exciting radiation techniques, laser-excited Raman spectra approaches that of an electronic are most commonly used to provide transition of a given chromophore within information on the ground-state a molecule $:e Fig. 4-1), the vibrational modes of systems spanning the intensities the vibrational size between diatomic molecules to displacement coordinates, within the macromolecular assemblies (Levin 1984). specified structural moiety, that are Raman spectra, arising from the intrinsic involved in displacing the ground state molecular polarizability changes that geometry to the excited state occur during nuclear motion, often conformation are selectively enhanced complement the vibrational information (Moore 1974). This phenomenon describes obtained from IR absorptionspectroscopy. the resonance Raman effect, where IR data, however, are generated by a increases in scattering enhancement can different molecular process in which the readily approach afactorof 104,allowing observed vibrational transitions are systems to be probed at 10-3to 10”7M determined by gradients of the molecular concentrations. Resonance Raman dipole moment treatedas functions of the approaches using both visible and UV molecular normal coordinates. tunable dye laser sources have been The Raman effect is described in exploited both in analytical chemistry, terms of the molecular vibrational where decreasing detection limits become levels, as shown in Fig. 4-1. That is, critical, and in macromolecular systems laser photons, whose frequencies VO are in which chromo-phoric groupings generally in the visible region (for representing loci for biological example, 514.5 nm or 19,436 cm-’) and functions are convenient y probed. nonresonant with any electronic level, Although the Raman effect was excite the sample and create a “virtual” discovered in 1928, the versatility of state from which photons are the technique -- coupled with the advent spontaneously (and inelastically) of a variety of laser sources, CCD, and scattered at frequencies VO t v,. Raman- diode-array detectors -- has literally shifted radiation at VO - vi, which is led to multiple renaissances in the recorded in “wavenumbers” (cm’), is application of the method to problems in termed Stokes-scatteringand is the event both physiicsand chemistry, aswell as in that is commonly measured; radiation at the multidisciplinary biophysical and V. + vi represents the weaker, blue- biochemici~lareas. Even today, we are shifted anti-Stokes scattering. The again witnessing an explosion in numerical values of the Raman shifts applications of the technique as near-IR correspond to the molecular vibrational Nd:YAG lasers are coupled with Michelson modes found in the IR region of the interferornetersin order to obtain Raman spectrum from about 4000 to 10 cm-’. spectra of highly fluorescing molecular Thus, we recognize that the interaction species. (In general, if fluorescence is of the laser radiation with the molecule induced by visible laser excitation, it is manifest by an increase in the natural will swamp the weak Raman signal and thus vibrations of the system and that the preclude an observation of the Stokes scattered radiation simply vibrational spectrum.) Since most specifies the normal modes of the biological materials exhibit strong vibrating molecule. Figure 4-1 also fluorescence signals in the UVRa~;~ depicts both the anti-Stokes transitions visible, Fourier-transform and the Rayleigh scattering process in spectroscopy using near-IR laser sources which the scattered light undergoes no offers an enormous potential for change in wavelength. investigating entirely new classes of The observed Raman intensity is molecules., In addition, the proportional to the fourth power of the spectroscopist now has at his disposal a

32 ......

Virtual ...... 4 Levels I ..

VJo h(vo+q) w w 1 1 V=l

1 r

v=o — Raman Rayleigh Raman Resonance Infrared Scattering Scattering Scattering Raman Absorption Stokes Anti-Stokes Figure 4-1. Illustration of several Raman processes, compared with Rayleigh scattering and infrared absorption. considerable number of new Raman allowing thleuse of defocused, low-power spectroscopes, other than spontaneous 1asers. Also, selectivity factors and resonance Raman scattering,involving approaching 10a are possible since higher order Raman effects (Harvey 1981). multiphoton absorption only occurs when These novel techniques have been used for the wavelength of the incident laser a variety of diagnostic purposes and for field is in proper resonance with the observing spectral transitions not target atom. Using commercially amenable to conventional Raman available pulsed dye lasers, nearly 70 approaches. different elements have been ionized in the RISmode (Parks and Lucatorto.1988). In many of these cases, the efficiency and selectivity have been shown to be 4.5. RESONANT AND MULTIPHOTON IONIZATION near theoretical limits. There is also OF ATOMS much excitement about recent successful attempts to resonantly ionize atoms with Laser spectroscopy has dramatically cw lasers (Miller et al. 1982). Although improved the prospects for efficient the efficiencies are not as high as for ionization of atoms in the gas phase pulsed lasers, with narrow bandwidths it (Hurst and Payne 1988). Since these has been pc)ssibleto selectively ionize ions may be counted with essentiallyunit different iisotopesof the same element efficiency, a broad range of (Mill~~of~e;l. 1985). spectroscopic methods aimed toward type of laser-based exploiting this remarkablephenomenonare ionization process often used with atoms currently under development. These is illustr~atedin Fig. 4-2(d). This include basic spectroscopic studies of process involves the use ofa high-power, free atoms, detection of very small tightly focused pulsed laser to numbers of atoms in the gas phase, and, unselectivelypump enough energy in’tothe perhaps most importantly,development of target atom to cause photoionization methods to create gas phase atoms from (Becker andlGillen 1984). Usually, the solid or liquid targets which may then be MPI process of case (d) involves no subsequently counted by laser-induced resonance. However, in other cases, ionization. resonances may be accidental or Figure 4-2 illustrates four deliberate to increase sensitivity and elementary forms of laser ionization selectivity. Resonance-enhanced MPI, which are used for atoms and for illustrated in (c), is an example of both molecules. For comparison, note that RIS and MPI. Fig. 4-2(a) illustrates the case of Multiphoton techniques provide new nonselective ionization with a single opportunities to learn about the basic photon. In order to employ this process behavior of atoms as they interact with for the ionization of most atoms, itwill strong laserfields. Line splittings due be necessary to develop soft X-ray lasers to the ac Stark effect, line broadening, with energies exceeding 10 eV. Process and excited state lifetime measurements 4-2(b) is the most elementary example of are leading to a more detailed RIS. In this case, it involves the understanding of nonlinear optical excitation of a ground-state electron to phenomena. For example, with this a bound excited statex followed by the important information, it is now possible absorption of another photon to induce with near resonant transitions to electron ejection or photoionization. routinely generate 115 nm radiation using More generally, the RIS process can “4-wave mixing.” include excitation via one or more In 1977, it was first demonstrated discrete transitions, so that another that a single atom of cesium could be photon of some convenient wavelength can observed in a background of 10’9 argon complete the resonance ionization atoms, USiincjRIS and a proportional process. One remarkable feature of RIS counter tc~ detect the photoionized is that the photoabsorption cross electron. The possibility of one-atom sections are generally quite large, detection has createdan avalanche of new

34 a va) s m s o VI al L $-w”:

Q- 0 G .-0

s ti 0 I ■ - e-

LL applications to exploit this unique An importantapplicationtoanalysis experiment. It has been possible to of solids is emerging via the use of cw observe the fluctuations of a few atoms dye lasers. For atomized barium, it has in an inert gas and to examine a number been possible to achieve 6% ionization of statistical mechanical concepts efficiency, using a doubly resonant, resulting from a small collection of three-photcm scheme where the ionization atoms, including the ergodic hypothesis. step was performed using a 1.7 W C021aser Other applications include the detection (Bushaw et al. 1985). The narrow-band of daughter atoms resulting from properties of cw lasers may be exploited radioactive decay, the detection of to achieve a degree of isotope short-lived radioactive isotopes such as separation,using the hyperfinestructure “Frwith a half-lifeof4.8min, and the to distinguish each component. It has determination of the solar neutrino flux. been possible to measure the isotope In this latter experiment, neutrino ratio of’73Lu and ‘74Luon samples of less absorption by 862 tons of ethylene than 10-’0grams in which these isotopes bromide is expected to yield 500 atoms of are 106times less abundant than the more “Kr. Because of its long half-life of stable isotopes (Miller et al. 1985). 2.1 x 10’ yrs, *’Kr may be counted The cw measurements are clearly exciting directly by RIS rather than by because of their enhanced selectivity and radioactive decay methods. The “Kr their improved “dutycycle’’when compared isotope has also been used in a number of to pulsed llasers. Developments in this geological applications. area are particularly important to drive The high selectivity and efficiency even more sensitive and demanding of RIS for atoms has spurred a great deal measurement. applications. One-atom of research into controllablemethods for detection is of great importance to atomizing solids. The earliest analytical chemistry. A related but experiments involved simply heating a inherently less sensitivemethod than RIS solid matrix in contact with a heated is the optogalvanic effect (OGE), which filament. The atomized sample is then refers to the effect of light upon the ionized via RIS and detected with a mass electrical properties of a plasma. spectrometer. The major problem with Though the earliestrnanifestationsof the this scheme is that there is poor spatial effect were observed in the 1920’s and and temporal overlap between the pulsed 1930’s, it has been rendered far more laser and the continuously evaporating general and useful by the advent of source. Atomization using pulsed laser tunable lasers (Travis and Devoe 1981). resorption (Hahn et al. 1988) or pulsed The effect is functionally equivalent to keV ion bombardment (Winograd et al. collisionally ionized RIS, where the 1982; Parkset al. 1983) has improved the final ionization step is attributed to efficiency tremendously. The latter collisional effects in a buffer gas method yields only atoms from the target instead of photoionization. However, surface, allowing the original gas-phase since ions are produced in the buffer atom-counting experiments to be extended gas, the attendant electrical background to surfaces (Pappas et al. 1989). These ensures that single atom events cannot be atomization techniques have also been observed as they can with RIS in coupled with nonresonant atomic favorable environments. ionization experiments. Measurements of The OGE in analytical flames is this sort have proven to be important for normally referred to as laser-enhanced the analysis of many chemical species ionization (LEI) in flames, and is sensed simultaneously and are highly by impressing a high voltage across the complementary to RIS (Becker and Gillen flame and monitoring the increase in 1984). All of these methods have not current synchronous with 1aser only been important new materials irradiation (Green 1986). For common characterizationtechniques but have also 5 ns lasers, with powers capable of allowed fundamental studies of the atomic optical saturation, a practical mechanism of laser ablation and ion/solid rule of thumb guarantees total ionization interactions. for population of an energy level within

36 1 eV of the ionization limit. For the be useful for solid sampling in chemical large portion of the periodic table with analyses. ionization potential (1P) values between Molecular and radical OGE signals 7 and8eV, this normally mandates a two- have been obtained in both flames and step excitation with aUV photon followed discharges. Many radicals are most by a visible photon. Best-case readily formed in discharges, and are detectabilities are about 1 pg/mL for thus particularly amenable to OGE meta?s in water aspirated into the flame, spectroscopy. Similarly, many refractory corresponding to number densities of metals form metal oxides in the around 10’ atoms/cm3 in the flame. analytical flame. Infrared, Detectability is degraded by sample rovibrational transitions have also been components which ionize readily and detectedoptogalvanicallyin low-pressure increase the limiting backgroundcurrent, discharges, including laser plasma tubes. or by nonresonant MPI of species in the flame. Unfortunately, LEI has not proved as adaptable to the atom reservoirs in 4.6. MULTIPHOTON IONIZATIONOF MOLECULES current vogue in spectrochemicalanalysis as it is to the flame. The graphite tube Note in reference to Fig. 4-2(a), it furnace emits copious quantities of is possible to ionize many of the thermionic electrons (Sjostrom et al. molecules with one photon. Photon 1988), and the inductivelycoupledpl asma energies exceeding 8 eV or 155 nm are exhibits electrical incompatibilitiesand needed; therefore, the process must be low sensitivities. Perhaps the most performed in a vacuum. In practice, promising development for LEI since the lasers are used for MPI of molecules in flame is the thermionic-diodeapproachof similar to that in our Niemax et al. (1987). Sample ;iscl;~~;ilnfor atoms (Section 4.5) vaporization is from a resistive (Lubman 1988a, 1988b; Bernstein 1982; filament, and ions produced by laser- Gobeli et al. 1985; Grotemeyer and Schlag enhanced collisional ionization are 1988). In Fig. 4-2(d), where the photon detected with gains of several orders of is not in resonance with an electronic magnitude due to space-charge state in the molecule, the efficiency for amplification. The low-buffer gas ionization becomes smal1 and high pressure also permits high-resolution irradiance is needed to drive this isotopic selectivitywith Doppler-freecw process. laser spectroscopic techniques. The truly unique property of In a plasma, not in local resonanceenhanced multiphoton ionization thermodynamic equilibrium (LTE), such as (REMPI) or RIS for molecules is the the low-pressure glOw discharge, ability to obtain wavelength selectivity prediction of the sensitivity of theOGE in the ionization process. Although ions is not as straightforward as in the LTE are detected as the final product, the flame. Compared with the LTE model for ionization cross section may reflect the flames, transitions between excited absorption-excitation spectrum of the states and transitions involving low- intermediateresonantel ectronic stateof energy photons may be surprisingly the molecule in the absence of other sensitive in the glow discharge. The quenching mechanisms. In combination accessibility of high-lying transitions with the SSJ technique (see Section is attractive for cw laser excitation in 4.2.2) which provides sharp spectral the visible, and even IR diode-laser features, high optical selectivitycanbe excitation in the near IR. G1OW achieved in the ionization process prior discharge OGE provides a convenient to mass spectral analysis. Thus, the re-servoirfor atomic spectroscopybecause possibility exists for highly selective natural sputtering of the discharge optical-mass spectrometry which can be cathode by buffer gas ions may be used to used to solve problems where sufficient provide free atoms directly from a solid selectivity cannot be easily achieved electrode. The same feature might also with conventional techniques. Such

37 problems that are currently being schemes such as those demonstrated by investigated include trace detection of Gobeli et al. (1985), where the target compounds in a complicatedmatrix. fragmentation and spectroscopic states This would have direct implications for accessed can be selectively controlled detection of metabolizes in blood and for sequencing, based upon the precise urine or for detection of trace toxic energy of the photons used. impurities in pharmaceutical s ntheses Anothetrimportant trend is the use I where a discrimination of 1:10 may be of direct VUV ionization of large required against a complicated matrix. biomolecules and polymers. Although Selective MPI (or RIS) has also proven direct VUV ionization has been used for valuable for detection of both isomeric some time with flashlamp sources, the and isobaric interferences which are development of VUV lasers which have high difficult to distinguish by conventional peak power and are tunable have already methods. The most impressive impacted this field. In recent work by demonstration of selectivity, however, Becker at the Stanford Research Institute has been in the area of isotopic (SRI) (Schuhle et al. 1988), it has been detection, which is based upon the fairly shown that laser-generatedVUV radiation sizable (several cm”’)vibrational shifts at 118 nm can be used to obtain in the spectroscopy of isotopic relatively soft ionization of large molecules. Isotopic selectivity on the polymer units that have been desorbed order of 1:108 has been demonstrated in using electron beam bombardment. Thus, the case of 12 isotopes using laser REMPI . relatively large polymers up to several combination with mass selective thousand mass units can be studied in a ;;chniques. The use of highly selective mass spectrometer. VUV laser ionization laser REMPI (or RIS) methodology may yet may also allow direct examination of revolutionize current ideas held by the large biological molecules such as mass spectrometry community about how to oligonucleotide strands for defects and, perform mixture analyses. thus, may have significant impact on A second important trend in this biotechnology quality control. The field is in the study of MPI-induced direct VUV method, however, is generally fragmentation (Bernstein 1982; Gobeli et nonspecific and lacks selectivity (Arps al. 1985). The processes for MPI-induced et al. 198!)). fragmentation are still not wel1 A new direction that is being understood, although studied by many employed is direct atmospheric pressure groups. However, there may be an detection iinsitu, using MPI. Since near important opportunity here to use MPI- UV-light is transmitted through air, MPI induced fragmentation of biomolecules can be performed in air at 1 atm and the (Lubman 1988b; Grotemeyer and Schlag ions detected by charge collectors or 1988; Li and Lubman 1988), such as interfaced to MS. These atmospheric peptides and nucleotides, for structural pressure MS methods have been shown to be analyses and quality control in capable of ppt detection and thus may be biotechnology or for detection of very important for detection of mutagenesis. MPI-induced fragmentation pollutants from coal burning plants and patterns are caused by absorption of their subsequent transport through the photons by the molecular ion or by air as one example. neutral fragments of the parent molecule. Thus, the combination of tunable These fragment patterns are similar to selectivity, high sensitivity due to the those of electron impact (EI). MPI efficiency of ionization, controllable fragmentation in the near UV has been fragmentation for structural analysis, used to study aromatic-based peptides, and the ability to interface MPI to mass and unique structural informationcan be analysis will make laser-MPI methods an obtained that is not easily available by invaluable tool for providing novel alternate techniques. A real opportunity solutions Ito many problems in detection, may afford itself here with the use separation, and structural analysis in of multicolor ionization-fragmentation various areas of chemistry and biology.

38 4.7. PHOTOACOUSTICAL AND PHOTOTHERMAL achieved. PA deoth-prof linq may be SPECTROSCOPY uniquely suited for ‘certain‘in ‘vivo studies in medicine and biology (e.g., dermatology and cancer diagnostics), ‘4.7.1. Introduction while PA microscopy has been used for nondestructive imaging of a variety of These techniques represent subsurface features (e.g., holes in outstanding evidence of the impact of metals, defects in integrated circuits, lasers in absorption spectroscopy, foreign inclusions in biological allowing on one hand the attainment of samples). PA techniques are becoming unprecedented sensitivities and on the increasingly useful for the measurement other hand the study of many physioc- of spectroscopic properties of hemical properties of the sample. Both biomolecules in vitro and in vivo (Moore techniques are based upon the generation et al. 1983). of heat due to thermal de-excitation In fact, other techniques (e.g., processes followingan initial absorption transmission and reflectance) are not step. The heat generated may be detected suitable for the study of biological as a pressure change (optoacoustic or materials due to opacities, scattering, photoacoustic effect) or as a variation heterogeneous surface properties, etc. in the refractive index of the absorbing The PA technique can be used to measure (adjacent) mediurn(photothermaleffect). concentration gradients, e.9.9 the To the extent that the matrix sedimentation of humic acids (Wylie and containing the analyte is nonabsorbing, Lai 1986). we are dealing with zero-background The sensitivity of the PA signal to measurement techniques,with capabilities the size of particles (within a given of reaching absorption detection limits range) was exploited in the photoacoustic as low as 10* cm”’,which could translate immunoassay method (Kitamori et al. into concentrational detection limits of 1987), anticipatedas being applicableto 10-’2to 10-’3M, for strongly absorbing determinations for trace tumors and complexes. At these exceptionally low cancer marker antigens, such as a- absorbencies, solvent absorption and its fetoprotein, basic protein, and carcino- associated noise will limit the ultimate embryonic antigen. detection power available. In principle, Of particular relevance is the chemical speciation is also possible and possibility to couple PA measurements in some cases, e.g., actinide ions in with fluorescence to study the status of water, at concentration levels which are green matter, and thus the efficiency of inaccessible to other conventional, the photosynthetic system as a function nonlaser techniques. Although not an of various types of stress (water and intrinsic characteristic of the nutrient deficiency, herbicides, gaseous techniques, spectral selectivity can be pollutants, toxic metals). improved by coupling these methods with In the analytical spectroscopy gas, liquid, and thin-layer field, PA methods have been applied to a chromatographic methods. variety of gaseous and liquid samples, with sensitivities at the parts per billion/partspertrillionlevels. Asan 4.7.2. Photoacoustical Spectroscopy example,”the speciation of actinides in underground water (Pollard 1985) is of Several applications of utmost relevance for understanding ‘he photoacoustics (PA) have been described transport and migration of these spec es (Tam 1986). PA imaging is concerned with in the soil/water system (see a-so the detectionof subsurfacethermoelastic Section 5.2.3). property variations in a sample. If An extremely attractive way to little lateral resolution is required, enhance drastically the photoacoustic one speaks of PA depth-profiling, while effect is by initiating or terminating a PA microscopy is reserved for the case chemical chain reaction (amplificationor where 1ateral resolution is also quenching effect) (Choi and Diebold

39 1985). In this way, nanomole and copper, iron, and cobalt), the analysis picomole sensitivities have been of gases at sub-ppb concentrationlevels, demonstrated for several gases. and the quantification of proteins PA methods are well established. separated by polyacrylamide gel Discernible trends are found in the electrophoresis. efforts made to further improve the The same expectations and sensitivity and to increase the numberof conclusions made for photoacoustic applications, including spectral regions techniques hold here. A clear trend in other than visible or near UV. photothermal spectroscopy is to reduce the size and complexity of the experimental setup (e.g., compacting the 4.7.3. Photothermal SDectroscoRY apparatus and using single-beam configuration, in which pump and probe In these methods, the heated sample experiments are feasible). acts as an optical element, leading to a rich lexicon of techniques. For example, a lens in thermal lensing, a grating in thermal diffraction, or a prism in thermal deflection (Dovichi 1987). One or two beams can be used. REFERENCES Crossed beams provide excellent spatial resolution: for two beams crossing at right angles, the probed intersection volume may be as small as femtoliters if Arps, J. H., C. H. Chen, M. P. McCann, the laser spot size is of the order of and I. Datskou (1989) “Ionization of micrometers, leading to sensitivities in Organic Molecules Using CoherentA#;~:~~. the low femtomole/high attomole range Ultra- violet Light,” (e.g., for amino acids). Such resolution Spectroscopy (submitted~. ‘“ and detection capabilities are of clear value in chromatographic, Becker, C. H. and K. T. Gillen 1984) electrophoretic, and microscopic imaging Anal. Chem. ~, 11. and detection. Supercritical fluid chromatography Bernstein, R. B. (1982) J. Phys. Chem. is another promising area, in which more 86, 1178-11,84. than 100-fold sensitivity improvements have been reached. Surface analysis is Bushaw, B. A., T. J. Whitaker, B. D. possible by these-called “mirageeffect” Cannon, and R. A. Warner (1985) J. Opt. (photothermal deflection of the probe Sot. Am. W, 1547. beam) or by “reflection” measurement probing the surface deformation. Studies Choi, J. G. and G. J. Diebold (1985) of thermal properties of materials have Anal. Chem. ~, 2989. been reported. As in the photoacoustic case, Cooper, R. S., R. Jankowiak, J. M. Hayes, speciation of actinides and lanthanides L. Pei-qi, and G. J. Small (1988) Anal. in solution is possible. Colloidal Chem. 60, ;!692-2694. solutions can also be investigated. As a typical example, the absorption Dovichi, N. J. (1987) CRC Crit. Rev. properties for dissolved organic carbon, Anal. Chem,,17, 357. which plays an important role in the speciation of trace metals, was studied Evenson, K. M. (1981) Discuss. Faraday in fresh waters (Power and Langford Sot. 7_l,7<, 1988). Miscellaneous applications include Gehrtz, M., G. C. Bjorklund, and Edward the detection of ultratrace metals in A. Whittaker (1985) J. Opt. Sot. Am. Bil, 1iquids (e.g., phosphorous, cadmium, 1510.

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Power, J. F. and C. H. Langford (1988) Travis, JohnC. and JamesR. DeVoe (1981) Anal. Chem. ~, 842. in LASERS IN CHEMICAL ANALYSIS, ed ted by GaryM. Hieftje, JohnC. Travis, and Fred Rahn, R. O., S. S. Chang, J. M. Holland, E. Lytle (HumanaPress, Clifton, NJ), pp. and L. R. Shugart (1982) Biophys. Res. 93-124. Commun. ~, 262-268. Vo-Dinh, T. (1978) Anal. Chem. ~, 396-401. Sanders, M. J., R. S. Cooper, R. Jankowiak, G. J. Small, V. Heisig, and Vo-Dinh, T., and P. R. Martinez (1981) A. M. Jeffrey (1986) Anal. Chem. ~, Anal. Chiml.Acta~, 13-19. 816-820. Wehry, E. L. (1983) Trends Anal. Chem. Schuhle, V., J. B. Pallix, and C. H. (Pers. Ed.) ~, 143-147. Becker (1988) J. Vat. Sci. Technol. A Q(3), 936-940. Wehry, E. L. (1984) Anal. Chem. ~, 156R-173R.

Sjostrom, S., J. Lejon, and H. Winograd, N., J. P. Baxter, and F. M. Rubinsztein-Dunlop (1988) in RESONANCE Kimock (1982) Chem. Phys. Lett. ~, 581. IONIZATION SPECTROSCOPY 1988, edited by T.B. Lucatorto and J.E. Parks (Institute WyJie, 1. W. and E. P. Lai (1986) Rev. of Physics, Bristol, U.K.), pp. 151-154. Sci. Instr. ~, 1185.

Smalley, R. E., L. Wharton, and D. H. Zare, R. N. and P. J. Dadigian, (1974) Levy (1977) Ace. Chem. Res. 10, 139-145. Science ~1, 739-747.

42 ~. POTENTIAL IMPACTS ON THE OHER PROGRAMS

5.1. INTRODUCTION to be primarily qualitative (or semi- quantitative) instruments, at best, with In this section we discuss the reasonably high sensitivities, but impacts that the present status and subject to matrix effects and sensitivity especially the trends of laser technology variations among analyte species. A (Section 3) and laser spectroscopy significant advantage of the laser (Section4) are having on programs within microprobe over charged particle the OHER mission. A number of microprobe is the applicability to the suggestions are made which can be very study of nonconducting samples. useful to those engaged in laser-related The selectivity provided by the research and development. Further, most interaction of tunable laser radiation of the prioritized recommendations made with the characteristic energy levels of in this report are based on these atomic species is the critical aspect of suggestions. For convenience of many of the most important applications identification, the suggestions are of lasers to chemical analysis. RIS, italicized. LEI, and LIF selectively detect atoms of the analyte species in the presence of large excesses of concomitant atoms. 5.2. LASER-BASED METHODS FOR ELEMENTAL From the discussion of atomic ANALYSES ionizationmethods in Section 4.5, it may be seen that ongoing research needs for Samples which are normally solids, analytical application are: liquids, or molecular gases must be rendered into free atoms or atomic ions before identification of the atomic 1) Studies of the basic physical species is possible. Thus, among the mechanisms of the laser ab7ation process earliest applications of the laser to for its use in the laser microprobe. atomic spectrochemical analyses was the Laser ablation is widely researched for optical laser microprobe, in which laser avariety of applicationsbeyond chemical ablation was employed to sample a small analyses, but a thorough understandingof portion of a sample for selective the mechanisms for a wide variety of detection by optical emission complex materials is missing. Many spectroscopy (Laqua 1979), In spite of artifacts which hamper sensitive and continuing research in the use of laser accurate analyses may be related to ablation for spectro-chemical analyses fluctuations in energy per pulse, and in (Cremers and Radziemski 1987), the spatical and temporal mode instabilities. commercial optical laser microprobe has been displaced by the Laser Microprobe Mass Analyzer, commercially offered by 2) Adaptation of new laser sources, Leybold-Heraeus as the LAMMA and by especially continuous wave, to resonance Cambridge Consultants as the LIMA (Houk ionization. The widespread analytical 1986). In these instruments, ions application of RIS and resonance generated directly in the laser-induced ionization mass spectroscopy (RIMS), as plasma are mass-analyzed by TOF mass well as the commercialization of the spectrometry. Laser microprobe can be method,,has been severely limited by the used to advantage for microanalytical nature of available tunable lasers. With applications to biological specimens, the approaching revolution in tunable inclusions in alloys, ceramics, plastics, lasers, as envisaged in Section 3.2, a polymers, glasses, and to dust particles new era for analytical laser spectrometry (Piepmeier 1986).. They are considered may be beginning.

43 3) Development and characterization sources may need the assistance of high- of excitation and ionization schemes for powered IR lasers for efficient a77 elements amenable to spectrochemica~ photoioniz,;:~ (see Section 4.5) or may analysis. Broadly tunable, all-solid- benefit alternate ionization state systems with reasonably accurate methods. Pulsed-de field ionization, as and simple wavelength control would both widely practiced in the Soviet Union, is raise the demand for proven, tabulated of particular interest to reduce some of RIS schemes, and greatly assist in the the laser requirements. exploration of potential schemes.

7) Continued development of 4) Further development of techniques technology unique to RIS of rare isotopes for the vaporization of samples of of inert !gases. The “Maxwell’s demon” arbitrary composition in vacuum and approach to isotonically selective rare- contro? led atmospheres, yie7ding accurate gas detection has been successfully representation of the bu7k samp7e and adapted from the work at Oak Ridge efficient geometric and tempora7 over7ap National Laboratory (Hurst et al. 1985) with pu7sed 7asers as we77 as cw 7asers. by Atom Slciences(Lehmann.et al. 1987; To the extent that new laser sources can Thonnard et al. 1987), and is presently be continuous, instead of pulsed, the capable of quantifying a few hundred potential analytical impact is even atoms ofSIKr in the presence of 1011atoms greater. However, certain excitation of ‘Kr ancl10= molecules of water. The schemes will probably continue to require process relies entirely on mass the extended wavelength range and/or spectrometric isotopic selectivity, multiphoton capability of pulsed lasers. requiring several stages of isotopic For these cases, suitably pulsed sampling enrichment prior to the final atom count schemes are essential to attaining the using RIS. Future generation devices ultimate efficiency in sample use. Laser have been separately proposed by Payne ablation/desorption and pulsed ion beam (1988), Hurst et al. (1988), and Hardis sputtering are among the prime et al. (1988) to reduce the number of candidates. stages reqluiredfor the process.

5) Development of isotopica77y selective (high resolution) ionization 8) 0eve70p position sensitive methods for hybridization with mass ionization detectors. Ionization spectrometry and/or LIF for detection of detectors such as proportional counters, 70W abundance, 7ong-lived radioisotopes electron multipliers, and single-ion in the presence of 7arge excesses of detectors can be provided with “position stable species. Again, isotopic sensitivity”. Therefore, 1aser selectivity may be aided in many cases by ionization processes can, in principle, the use of new cw laser systems, with be used to construct spatial images implications for chemical analysis as showing the locations of the atoms or well as isotope separation (Bushawet al. molecules that were ionized. This 1987; Bushaw and Gerke 1988). Also, capability has not been exploited in the Niemax et al. (1987) propose the use of past, yet it illustrates another optical isotope-dilution quantitation interesting potential use of lasers in without the use of mass spectrometry. the future. Another major impact of lasers on atomic analysis is in the area of LIF or 6) Further implementation and 1aser excited atomic fluorescence characterization of a7ternate ionization spectrometry (LEAFS) as it is often known ~;~;~ion(e. g., dc and rf field when applied to analytical atom co71isional ionization) for reservoirs (see Section 4.2.2). RIS. Thou~h fully capable of saturating Consideration of Section 4.2.2 suggests resonant atomic transitions in vacuum, cw a number of important development areas

44 which should be supported to enhance the 5) Standard7ess ana7yses. In some analytical future of LEAFS: cases, modern graphite furnace technology and atomic absorption can provide 1) The development of new absolute (standardless) analyses in methodologies to avoid background several complex matrices to within 10%to emission and scattering with furnace 20% accuracy. The same technology could atomizers. The use of two-step excited possibly be applied to optically fluorescence, with detection at shorter saturated fluorescence measurementswith wavelength than either excitation,should calibrated detectors. be extended to more elements. Zeeman- effect background correction techniques 6) Continued development of the should be applied to a variety of real photon-burst method for single atom samples. Polarization-based rejection detection by LIF. Photon-burst methods and nonlinear techniques such as methodology is being developed with DOE intermodulated fluorescence, used widely funding at Los Alamos National Laboratory in laser spectroscopy, need to be fully and at Pacific Northwest Laboratory. implemented in the analytical context. Keller et al. at Los Alamos National Laboratory propose the use of isotonically selective photon burst 2) UVsources for initial excitation. detection on an ion beam which has Many elements of critical importance, already undergone magnetic mass such as arsenic, selenium, tellurium, and discrimination, thus convoluting the phosphorus, have evaded analytical laser selectivities of the two methods spectrometrists, because their principal (Fairbank et al. 1988). Whitaker and resonance excitation lines at energies Cannon (1988) at Pacific Northwest above 40,000 cm-’were not conveniently Laboratory are applying photon burst accessible to lasers. The recently methods to rare gases excited to developed BBO crystal is solving this metastable states in discharges. problem. This area is relevant toRIS as well as LEAFS. 5.3. ENVIRONMENTAL RESEARCH

3) Multiphoton fluorescence investigatio,ns. With pulsed laser 5.3.1. Global Effects and Active Remote excitation, the photon irradianceis high Smm9 enough for efficient excitation (oftento “virtual” levels) of many nonmetals The fact that human activities are (oxygen, fluorine, chlorine, nitrogen, now altering the content of the global and carbon) which can then be detected by atmosphere justifies increased measuring their red fluorescence measurement. Fortunately. measurements emission. can now be made that will allow us to investigate and learn about the global environment and perhaps eventually learn to model the global climate. The tools 4) New atomizers. Many elements, at hand are observation from aircraft and such as the refractory metals, are still space craft platforms, using both active not detectable at ultratrace levels. and passive monitoring approaches. NASA Atomizers such as pulsed glow discharges has recently completed a detailed study or atomization methods such as laser of measurements that should be performed ablation or ion sputtering should be from space platforms as part of the Earth tested with careful time-gated detection Observation System proposal. This study of nonresonant emission. Requirements was used as one scientific component of for atomization and light collection the proposal placed before Congress this should be met in one device, designed to year called the Mission to Planet Earth. provide the maximum efficiency for both. The program called for measurements on a

45 global scale not only of atmospheric desired, especially if wind velocity is parameters such as wind, temperature, to be determined from Doppler-shifted pressure, humidity, cloud height, carbon return si!~nals. dioxide and other gases, and atmospheric To date, remote sensing using turbidity but also for measurements of tunable laser sources has been limitedto global sea conditions, including ground-ba:sedor to aircraft platforms. temperature, sea state, and sea height, However, laser technology is now being and also for measurements of global developed by NASA that will allow early geodesy with a precision not before test measurements to be made from the possible. In other words, over the next shuttle bay. The laser technology is decade the earth would be studied as a based on flashlamp-pumpedNd:YAG followed system from aircraft platforms by harmonic generation to the green to orbiting polar satellites and from t% pump a tunable solid-state laser or a space station. tunable dye laser. It is expected that The Department of Energy may have the the 1aser source will transmit opportunityto cooperate with NASA in the approximately 1 J of energy at a Mission to Planet Earth proqram and repetition rate approaching 10 Hz. The support measurements of “at~ospheric receiver optics is a 1 M diameter parameters that are impacted by energy reflector mirror. The first system will generation. The goal of the program use direct detection of the received would be to provide guidance to long-term signal with time resolution providingthe energy production policy that is in the vertical spatial resolution for the interest of the United States and the measurement. world community. The wind sensing measurements are Active remote sensing includes being approached using coherentdetection microwave sounding, radar imaging of to resolve the Doppler-shiftedfrequency. clouds and rain, and height measurements The first generation system is based on to the ground. A great number of the CO, laser at the 10 pm wavelength parameters can be measured by active band. Studies based on ground sensing in which a signal of observationshave shown that the C021aser electromagneticradiation is transmitted, system is of adequate sensitivity for scattered or absorbed by the object, and wind sensing using currently available detected in either transmission or in laser amplifier technology. However, reflection. Opportunities for active there remains some question about the laser remote sensing include the feasibility of packaging the C02 laser detection of clear air turbulence (CAT) for satellite-based global observations from aircraft to enhance the safety of because of the operational 1ifetime of the vehicle and its passengers, the the laser, the high voltage required for measurement of wind shear near airports, the excitation of the gas, and the lack the measurement of atmospheric of return scattered signal at the 10 pm temperature, pressure, humidity, wavelength from particles in the turbidity, tropopause height, global dust atmosphere. These issues have opened the dispersion from either volcanic activity possibility that a solid-state laser or other large events, and global wind transmitter could be developed for global measurements. All of the laser remote wind measurements. sensing approaches involve either The diode-pumped, Nd:YAG laser inelastic scattering processes such as oscillator followed by a sixty dB gain absorption and remission of optical laser amplifier was the basis for the radiation on or off resonance, or elastic first coherent laser radar measurementin scattering as a return from a solid the 1 pm wavelength range. The first object such as the ground or from experiments of coherent laser radar used particles in the atmosphere. In the the coherence of the diode-pumped, former case, tunable laser radiation is nonplanar, Nd:YAG, ring-laser oscillator required to selectively monitor the in a ground-based demonstration. With molecule or atom of interest. In the the rapid!development of the diode-laser latter case, highly coherent radiation is technology, diode-pumped Nd:YAG lasers

46 now appear to be quite capable of making TheMission to Earth program proposed the transition to coherent laser radar in by HASA for the next decade is now both aircraft and spacecraft platforms. recognized as one of the most important However, the I pm wavelength of the space missions ever defined. By Nd:YAG transition is not eye-safe which participating with NASA to develop the places severe constraints on the ability remote sensing technology for global to use the laser in an uncontrolled remote sensing, DOE has the opportunity environment. Work began three years ago to play a major role in seeking answers to define a diode-pumped laser system to questions of global importance related that would meet theeyesaferequ irements. to the effects of energy on the climate. The work led to the demonstration of a An attractive application of active diode-pumped Tm:Ho:YAG 1aser which remote sensing methods is the use of LIF operates at the 2.1 ~ wavelength region. of green plants in order to gain some Also Er:YAG lasers operating at 1.54pm insight about the functioning of their are currently used for eyesafe photosynthetic apparatus, i.e., the applications. This wavelength is eyesafe possibility to challenge LIF as an early and is transmitted through the atmosphere detector of a potential status of stress. free ofabsorpti on. Further, work on the In fact, under optimum conditions of laser system showed the potential for photosynthesis, the proportion of efficient operation and for high peak absorbed light which is emitted as red power operation in a Q-switched mode. chlorophyll fluorescence is rather low, There is now considerable effort to while under various conditions of stress develop both the efficiency and the (e.g., water deficiency, 1ack of coherence of the diode-laser-pumped nutrients, presence of herbicides, toxic Tm:Ho:YAG laser source for wind sensing metals, and gases) the fluorescence or clear air turbulence applications. In emission increases considerably. There addition, this laser source could also is, therefore, an inverse relationship form the basis for coherent laser radar between in vivo chlorophyll fluorescence for near-distance measurements of both and photosynthetic activity speed and target size and location. (Lichtenthal er and Rinderl e 1988; In summary, the motivation fordiode- Chappel le and Nil 1 iams 1986). pumped, solid-state 1 aser research The incorporation of lasers into conducted in the early 1980’s was global oceanographic technology is wel 1 remote sensing of atmospheric parameters documented ina NASA report (Kim and Ryan including chemical constituents, density, 1975) where several hydrographic light temperature, humidity, aerosol detection and ranging (LIDAR) concentration, and wind velocity. The applications involving the fluorescence requirements for the laser transmitter technique are reported, including the operating from an 800 km altitude mapping of phytoplankton and oil spills. orbiting satellite include a 4 year Airborne fluorescence emission operational lifetime, greater than 10% signatures obtained from vegetation were electrical efficiency, 100 W of average also reported in the literature (Iioge et output power in a linewidth of less than al. 1983). Work at NASA/Goddard Space 10 kHz, and operation at an eyesafe Flight Center (Chappelle and Williams wavelength. With the recent 1986) involved the use of a nitrogen demonstration of a diode-1 aser-pumped 1aser (337 nm, 30 Hz, and 9 mJ/pul se with TIII:Ho:YAG operating at room temperature a pulse width of 10 ns) to excite, in a at the eyesafe 2.1 pm wavelength, a laboratory configuration, the crucial step has been taken to achieve fluorescence spectra of all major plant the above goals. It now appears feasible types (i.e., herbaceous, di cots, to define the all solid-state laser monocots, conifers, hardwoods, and algae} transmitter that will allow depth- with the aim to correlate the resolved global wind measurements by the fluorescence measurement with the mid 1990’s. manifestations of specific environmental

47 changes. Fluorescence spectra not only Large quantities of transuranic provided a means for plant type elements are produced from commercial identification but also reflected the nuclear technology as burn-up products of status of physiological stresses. uranium fuel. The disposal of long-lived radioactive waste has largely involved the need of finding deep, geologically Since LIF is amenable to aircraft stable sites (e.g., clay and granite) and, potentially, to space platform with long pathways to the biosphere and observations, there is an opportunity to good absorptive properties for the complement already existing standard actinides that might be leached out of remote sensing methodology, e.g., passive the waste itself (Kim 1986; Saunders and reflectance measurements, with the Wilkins 1987). Any assessment of the addition ofa fluorosensor package (which consequences of a potential release of canbe operated with a fixed frequencyllV actinides from such repositories relies laser as excitation source and an optical upon the ability to predict the migration multichannel analyzer for the behavior of these elements in soil/water simultaneous detection of the spectral systems, i.e., it requires the information.) characterization of their absorption process Iby many potentially receptive organisms. The keyword common to these studies 5.3.2. Transport Studies Involvinq is “chemical speciation,” i.e., in its Actinides and Heavy Elements broadest meaning, the capability to identify and quantify inorganic and organometallic compounds, as well as As noted in the previous section organic compounds, present in the (5.3.1), modern laser spectroscopy and environment (Braman 1983). Speciation laser technology can make a significant studies should be stressedwhen preparing impact on the important studies environmental impact statements, since, concerning the fate and transport of many due to large differences in volatility elements and/or chemicals, both natural and volubility, some chemical forms can and synthetic, throughout the dominate the physical exchangesoccurring atmosphere/soil/water system. This is in global geochemical cycles so that the essentially due to the fact that lasers rates of decompositionand transportation provide an analytical laboratory of the various elements are strongly capability, as described in Section 5.2, affected. Moreover, the biological with much improved characteristics of activity (e.g., toxicity) of an element sensitivity and selectivity. As a can vary significantly from one compound result, the range over which measurements to anothe!r. Since the volubility of can be made is considerably extended. transuranic compounds in near neutral Laser techniques can be used in solutions is very small (<1O* mol/L), remote sensing to trace directly gaseous their direct speciation under natural pollutants (e.g., S02, NOX, OS); these conditions seems to be feasible only by measurements help to elucidate the 1aser spectroscopic methods. dynamical response patterns of the global Thermal lensing spectrophotometry tropospheric chemical system (National and photoacoustic spectroscopy (see Academy Press 1984). Here, many Section 4.7) have been applied to the additional opportunities exist to determination of uranium, plu~::j::i increase the selectivity. For example, americium,, and neptunium in the use of artificial intelligence to waters under varying conditions of pH, unscramble complex signals may be of Eh, and carbonate concentrations with particular value. loreover, air samples sensitivities attaining 104 mol/L, i.e., can be collected at various sites and absorptioncoefficients of approximately ,, ana7yzed in the 7aboratory, producing 10”’cm”’. data from which atmospheric transport The combination of laser patterns can be discerned. spectroscc~pyand chemical sensors seems

48 to offer the most promising new method of employed in these applications. For . situ chemical analysis of the instance, ‘Krhas been used to date polar ~~vironment (see Section 5.3.4). The ice caps to gain information on climatic inherent miniature dimensions of these history, and “Kr could be used in a very sensors have elicited considerable similar way for extensive analyses of interest not only in the environmental groundwater. These methods could be used field (e.g., air samples, potable waters) on a much wider scale for tracing the but also in the biomedical field (e.g., migration of po77utants which could body fluids) where useful applications affect our water supply. Of particular for direct “dip-in” operations have been importance is the use of 8’Kr to date found. water in wells up to 1 million years. Remote monitoring of groundwater The question of whether these we~ls have contaminants can be$uccessfully tackled been recently vented to modern sources with fiber optic-based probes and can be addressed by measuringmKr with a fluorescence detection, especially when ha7f-life of 10.7 -yrs. - coupled with time resolution (Kenny and Jarvis 1987; Vodacek and Philpot 1987). In this case, naturally fluorescent contaminants can be detected directly; in 5.3.3. Fossil Fuel Pollutants situ, specific fluorescent-basedoptrodes can be developed; or, finally, a fluorescent tracer can be injected in the As a result of either natura- water and the motion of the fluorescent mrolvsis Drocesses or fossil fue” plume followed. In the case of uranium, bfirni~g,PNAH’s are widely distributed in the fluorescence lifetime persists for a our environment. Their release into air comparatively long time (few tens of and water can directly affect the health microseconds) as opposed to that of most of individuals since the formation of organic molecules present in natural adducts between DNA and PNAH’s may be waters. Therefore, time gating of the important in initiating carcinogenesis signal al1ows rejection of the (Butler 1979; Reddy et al. 1984). interfering species with very high Although the level of PNAH concentration detection sensitivities. Aromatic can be monitored in the environment, a groundwater constituents (e.g., barkpile more useful indication of the risks of and landfill leachates, phenol, cresol, mutagenesis and carcinogenesis can be gasoline, and organochlorides) can be obtained by actually measuring the level detected. of DNA-PNAH adducts found in the body. Particularly important are the In particular, the ;~~~~y to accurately complex organic acids found in natural identify the can tel1 waters and variously called humic investigators something about the source substances, dissolved organic carbon, of the contaminant, i.e., whether it came Gelbstoffe, or, more specifically, fulvic from a coal-burning plant, an oil spill, or humic acids. Because of their a forest fire, etc. In turn, the major affinity with metal ions in general, they source of mutagenesis in any one region provide an important mechanism for the may be pinpointed and controlled by such transport, dispersion, and sedimentation methodology. of metals in natural waters. The A number of chemical methods tiave biological availability of these ions, been developed to study the extent of both as nutrients and as toxins, is PNAH-DNA adduct formation, including altered by complexation to humic radiolabelling methods (Reddy 1984). substances. However, the accurate identification of RIS (see Section 4.5) has provided DNA-PNAH adducts requires high a new method for dating water which has selectivity using ultrasensitive applications to both marine and detection techniques. One laser-based terrestrial environments. Very sensitive method which has been used to achieve methods for counting individual atoms this goal is synchronous fluorescence with isotopic selectivity have been spectroscopy (Vo-Dinh 1978). This method

49 can provide enhanced selectivitybased on Recently published work (Nuwaysir relatively narrow linewidths for and Wilkins 1989; Lam et al. 1988) has absorption and sensitivity based on demonstrated the ability to volatilize, fluorescence detection to allow the detect, an~didentify nuclei acid strands measurement of a single adduct in 107DNA up to 10 base sequences long. In bases. However, the measured linewidths combination with enzymatic methods that are still too broad to distinguish are selective towards specified between chromophores that have severely cleavages, this method may yet provide overlapped absorption bands. A more the additional speed needed to achieve selective method is the use of rapid sequencing. fluorescence line narrowing spectrometry in combination with liquid helium or glass matrices at4.2K. This method was 5.3.4. ~mical Sensors (Ootrodesl used to distinguish DNA-PNAH adducts ind various mixtures with an estimated As a“lready reported in Section detection of 5 bases in 10’ (Sanders et 5.3.2, there is a significant need for al. 1986). The latter has also been accurate measurement of the combined with enzymatic degradation and concentrations of a variety of chemical TLC separation to further separate the species associated with energy nucleotides from the adduct-nucleotide production,, Lasers and fiberoptic led pairs (Cooper et al. 1988). to the invention of the chemical optrode, The methods described above may be the optical equivalent of a chemical sufficiently selective for a limited sensing electrode. The optrode concept number of DNA-PNAH adducts; however, if is simple. The tip of a fiber optic is a large variety of adducts from different coated with a chemical that undergoes fossil fuel sources are present, then some optically measurable change on more selective techniques need to be exposure to another chemical in its developed. This may require actual DNA immediate environment. This change is sequencing procedures (see Section 5.4). measured, for example, by transmitting a Because of the 7argenumber of base pairs light beam down the fiber and sensing the in human DNA (3 x I&’), rapid sequencing amount of light at a specified wavelength methocfsneed to redeveloped which should that returns from the tip. be able to provide unambiguous Optrodes have been developed to identification and to distinguish against measure the concentrations of a wide modified nucleosides, which is difficult variety of chemical species (Goldman by conventional methods.The abi7ity to 1988; Saari 1987; Wolfbeis 1986). Many obtain unique identification and to optical parameters, such as absorbance distinguish modified nuc7eosides can be reflectance,chemiluminescence,and Raman obtained using mass spectrometric scatter have been used up to now; others, methods. Future directions in this field includingIight polarization,decay time, wi17 require development of efficient and refractive index can still be introduction and volatilization methods exploited. Metal ions such as beryllium, for these highly po7ar molecu7es aluminum, magnesium, zinc, and cadmium fo170wing enzymatic degradation. New can now be detected using fluorescence- developments in continuous flow FAB are based optrodes. Concentrations of now having a major impact on peptide oxygen, carbon dioxide, and hydrogen ion sequencing and identification and with (pH) can also remeasured. Optrodes have modification shou7d be applicable to been developed to measure hydrogen nucleotide sequencing (Caprio7i et a7. sulfide,methanol, formaldehyde,phenols, 1986) . Other techniques to be and various chlorinated hydrocarbons. investigated inc7ude various particle It is now possible to measure the resorption methods which can be used to concentrationsof chemicals at sites that volatilize nuc7eotide strands. These can were not feasible only a few years ago. thenbe ana7yzed by a combinationof mass spectrometry and fragmentation pattern Optrodes cou7d be p7aced in water we71s recognition. to continuously monitor the

50 concentrations of a variety of 250 @l of penicillin G. There are pollutants. Optrodes couldbe buried at opportunities for development of new various depths surrounding a chemical sensor types to add more selectivity to waste disposal site to monitor the the measurement (while stil? maintaining concentrations of the waste chemicals and adequate sensitivity) and to increase the track their movement in the soi7. Some number of applications, especially since po17utants are transported inthe soilby small, tunable lasers and high anaerobic bacteria. lfovement of these transmittance of fiber optic cables have pollutants might be monitored by using made many new wavelengths accessible. In buried optrodes to measure waste products particular, the development of new of bacteria that are transporting to the enzyme-based optrodes should be pollutant. Optrodes could be implanted encouraged. Many enzyme-based reactions in Laboratory animals exposedto chemical are well understood. In addition, the pollutants and the concentrations of the enzyme attached to the end of the f ber po77utants and their metabolizes is not consumed in the reaction, so the monitored in vivo. This might detectors should have a reasonably “ong significantly reduce the number of lifetime. animals needed in such research projects. Enzyme-based optrodes, which are a recent development, greatly increase the specificity of chemical measurements. As 5.4. BIOLOGICAL RESEARCH an example, an optrode has been developed for measurement of the concentration of p-nitrophenylphosphate (Arnold 1985). 5.4.1. [email protected] Alkaline phosphatase is immobilized on a nylon mesh at the tip of a fiber optic. Biological and medical sciences have The alkaline phosphatase catalyzes the made the greatest advances when a new hydrolysis of p-nitrophenylphosphate, tool was developed to reveal biological producingp-nitrophenoxide,which absorbs structures in more detail. The first light at 404 nm. Light is transmitted major advance was the invention of the down the fiber optic and the amount of microscope which revealed for the first 1ight backscattered at 404 nm is time the world of microorganisms and the monitored. When the enzymatic reaction cellular nature of living organisms. The occurs, some of this light is absorbed by next leap was the discovery of X rays the p-nitrophenoxide, enabling the which permitted a view of the internal measurement of the concentration of structure of large organisms. This was p-nitrophenylphosphate. followed bytheel ectron microscope which The concentration of penicillin can permits a view of external structures on now be measured with an enzyme-based a much finer scale but only after the optrode (Kulp et al. 1987). structures are immobilized and stained or Penicillinase and the fluorescent coated with metal films. The organisms compound fluorescein are mixed in are no longer in their living form, but polyacrylamide gel covalently attached still great progress has been made with to the end of a fiber optic. When this very important capability. A more “penicillindiffuses into the gel from the recent advance is the MRI technology bulk solution, penicillinase catalyzes which has similar or somewhat better the cleavage of the 13-lactam ring of resolution than does the medical X-ray penicillin, producing penicillin acid technology, but it permits the which dissociates into penicillate and a visualization of the internal structure proton, producing a pH change in the of hydrogen-containing or soft tissues sensor medium and a consequent decrease and organs of the human body. The most in the fluorescence of the fluorescein. recent advance is the scanning tunneling This change in fluorescence is detected microscope which can determine surface synchronously, using a chopped light profiles to subatomic dimensions. source and a lock-in amplifier. The A new technology is now under minimum detectable concentration is development which should permit the next

51 stride in biology and medicine -- that of pinhole or by the use of a metal X-ray holography. Holograms in the scattering sphere of the appropriate size extreme ultraviolet (XUV) region in the to give the desired reference wave water window with resolution of3 to 5 nm intensity. .Pinholes and zone pJates for cou7d revea7 interna7 structure of 7iving focusing limits resolution to 20 nm. matter the size of a ce71. Later However, the resolution of 1 nm with a developments shou7d permit ho70grams with source wavelength of .6 nm should be .5 to 1 nm resolution (at the level of obtainable with the sphere technique. A larger molecules). However, the system is under development by a group in difficulties increase very rapid7y below Chicago which uses a reference sphere, an 1 nm, making the achievement of such X-ray sensitive array CCDwith a computer resolution very speculative. readout, and appropriate software to Even the achievement of resolutions reconstructthe desired video images from of 1 to 10 nm requires the development the resulting hologram. Optical of two separate technologies. The first reconstructiontechniques have also been is an X-ray source of the appropriate developed. intensity and coherence length. The If successful, this technique could second is an imaging system based on be used on many other important problems holography. biology. The most important The brightest synchrotrons light ~~velopment can only be achieved if both sources now planned have enough the X-ray laserand the higher resolution intensity, even after selecting a narrow holographic schemes are developed and enough spectral width with used to obtain three dimensional monochromator, to give the desire: structural information on biological coherence length (about 1 #m) by using an molecules and living systems such as expanded exposure time (-5 rein)for 1 nm viruses and organelles. By selecting the resolution. This means, of course, that wavelength, the resulting image would be any sample must be completely immobilized atomic-species selective, greatly for that time, and radiation damage will increasing the information obtained and be limiting in many cases. X-ray lasers improvingthe effective resolution. This under active development,as discussed in technique would also freeze the action Section 3.2.4, should be able to provide (-lo” see) and thus reveal structure and a source capable of producing a hologram conformatialnalchanges. These appear to of living material in a single exposure be a key to the understanding of which is too short in duration to permit biological activity. Other groups are blurring (the order of -10-’3s). known to be exploring direct imaging Holographic systems are of two techniques, but information on these general types. The first is the Fresnel activities is not generally available. or the Gabor hologram which uses the original wave as a reference to interfere with the scattered wave from the target to form the hologram and so produces no magnification. This means that the 5.4.2. ~t Chemical Kinetics recording medium or device must have the desired resolution. Photo resists have the best resolution (-10 rim), a major The development of mode-locked limitation. The second is the Fourier lasers at wavelengths ranging from IR to transform hologram. This uses a the UV; fast, high sensitivitydetectors; spherical reference wave so that the and picosecond streak cameras have made magnification can be chosen to match the it possible to investigate chemical recording medium. The resolution is set kineticson the nanosecond to feintosecond by the magnitude of the scattering angle time scales. These developments have the accepted by the recording mediurn potential for making major contribution (a = A/2fl). The required reference wave toward understanding some of the basic can be generated by focusing part of the physics, chemistry, and biology programs primary beam through an appropriate of OHER.

52 In general, the techniques are the rhodopsin, the initiation of proton and time-dependent measurements of optical ion transport across a membrane -- which absorption by use of variable ‘delay is essent-ialfor biological activity -- between an exciting beam and a is shown to result from structural-and time-locked probe beam to determine confirmational changes caused by light saturation and relaxation rates, or by absorption in which a trans- to photon-excited fluorescencewhich is then cis-isomeric transition takes place in a observed by streak cameras to resolve few picosecond. Much has been learned very short time scales. Channel plates about vision mechanisms by such studies. and diode detectors are used for longer Subpicosecond absorption time scales, together with a sampling spectroscopy is used to study the oscilloscope or transient digitizers. interaction of hemaproteins with 02, NO, Areas of biological interest where and CO in which it is found that the such fast time scales are important are photodissociationof the ligand from the the end use of photon energy in hemoprotein occurs in less than 250 fs, photosynthesis,the mechanisms of vision, while the recombination occurs on the electron transfer reactions, the subnanosecond timescale for 02 and NO. transport of oxygen and CO, used in Femtosecond IR spectroscopy is used to metabolic processes, and structural or observe directly ligands trapped in the configurational changes by isomeric protein. Subpicosecond resonance Raman transformations induced by sudden energy and IR spectroscopy is used to monitor additions. structural changes in the heme after The photosynthetic process is under photodissociation and to study ligand intensive study in order to understand dynamics. Prior to the advent of current the various mechanisms by which photons short-pulse laser methods, it was are harvested by antenna fields, the necessary to carry out low temperature energy transport to the reaction center, experiments which showed there are and the energy use in photosynthesis. An confirmational substates, each with a example is photosynthetic bacteria which different barrier height, of the absorb photon energy in carotenoid photodissociated hemo-protein which proteins to produce singlet state controls the rate of ligand bonding. At excitation which is transferred by higher temperature,all the substates are singlet-to-singlet excitation to the sampled to give an average reaction rate. reaction center of bacteria chlorophyll. Picosecond studies have been madeon A separation of charge would then exist internal motion in DNA molecules, using across a transmembrane of the laser-excited fluorescence and streak chlorophyll-protein complex. The charge camera detectors. The ultrashort (ea. 1- separation then drives proton transport 3 ps) excited state lifetimes of the across the membrane which powers the purine and pyrimidine bases in DNA photosynthetic process. The reaction indicate that nonradiative decay kinetics, which is extremely complex, is dominates over energy loss by studied by the bleaching rates and fluorescence. recovery time for different wavelengths, Most of the DOE-sponsored work in using double-pulse, femtosecond laser fast chemical kinetics for biology is systems. Since some of the electron supported in the Chemical Division of the transfer times have been shown to be less Basic Energy Sciences Office. The than50 fs, the development of ultrashort supported work is quite substantial in pulse lasers is very important. Studies photosynthesis,mechanisms of vision and include attempts to develop synthetic heme-protein, and porphryn dynamics in photosynthesis for energy production. oxygen and carbon dioxide transport. Rhodopsin found in the rods of the retina While the area of structural changes and and three similar compounds found in the excited state chemistry using 7aser cones form the reaction centers for excitation have received somewhat 7ess vision. The simplest to separate and use emphasis, the latter area shou7d be is rhodopsin which also occurs in purple important in studies of radiation effects bacteria. In studies of bacterial on bio70gica7 systems, particu7ar7y with

53 regard to possible prophylaxis against dream. Other model developments are excessive @xposure to radiation. leading to a deeper understanding of Although the structure of the basic photalbiological processes. chemical bond has been well characterized Certain aromatic compounds have in a variety of systems, there is a clear bimodal fluorescence emission spectra. need to understand the dynamics of The bimodality has been explained by tra~sition states in chemical reactions. recent models in terms of the twistingof Achieving this goal will help answer single and double bonds in excited-state quest ions about how enzymes work and wi 11 molecules (Rettig 1986). These models contribute to making enzyme engineering have increased our understanding of the a reality. photophysical behavior of many organic, Transition states in chemical inorganic, and biologically relevant reactions can now be analyzed compounds. Continued research in these experimentally using femtosecond areas is warranted. transition state spectroscopy (FTS]. Excited states of molecules of all This is a technique for observing tvDes have well defined lifetimes and reaction fragments on their way to d&ay funct’ ons that span a range of 12 becoming products (llantus et al. 1987). decades in time. By means of laser For a dissociation reaction, a excitation, these excited states can be femtosecond Iaser pulse is used to excite used to c- ock the dynamics of the the reactant to an energy level above interactionl of a fluorophore with its that needed for bond dissociation. A environment The fluorescence second collinear laser pulse is used to measurement. can also provide information search for absorption by one of the on orientational changes, collisional reaction products. The time delay effects, energy and electron transfer, between the excitation pulse and the and the rates of chemical reactions second (probe) pulse determines the specific to a particular location of the duration of the bond breaking event. The biomolecule!. The major ways of measuring FTSmethod has been used recently (Rosker fluorescencedecays involve synchronousl y et al. 1988) to measure the dissociation pumped dye lasers, streak cameras, or of ICN into I and CN at 205 ~ 30 fs. nonlinear optical (upconversion) Theoretical work on transition-state techniques, depending on the required dynamics has also moved ahead rapidly. time resolution. The principle governing the A great deal has been learned about interactions of enzymes and their fast chemical kinetics using the methods substrates is now well established (Kraut discussed here. Advances in technology 1988). The principle of transition-state can be expected to open many new areas stabilization asserts that an enzyme for research and to lead to fundamental binds most strongly to the reactants in understanding of the kinetics of chemical their activated transition-state reactions. geometry. This significantly increases the concentration of the transition state and drives the reaction forward. The transition-state stabilization principle 5.4.3. mow Cytometry has been used to assist in the development of a model in two zinc proteases. Carboxypeptidase and F1 OW cytometry (FCM) is a thermolysin are zinc proteases that have laser-based method for making rapid very different amino acid sequences and multivariate measurements on biological three-dimensional structures. Yet they cells (Steinkamp 1984). In a flow appear to have a common transition-state cytometer, one or more laser beams are geometry. It is likely that numerous focused into a flow chamber contain;;~l~ other enzymes will be found to have sample stream of biological similar transition-state geometries. As surrounded by a cell-free sheath. The more of these are understood, enzyme sample stream is a suspension of engineering will become more than a biological cells that may be stained with

54 fluorescent dyes wtiichare stoichi ometric agents * and sometimes bacterial with respect to various subcellular identification can be done with FCM organelles or surface antibodies. The (Bailey et al. 1977; Bercovier et al. sheath and sample streams pass through a 1987; Boye et al. 1983), using DNA 50 to 100 P diameter orifice and into content measurements. the optical measurement region of the Laser-based flow cytometry has had flow chamber. The orifice constricts the a major impact on the study of the immune sample stream to a diameter of around system. Qmphocytesc anbedistinguished 10 P so that the cells pass through the from other formed elements in the blood laser beams one at a time at rates of using combined forward (parallel to the 1000 to 5000 per second. As the sample incident laser beam) and 90° light and sheath streams leave the flow scattering from cells in a flow cytometer chamber, a piezoelectric crystal causes (Salzman 19822. With the lymphocytes the stream to break up into uniform selected two-parameter light droplets, some of which contain cells. scattering, fluorescent monoclinal Droplets containing cells which have antibodies to various lymphocyte subsets given rise to specific signals can be can be used to identify the subsets charged and electrostatically deflected (Hoffman et al. 1980). FCM is the major onto slides or into special containers research tool for modern immunology. for further analysis. This is the Continued development of FCM technology process of cell sorting. Multiple laser is likely to provide more new approaches beams are focused along the sample stream to studies of the immune system. at different points within the optical The time dependence of the effects measurement region. Flow cytometry has of physical or chemical agents on been a major user of large frame gas biological cells can be studied using 1 asers. FCM. The time at which a cell is Typical flow cytometers have one or analyzed in a flow cytometer can be two cw gas lasers. One of these lasers measured as part of the multiparameter is usually an argon ion laser with a data (Martin and Swartzendruber 1980). total power output of5to 10 U. Typical Enzyme kinetics and drug transport in laser 1 ines used are a group in the UV cells have been studied, using time as a near 360 nm, 457 nm, 488 nm, and 514 nm. parameter in FCM measurements. FCM is Other gas lasers used are krypton ion being used in studies involving trans- (413 nm, 531 nm, 568 rim, 647 nm, and membrane signaling in cells (Lazzari et 753 nm), helium-cadmium (325 nm and al. 1986). Other functional studies have 442 nm), and helium-neon (633 nm). An involved calcium ion concentration argon- or krypton-pumped dye laser is (Rabinovitch et al. 1986) and used when excitation wavelengths are intracellular pti measurements {Bassoe et needed that fall between the laser lines. al. 1983). Flow cytometers are being Mercury arc lamps are also sometimes used used for preparation sorting of human on smaller flow cytometers. chromosomes as part of the human genome The most common measurement made with initiative (Gray et al. 1987). They have a flow cytometer is the DNA content of a also been used to detect specific DNA manmalian cell (Merkel et al. 1987; sequences (Trask et al. 1985). Barlogie et al. 1980). In normal cells, FCM technology istightlycoupl edto DNA content is precisely regulated as research into new types of lasers and cells grow through the cell cycle and photodetectors. Research support of divide. In cells perturbed by mutagens these technologies has had a broad and or that have progressed to cancer, the beneficial impact on biological research DNA content may no longer be euploid. and on human health care. This aneuploid state is readily detected A laser technique for the rapid and quantitated by FCt4 measurements sequencing 1 arge fragments (Andreeff et al. 1980). Common laser [-40 kilobases ‘ffkb) ] of DNA based upon wavelengths used for these measurements the detection of single, fluorescently are360 nm, 457 nm, or488 nm. Bacterial tagged nucleotides cleaved from a single growth, the effects of antibacterial DNA fragment has been proposed. The

55 projected rate of sequencing is -1000 an area c}f potential importance to bases per second. The technique is based several of the OHER programs. Potential upon the projected ability to detect biomedical sensor applications include single chromophores by laser-induced- critical care (in vivo determination of fluorescence in flowing sample streams o C02, pHl, temperature, K+), chronic (Nguyen et al. 1987a, 1987b). The maintenance (in vivo and in vitro technique involves (a) labeling the determinations of glucose, therapeutic nucleotides with base-specific tags drugs), and acute diagnosis (in vitro suitable for fluorescence detection, (b) studies of blood chemistry, abnormal selecting a desired fragment ofDNA, (c) function, infection, diagnostic suspending the single DNA fragment in a metabolizes) (Wolfbeis 1987). flowing sample stream, (d) sequentially Laser-induced fluorescence is considered cleaving labeled bases from the free end to be one of the most promising of the DNA fragment using an exonuclease, approaches in this field because of its and (e) detecting and identifying the detection sensitivity and versatility. cleaved, labeled bases as they flow For exampl[?,with fiber optic sensors through a focused laser beam. The rate based on fluorescence,in situ determine- that bases can be sequenced is determined tion of immunoglobulin, IgG, and by the kinetics of the exonuclease benzo[a]pyrene was performed with cleavage reaction and the time required detection sensitivitiesof25 fmol and 1 for detection and identification of the fmol, respectively. Among the bioanaly- labeled bases. Based upon results for tical applications of the fluorescence the detection of rhodamine-6G18, the technique (Bright 1988), one includes (a) investigatorsanticipatesequencingrates assays directed at the determination of of -1000 bases per second on a single DNAand RNA; (b) enzymatic m;}hods, e.g., strand of DNA several tens of kilobases for the determination glucose, (kb) in length. In order to detect true sucrose, and fructose and for the single molecules, it is necessary to evaluation of kidney function; (c) improve the sensitivity for fluorescence studies of protein-ligand interactions, detection. With improvements in the e.g., the understanding of the apparatus, the investigators envision stoichiometry and affinity of single mo7ecu7e detection with exce71ent protein-binding sites is of critical signal-to-noise ratio. importance in elucidating protein functions amd dynamics; and (d) fluoro- immunoassays (FIA). This last concept deserves special attention. In its 5.4.4. Biosensors simplest form, it involves a competitive binding reaction between an antigen (Ag - the target analyte), a labeled antigen Fiber sensors find their greatest (Ago), and a highly selective antibody use in monitoring clinically and (Ah) to form AgAb and Ag”Ab. Basically, biochemically important analytes such as the immunoassay exploit some differe~ce serum electrolytes, metabolizes, between the free labeled antigen (Ag), co-enzymes, immunoproteins, and and,the antibody-bound labeled antigen inhibitors. Biosensors are available (AgAb) to quantify the target analyte. which respond selectively and reversibly FIA isexpe~ctedto be more sensitive than to the concentration or activity of is radioimmunoassay (RIA), where radio- chemical species in biological samples. isotopes are used as labels. In some cases, a biologically active Polarization-basedfluoroimmunoassay material is incorporatedto make intimate (PFIA) and fluorescence anisotropy contact with an appropriate transduction selective technique - fluoroimmunoassay element for detecting, reversibly and (FAST-FIA) constitute attractive selectively, a chemical species in any detection methods in FIA. In the first type of sample (Arnold and Meyerhoff case (PFIA) developed for many drugs, 1988). New breakthroughs in optical antibiotics, and steroids, the sensor technology are likely, and this is fluorescent label can be used to track

56 rotational diffusion which manifests measurement that one can make in a itself in a large polarization for a cuvette. larger species versus a small species. Thus Ag” and Ag4Ab are easily distinguishable by measuring the 5.4.5. Ultrasensitive Detection and fluorescence polarization. In the last Analvsis for Biological Research detection method (FAST-FIA) applied to the determination of bovine serum Many biological research activities albumin, the individual rotational rates require the detection and analysis of of the molecular species Ag” and Ag”Ab micro-quantitiesof biologicalmaterials. could be directly resolved. Examples include FCM, cytochemical Time-resolved fluorescence (TRF) is analyses of single cells, detecting trace a key technique for discriminating chemicals or particulate of health and against background fluorescencewhich can environmentalconcern,measuring cellular be due to other matrix components. For uptake of chemicals, and analyzing micro- example, fluorescein in serum-containing quantities of proteins or nucleic acids samples suffers from interference of via spectrophotometry,chromatography,or bilirubin because of spectrally electrophoresis. overlapping emissions. However, Several 7aser-based technologies fluorescein and bilirubin decay times are could usefu17y be considered for these about 4 ns and 0.1 ns, respectively, so applications. Zeeman interferometry that time gating can be successfully (Johnston 1987, 1989; Johnston et a7. used. 1988) is a relatively new technique that Rare earth chelates are of special looks particularly promising. It interest. FIAwith lanthanide labels is exploitsthe two-frequency,Zeeman effect a fast moving research field and is laser for simple and high resolution likely to grow in the future (Soini and interferometry. Many of the stability Lovgren 1987). A directly fluorescent and complexity problems that plague lanthanide label can be used in which conventional interferometric or optical both the fluorescent and the chelating heterodyne techniques are not present properties are incorporated into one (Johnston 1987, 1989; Johnston et al. ligand-lanthanide complex. When such 1988). A suitably developed Zeeman complex is bound to an immunoreactive interferometer could be used as an component, completely new assay designs ultrasensitive refractive index detector based on TRF become real. In other for chromatography or electrophoresis. cases, the lanthanide is bound to one of The Zeeman interferometer is the immunocomponents in an essentially substantially superior to conventional nonfluorescent form. After completionof refractive index detectors in its the immunoreaction, the lanthanide is sensitivity, inherent absence of drift, dissociated from the immunocomponentand and its ability to be applied to brought into solution, where a highly extremely small volumes. fluorescent chelate is formed. This Micro-quantities of proteins or nucleic technique is called dissociationenhanced acids could be detected in real time as fluoroimmunoassay. they are eluted from chromatography or Presently, fiber optic sensors have electrophoresiscolumns,without the need some ?imitations, associated mainly with for fluorescent or radioactive tags or matrix effects. Nevertheless, the future the damaging effects of UV radiation. of fiber optic sensors is extremely Phototherma7 spectroscopy (Harris promising. The ultimate goal would be 1982; Dovichi 1988) “ another the development of real time, in vivo laser-based technology th~~ warrants chemical and immunochemical sensors of consideration. The technique measures the determinationat trace and ultratrace the small temperature rise (or refractive levels of specific biomolecules and index change) in a sample when it absorbs bioactive species, with simple sensors small amounts of incident light (Johnston taking over essentially any optical 1987; Harris 1982; Dovichi 1988). Zeeman

57 interferometry may again play an understanding protein thermodynamics, important role “ photothermal biopolymer chemistry, and protein-drug applications (Johns~~n 1987, 1989; interactions. Johnston et al. 1988). Finally, experimental advances in Photothermal spectroscopy has many pulsed-lasertechnologymake impractical experimental advantages (simplicity, to study the elastic and Raman scattering sensitivity, relative freedom from model of femtosecond 1aser pulses from dependence) over photoacoustic biological cells (Chang and Chang 1988; spectroscopy and can be better applied to Biswas and Chylek 1988). Both polarized biological samples and small volumes. and unpolarized measurements are of Photothermal spectroscopycan be used for interest. Such measurements can provide microchemical analyses (on small volumes additional size and compositional and or on 1arge volumes with trace structural information about a cell not contaminants) and for linear or circular readily accessible from steady-state dichroism measurements. scattering measurements. Surprisingly, dichroisnrapplications of photothermal spectroscopy have received 7itt7e attention. They are, however, of great potential va7ue for 5.4.6. Structural Analysis and studying trace samp7es and for dichroism $eauencinq of Biomolecules measurements at very short wavelengths. The latter are of much structural interest, but are difficult due to the Structural analysis and sequencing strong absorption of water and the of nucleic acids and peptides (Landegren shortage of suitable light sources at et al. 1988; Maugh 1984; Garnick et al. wavelengths below 180 nm (Williamset al. 1988) will be a key accomplishment in 1986). biological research over the next decade. Light scattering has always been a The ability to rapidly determine DNA powerful tool in biological studies. In sequences would facilitate the recent years, there has been increased identification of disease-associated interest in the polarization properties genes whilch cause inherited disorders of scattered light (see, for example, such as cystic fibrosis, phenylketonuria Johnston et al. 1988; Bustamante et al. and Huntington’s disease. In some of 1982). This is evidenced by measurements these illnesses, large deletions of of the Mueller scattering matrix and sequences in the genes may occur or just amplitude scattering matrix for a single nucleotide substitution may be biological samples and by polarized Raman responsible for the disorder. Nucleic scattering experiments. These acid sequencing is also important for measurements are all experimenta77y determining damage to the DNA due to difficult, yet should be encouraged mutagenesis from external agents such as because of the 7argeamount ofstructura7 chemicals or radionuclides. Peptide information that is theoretica77y sequencing and identification is an availab7e. A third area, polarized equally important problem for studying dynamic 7ight scattering, may not have critical metabolic disorders in the body received the attention it deserves. and for quality control in the Autocorrelation (unpolarized)scattering biotechnology industry (Garnick et al. measurements have proven to be useful for 1988). understanding translational and Both the peptide-proteinand nucleic rotational diffusion coefficients (Berne acid sequencing problems are complicated and Pecora 1985). Polarized dynamic by the large number of units to be scattering measurements have the sequenced. This is especially true in potential for providing important the nucleic acid sequencingproblem where information on structural and there are >10’ base pairs in DNA. Some confirmationalfluctuationsin biological DNA sequencing techniques have been macromolecules. Such informationis, for automated (Landegren et al. 1988; Maugh example, of great importance in 1984; Garnick et al. 1988); however,

58 these are too slow and are prone to a hundred nanometers. However, recent number of errors. Also, these techniques work by Kopelman (Michigan) and Lewis fail when there are a large number of (Jerusalem) has utilized the properties modified or mutant nucleosides as in RNA, of exciton migration totransfer UV light for example (McCloskey and Nishimura energy to a target fluorophore in contact 1977). In order to identify uniquely with the crystal in a region <100 nm. each of the four base units and to distinguish them from the modified or mutant nucleosides that may be present, a technique such as mass spectrometry or 5.4.7. Genome Sequencing 1aser spectroscopy “ required. Fluorescent tags can be u~~d to identify each of the four regular base pairs, but Laser technology could have adirect cannot easily identifymutant or modified and impressive impact on genome species in the sequence. Mass sequencing. Several approaches are being spectrometry is universal in that any taken, including various imaging substituted unit can be identified. methodologies. In order to sequence the To accomplish sequencing rapidly, base units along aDNA segment, a spatial continuous flow techniques need to be resolution of 1 nm or better is required developed (Covey et al. 1986). This well as a tagging scheme to would involve biochemical degradation of ~~stinguish the base units from each a nucleic acid strand followed by a other. Several methodologies are being subsequent continuous injection of the explored, including coherent electron effluent into a mass spectrometer. microscopy, acoustic imaging, and Continuous flow FAB (Caprioli et al. scanning tunneling microscopy which may 1986) has proven to be valuable in the be able to provide the desired sequencing of peptizes. As solutions are resolution. injected into the MS, FAB is used to One very promising method for vaporize and ionize the peptides for sequencing may be X-ray microholography. identification. Techniques such as This novel X-ray technique, under laser-induced MPI may be interfaced to development by a research group at the this method for enhanced selectivity University of Illinois at Chicago (Boyer against the liquid matrix which often 1989), is performed by tagging each type provides background interference in of base unit with a separate atomic FAB-MS. Laser-induced MPI can also species. The prepared tagged DNA segment provide versatility in identification is exposed to a specific X-ray radiation based upon induced soft ionization or wavelength from a synchrotrons light extensive fragmentation. source or an X-ray laser, corresponding Lightmicroprobetechniques (Kopelman to a resonance (near an absorption edge) and Lewis, unpublished)might be a second of each atomic tag labelling each base possible method for DNA sequencing. Each unit. By using an X-ray CCD for different nucleic acid base would be detection, it is estimated that a Fourier labeled with a different fluorescent transform hologram could reobtained with probe. If a light source that could be 1 nm resolution with a 200 s exposure confined to .5 nm (the dimension of the time for each type of base unit of a nucleoside plus fluorescent probe) were 100,000 string. A reconstruction developed, then the light probe could be algorithm has been developed which scanned along the intact nucleic acid permits a computer-generated video image strand to obtain sequence information. to be obtained. Optical reconstruction This procedure would obviate the problems techniques have also been developed. of chemical degradation, but would be Superimposing four video images for each accurate if only the four usual bases base type at the appropriate wavelength were present. Development of a .5 nm would provide the sequence for the DNA light source would be the main problem string at an overall rate of 100 base since light in the near UV and visible units per second or greater. Such a rate would be diffraction limited by several is essential if DNA sequencing is to

59 become a general tool for biology and a mixture of peptides is separated by medicine. The DNA string has to be microbore HPLC and the eluted product fossilized and contained within mixed witlhglycerol. As the solution is supporting films (probably diamond) to injected into the MS, FA8 is used to limit the effect of radiation damage from vaporize and ionize the peptides for the synchrotronsbeamso that the relative identification. Techniques such as positions of the markers are not laser-induced MPI may be interfaced to affected. The use of a short-pulse, X- this method for enhanced selectivity ray laser would eliminate the damage against the liquid matrix which often problem and the necessity of protecting provides background interference in FAB- the DNA string. MS. Laser-induced MPI can also provide Other interesting methods forgenome versatility in identification based upon sequencing include light microprobe induced :Soft ionization or extensive techniques (Kopelman 1989). This method fragmentation. Using high repetition involves exciton migration and rate exci{merlasers (>100 Hz), the main fluorescence in solid-state materials. 1imitation for speed in sequencing DNA This technique can be coupled with strands will be the degradation and micropipette technologyto confine energy introduction processes. to a very small (<100 nm) spatial volume from which the energy can be transferred directly to another species. If this technique can be further developed, such 5.5. NUCLEAR MEDICINE USING STABLE that energy transfer can be confined to ISOTOPES a probe tip of <1 nm, the required resolution would be available for distinguishing tagged fluorescent probes Isotopes have enormously important specific to each base in a long DNA roles to play in the diagnosis and strand. The probe could be scanned treatment of human diseases. The basic rapidly along the DNA molecule to obtain idea is to attach these isotopes to such the specific sequence. diverse tags as neuroreceptors and One of the most powerful methods monoclona”l antibodies and to use the available for sequencing is analysis via characteristic radiation emitted from mass spectrometry, especially when a these isotopes to trace their flow within 1arge number of modified or mutant the human organism. Radionuclidesare by nucleosides are present, as, for example, far the most commonly found agents in RNAth~scCloskey and Nishimura 1977). employed in these medical applications In “ case, positive mass since their location may be directly identification of the nucleic acid imaged. There is currently an important components is required. The problem of thrust within DOE, for example, to rapidly sequencing a large number of develop radiopharmaceuticals including nucleic acid base pairs still remains. monoclona-l antibodies for improved One developing trend in mass spectrometry methods of diagnosis and treatment. This is the use of exonucleases to strategy may assist the early detection sequentially break down the DNA strand of cancer:,for following the response of followed by continuous flow injection patients to treatment, and for general methods into a MS. As each base pair is cancer th~rapy. Radionuclides injected into the MS device, it can be themselves, however, pose a significant sequenced rapidly and accurately. health threat to the patient. Current research is actively engaged in With the advent of advanced laser continuous liquid injection methods spectroscopic techniques, it appears interfaced to various MS ionization feasible to find a number of sources. A relatively new method which complementary approaches to utilize has proven to be valuable in the stable iisotopes to examine these sequencing of peptides and processes,, completely eliminating the oligonucleotides is continuous flow FAB risks of radiation. There are currently (Caprioli et al. 1986). In this method, few examples of the use of stable

60 isotopes for these medical applications. developed for many stable isotopes. These tags are not as convenient to trace Recently (Moore et al. 1987) this method through the body since they must be has been used to determine the copper and examined from blood, urine, or other molybdenum concentrations in microliter samples extracted from the body. The samples of blood by an isotope dilution ultra70w detection limits and the procedure. The present accuracy of 15% isotopic selectivity of laser-based at the femtogram range suggests this analytical schemes, however, offers approach may be important in fo]70wing potential for major new clinical isotopic enrichments in small samples for approaches. metabolism studies. A critical issue among health As research into the laser detection physicists is to provide rapid, low-cost of stable isotopes continues to increase methods for the detection of uranium and in sophistication, there are many new transuranic species in biological avenues of application which may open. samples. Currently, uranium or plutonium Direct ana7ysis of inhaleable analyses in urine require extensive particulate may provide ear7y warnings sample pretreatment, long countingtimes, of toxic dust. The role of trace metals or an intense source of neutrons. In a in nutrition, toxicity, and disease series of preliminary experiments (Gerke etiology could be more clear7yeva7uated. et al. 1988), it has been possible to volatilize uranium atoms directly from a urine sample and to collisionally ionize 5.6. RADIATION DOSIMETRY the atom after 2-photon resonant excitation to a Rydberg level using Laser technology and spectroscopy inexpensive N2-pumped dye lasers. The make it attractive to consider new experiments were performed at low methods for the solution of old problems pressure (1 to 10 torr), and the ions in radiation dosimetry. Actually, laser could be counted with high efficiency technology has already been used to make using just a thermionic diode. Detection more convenient systems for gamma ray limits in the few femtogram range have dosimetry. Thermoluminescentdosimeters been demonstrated for calcium, and these are used to assess the exposure of limits appear feasible for theactinides. personnel who wear these devices. After /Yuch research into this promising exposure the small samples are heated approach is needed to improve isotopic with a 1aser beam to initiate selectivity, samp7ing methods, and laser fluorescence from thedosimeter. Herewe excitation schemes. If these approaches explore laser spectroscopy and speculate cou?d be generalized, wide screening of on more novel approaches by considering patients for a wide variety of stable two examples. The first example is isotopes may be feasible. appropriate for new research in radiation Sampling is a continual problem for dosimetry based on the use of resonance any biological assay involving heavy ionization spectroscopy. When ionizing atoms. Converting the target isotope radiation of any type (X rays, gamma quantitatively into the gas phase with rays, neutrons, etc.) interact with high efficiency from a variety of matter, both excited states and ion pairs matrices is a topic of intense research are created and in about equal numbers. interest. The methods which utilize However, nearly all systems of radiation laser resorption or keV ion-resorption dosimetry, and even the most from a complex solid matrix are sophisticated of detectors like those particularly attractive since there is used in particle physics, measure the efficient temporal and spatial overlap ionization created and neglect the rich between the atomized sample and the laser amount of information which could be ionization probe which makes it possible retrieved from a study of the excited to use smaller samples. Further, with species. RIS the problem of interferences is In radiation dosimetry, even in the greatly reduced, allowing more most advanced form such as quantitative schemes to be easily microdosimetry,the entire emphasis is on

61 the creation of ion pairs. However, it been shown that the incorporation of is usually assumed that when charged par- xenon atoms into amorphous metal alloys ticles interact with matter, e.g., in a should lead to a personnel dosimeter gas, the particle loses as much energy in which would have excellent energy creating excited states as in ion pairs. dependence from about 1 keV to about Further, in most theories of charged 20 MeV and would be so sensitive that a particle interaction, the assumption is few mrad of exposure could be reliably made that the excited state population recorded. In practice, amorphous metal compared to the number of ion pairs is alloys containing xenon would be ground independent of the energy of the primary into fine particles and enclosed in a charged particle or the secondary smallenvelope which would retain the particles created. These assumptions sputtered xenon atoms. could be tested in actual experiments by The above examples indicate that using the techniques of laser methods clf laser spectroscopy could be spectroscopy. used for more definitive research on the We note that, historically, the RIS interaction of radiation with matter, and method was invented for just these there is every reason to believe that studies on excited states. However, the such research would 7ead to improved demand forRIS in analytical chemistryto practices in personnel dosimetry, evenin detect atoms and molecules in their the near term. ground states diverted attention from these studies. There is considerable evidence that the study of excited states would yield new information. Certainly, as electrons slow down in matter, they reach an energy where low-lying REFERENCES metastable states become more important than other types of excited states or ion pairs. To the extreme, it is noted that Andreeff, M., Z. Darzynkiewicz, T. K. the magnetic monopole (if it exists Sharpless, B. D. Clarkson, and H. R. according to current grand unification Melamed (1980) Blood ~, 282-293. theories) would interact, for example, in helium gas to create only metastable Arnold, M. A. (1985) Anal. Chem. n, 565. states. ln research where attempts are Made to correlate track structure with Arnold, MI.A. and M. E. Meyerhoff (1988) the effects of radiation on biological CRC Crit. Rev. AnaJ. Chem. ~, 149. cells, it is appropriate to consider the role of excited states. Bailey, J1.E., J. Fazel-Madjlessi, D. N. Another example where the quantum Quitty et. al. {1977) Science ~, 1175. state selectivity of RIS could be used is found in neutron dosimetry (Hurst et al. Barlogie, B., B. Drewinko, J. Schumann 1987) . Workers at power reactors are et al. (1.980) Amer. J. Med. 69, 195. possibly exposed to neutrons inan energy range (about 10 to 100 keV) which cannot Bassoe, C. F., O. D. Laerum, J. Glette be detected with any presently available et al. (1.983) Cytometry ~, 254. personnel badge. It turns out, however, that charged particles sputter atoms from Bercovier, f-i., M. Resnick, il. Kornitzer, a solid target, with the cross section and L. Levy {1987) J. Microbiol. Methods for the process peaking in the few ken ~, 167. region. This process could be incorporated into a neutron badge to give !3erne, B. and R. Pecora (1985} DYNAMIC just the desired response in the energy LIGHT SCA,TTERING (Ple~um Press, New York, region that is most critical for the NY] . power reactors. The ability to measure small numbers of these sputtered atoms is Bi swas, A. and P. Chylek (1988) Appl. provided with RIS. For instance, it has Phys. Lett. !52, 1642.

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65 ,, ./‘

APPENDIX I

LASER ASSESSMENT STUDY ACRONYMS

BER Biological and Environmental Research CAT Clear Air Turbulence CCD Charge-CoupledDevice CID Charge Injected Devices Cw Continuous Wave CZE Capillary Zone Electrophoresis DOC Dissolved Organic Carbon DOE Department of Energy IIOM Dissolved Organic Matter EI Electron Impact ESD Electron-StimulatedResorption FAB Fast Atom Bombardment FAST-FIA FluorescenceAnisotropy Selective Technique - FIA Fluoroimmunoassays FCM Flow Cytometry FET Field Effect Transitor FIR Far-Infrared FLN FluorescenceLine Narrowing FM Frequency-Modulating FTS FemtosecondTransition State Spectroscopy FTMS Fourier-TransformMass Spectrometry GC Gas Chromatography HDTV High-DefinitionTelevision HPLC High Performance Liquid Chromatography ICP InductivelyCoupled Plasma ICR Ion Cyclotron Resonance 1P Ionization Potential Infrared lP Potassium Di-hydrogen Phosphate LEAFS Laser Excited Atomic Fluorescence Spectrometry LEI Laser-EnhancedIonization LET Linear Energy Transfer LIDAR Light Detection and Ranging LIF Laser-InducedFluroescence LOD Limit of Detection LP Longation Potential LTE Local Thermodynamic Equilibrium MCP Microchannel Plate MCP-PMT Microchannel Plate Photomultipliers MPI Multiphoton Ionization MRI Magnetic Resonance Imaging MS Mass Spectrometers NASA National Aeronautics and Space Administration

66 OGE Optogalvanic Effect OHER Office of Health and Environmental Research OPO Optical Parametric Oscillator PA Photoacoustics PET Positron Emission Tomography PFIA Polarization-basedFluoroimmunoassay PMT Photomultiplier PNAH PolynuclearAromatic Hydrocarbons RBE Relative Biological Effectiveness REMPI Resonance Enhanced Multiphonic Ionization Radio Frequency ;;s Remote Fiber Spectroscopy RIA Radioimmunoassay RIMS Resonance Ionization Mass Spectroscopy RIS Resonance Ionization Spectroscopy SERS Surface Enhanced Raman Spectroscopy SF Supercritical-Fluid SIRIS Sputter-InitiatedResonance Ionization Spectroscopy SIT Silicon-IntensifiedTarget Sos Silicon-On-Sapphire SRI Stanford Research Institute SSJ Supersonic Jets STEM Scanning Transmission Electron Microscope TOF Time-of-Flight TRF Time-Resolved Fluorescence Ultraviolet ;Iv Vacuum Ultraviolet Xux Extreme Ultraviolet

67 I APPENDIX II

INTRODUCTION TO TERMINOLOGY

LASER: The acronym laser is derived the scattered energy, and the transmitted from “Light Amplification by Stimulated energy. Emission of Radiation” and is a light source with some unique properties. Most EXCITATION: The energy absorbed by are familiar with the directional a material may cause the excitation of property of a laser beam and know that a the material. Of course, the nature of laser beam has such a small divergence this excitation depends critically on the that it can be used to illuminate a small type ofma~terial. For an atomic gas this spot on the moon from a point on the excitation is simply the promotion of an earth. For the purposes of this report it electron iinits lowest or ground state to is even more significant that lasers are some higher state allowed by a quantum a very intense light source and that they mechanical description of the atom. For can be monochromatic, having a single molecular gases, this excitation process wavelength or color. A variety of laser is more complex; in addition to systems are now commercially available electronic excitation, there is that can be “tuned” to almost any desired vibrationi~lexcitation as atoms in the color. Currently these tend to be “dye” molecule move with respect to each other, lasers which use liquids similar to those and there is rotational excitation as the used to dye clothing. However, these are entire molecule rotates about some axis being replaced with solid-state lasers of symmetry. In condensed matter such as which are much more convenient to use. a liquicl or a solid, collective processes--often described in terms of ABSORPTION: When alaser beam strikes phonons or plasmons--add to the a material, some of its energy may be complexity of the excitation processes. absorbed by the material. As a consequence, the material may be heated FLUORESCENCE: When matter in any of or other physics” processes as described its forms is excited, some of the energy below may occur. can subsequently be emitted as fluorescence due to the tendency for matter tc) relax to its ground state SCATTERING: Some of the energy following excitation. The lifetime for striking a mater al can be radiated away excited states depends critically on the immediatelyor scattered from the target-. type of Imatter and on the particular In other cases (see below), this state which was excited. With most forms radiation can be delayed and is not of radiation, such as electrons, X rays, usually considered to be a scattering or gamma radiation, a variety of types of process. excited states are formed during the excitation process and the specturm of TRANSMISSION: When a radiation beam fluorescence radiation is complex. With such as a laser strikes a material of monochromatic lasers it is possible to finite thickness, some of the beam can select just one of the many types of pass through the material without excited-states that may be desired. This interaction. Consistent with these choice depends on- the wavelength definitions and the conservation of tunabil ty of the laser source. In this energy principle, the incident energy is manner, the fluorescence spectrum may be equal to the sum of the absorbed energy, simplif ed with laser excitation.

68 IONIZATION: Another physical process is even more directly indicative of the which may occur when radiation is energy absorbed than is the case just absorbed in matter is ionization, i.e. described, where the transmittedlight is the complete removal of an electron from measured. Photoacoustical or PA the atom. The process of excitation, spectroscopy is based on the measurement usually refers to the promotion of an of pressure changes with a sensitive electron only to discrete levels allowed microphone. by quantum mechanics, while for ionization the electron is removed from RANAN SPECTROSCOPY: When light the atom and can have a continuous range scatters from a molecular material, the of energy. Sometimes, the ionization scattered light can carry away the process is referred to as a process of fingerprints of the molecules present. excitation into the ionizationcontinuum. This is a weak process but provides a valuable method of spectroscopy when SPECTROSCOPY:Any light source,other using the light intensities available in forms of radiation, or beams of charged lasers. The spectrum of scattered light particles such as electrons or positive consists of the fundamental or incident ions can be characterizedwith an energy wavelength modified to contain other associated with the particles. These wavelengths that depend on the structure energies can be discrete or have a of the molecules. Raman spectroscopy is continuous range. In any case the frequently incorporated into 1aser description of the number of particles systems for remote sensing. having particular values of energy is referred to the spectrum. SURFACE-ENHANCED RAMAN SPECTROSCOPY Spectroscopy in it~~any forms is derived (SERS): When a molecular sample is from this basic concept of the spectrum. prepared to be in contact with certain Spectroscopy is extremely useful because materials, the intensity of the Raman the spectrum can serve as a most valuable process is greatly enhanced. This effect fingerprint. For instance, the spectrum is believed to be due to the inductionof of the light emitted from a gas discharge surface excitations called plasmons when lamp such as a neon sign gives a positive the laser beam interacts with the identification of the types of atoms in surface-sample interface. The SERS the lamp. Lasers are used in many forms process is being studied in a number of of spectroscopy based on absorption, laboratories in an effort to develop emission, scattering, and ionization improved analytical methods for the phenomena. Some of these are described analysis of samples such as complex more fully below, and all of the organic molecules. principal forms of laser spectroscopyare mentioned frequently in this report. FLUORESCENCE SPECTROSCOPY: In short times following the absorption of energy ABSORPTION SPECTROSCOPY: If a by a sample, some of this energy may be continuous spectrum of light illuminates emitted as radiation characteristic of a sample, the transmitted light may be the material. Fluorescence spectroscopy modified; in general, some wavelengths based on this process is widely used for will be more strongly absorbed than detection of atoms and molecules. It is others. This absorption spectrum is very a preferred method for remote sensing and useful in identifying the types of atoms can work even when the target material is or molecules making up the sample. being excited by some other agency such as a gas discharge, and in this case PHOTOTHERMAL AND PHOTOACOUSTICAL provide information on the transient SPECTROSCOPY: Intense laser beams have species. made possible a form of spectroscopy based on the selective heating of a SYNCHRONOUS FLUORESCENCE sample due to the absorption of light at SPECTROMETRY: In conventional different wavelengths. Actually, this is fluorescence spectrometry, two basic a subset of absorption spectroscopy and types of spectra can be recorded, i.e.,

, 69 ,. , an excitation spectrum (fixed excitation OPTOGALVANIC SPECTROSCOPY: When atoms wavelength, variable fluorescence are already excited, for instance in a wavelength). With the synchronous flame, laserscan be directed through the fluorescence technique, the fluorescence sample Ito enhance the amount of signal is recorded while simultaneously, ionization. This process can also be iie., synchronously, scanning both selective as in RIS. Optogalvanic excitation and emission wavelengths, spectroscopy, is commonly used with maintaining a constant wavelength flames an~dcan be another very sensitive interval. This technique is method of ionization spectroscopy. characterized by an improved selectivity in the analysis of multicomponent MASS SPECTROSCOPY [MS): When mixtures. electrons are used to ionize an atom or a molecule, the positive ions produced FLUORESCENCE LINE NARROWING are, of course, quite representative of SPECTROSCOPY: This technique, which is the samp’le present before ionization. also described in the literature as This has led to the discipline of mass “site-selectionspectrometry,” is largely spectroscopy, where the mass of the ion used for the characterization of is determined by a mass spectrometer. MS polyaromatic hydrocarbons in view of its is one of the most powerful and sensitive special selectivity, since it is analytical methods in existence. When performed at very low temperature (about electrons ionize complex molecules, 4 K) in organic glasses, where molecules several mlass peaks are formed for each occupy different microenvironment or molecule. In MS, mass analysis is done sites. Excitation by a spectrallynarrow by deflection in a magnetic field, by laser gives sharp-lined fluorescence time-of-flight, or by a variety of other spectra, greatly improving resolution. methods. However, regardless of the method, the presence of various mass RESONANCE IONIZATION SPECTROSCOPY peaks causes interferences, related to (RIS): One of the consequences of energy and exacerbated by the fact that no absorption in a sample is ionization of method has perfect mass resolution. If atoms or molecules. Lasers, especially two atomls have the same mass but carefully tuned lasers, can be used to different atomic numbers, aMS system may selectively ionize the species. In the not be able to differentiate the two. RIS process, photons are first absorbed This is usually referredto as the isobar to excite the atom, generally according problem. Since atoms can have a number to one-photon selection rules of quantum of stable isotopes, it is possible for mechanics. Then, photons are absorbedby two types of atoms to have nearly the the excited atom to produce a free same mass and identical mass numbers, electron and a positive ion. Since the i.e., they are isobars. High resolution first step is quite selective the entire MS can resolve many of the isobars, ionization process is selective. In provided they are of relatively similar practice, laser systems are now used to abundance. selectively ionize almost any atom in the periodic table. RESONANCE IONIZATION MASS SPECTROMETRY (RIMS): Lasers can be used MULTIPHOTON IONIZATION (MPI) is as an ionization source in a mass closely related to RIS but is more spectrometer system. When the laser is general since lasers are now so intense used in the selective RIS process, the that several photons can be absorbed strongest features of both the laser and simultaneously. MPI, therefore, may not the mass spectrometer are combined. For be as selective to the type of atom instance, the isobar problem is present as is the RIS case. REMPI refers eliminated when detecting atoms. to resonance-enhanced MPI and, thus, is Further, the resulting mass spectra (of indistinguishable from RIS. organic compounds) are much simpler with

70 the resonance ionization process. RIMS metallic, and organic compounds present systems have been developed which combine in the environment, and emphasizes the Iaserionization with virtually all types fact that one needs to make a clear (magnetic sector, time-of-flight, and distinction between the capability of a quadrupo?e) of mass spectrometers. technique of measuring the total concentration of an element and that of ATOMIZATION: In many 1aser measuring the concentration of each of spectroscopy approaches to trace element its chemical forms. analysis, it is necessary first to atomize the sample. For instance,,touse RIS, free atoms, representative of the BIOSENSOR: This term, which can be solid, must be produced. Thermal sources understood in analogy with the term in which the sample is heated in a small “chemical sensor,” can be defined as an graphite furnace is often used. In other analytical device which incorporates a cases,” atoms can be ablated from the biologically active material in intimate solid by focusing a strong laser beam contact with an appropriate transducer onto the sample. Another method which is for the purpose of detecting, reversibly better understood and under better and selectively, the concentration or control than laser ablation is sputtering activity of chemical species in any type of atoms using a beam of charged of sample. It can also be defined as an particles. In principle, the sputtering analytical device that responds process is simple: a beam of charged selectively and reversibly to the particles imparts considerable kinetic concentration or activity of chemical energy to the atoms of a solid. These species in biological samples. This last hot atoms impart energy to several other definition refers to the type of sample, atoms in a collisional cascade, resulting while the former refers to the nature of in the emission of several neutral atoms the chemical reaction that generates the for each energetic charged particle analytical signal. striking the sample.

SPUTTER-INITIATED RESONANCE DISSOLVED ORGANIC NATTER (DOM):This IONIZATION SPECTROSCOPY (SIRIS): Sputter term refers to the large and complex initiated resonance ionization organic acids found in natural waters. spectroscopy is the combination of a It is variously called dissolved organic sputter source for atomization, RIS for carbon (DOC); humic substances; and selective ionization, and a mass Gelbstoffe or, more specifically, fulvic spectrometer for mass selection. Systems or humic acids. Collectively, these of this type are being used for the dissolved organic materials represent an understanding of surfaces and solids and important component of the global carbon are being developed for routine trace cycle. element analyses of solids. REMOTE SENSING:This term is used OPTRODES: Chemical substances can be for all types of analytical measurements deposited on the ends of optical fibers where the sample stays in one location so that when specific other chemicals, and the measuring instruments stay in such as pollutants interact, light another location. Therefore, it refers changes in passing through the optical to “real time” acquisition of the fiber are produced. These detectors, or analytical data. In environmental optrodes, are being developed for a wide applications, remote sensing with laser variety of applications, and they are sources is called with the acronym LIDAR becomingan interestingclass of specific (1ight detecting and ranging). With the chemical detector. advent of fiber optics and chemical sensors, remote measurements of many CHEMICAL SPECIATION: The term means physico-chemical parameters have become identification of inorganic, organo- feasible.

71 SENSITIVITY AND SELECTIVITYUNITS: q (10” g); nanograms, ng (lO+g); Traditionally, a variety of units have wcopranw P9 (10”’2 g); femtoqrams~ fg been used for expressing both the (10’ g); or attograms, ag (10”0 g) are sensitivity and the selectivity of common. For example, the expression analytical methods. With 1aser 1 pg/g means.10”’2g of an analyte in 1 g spectroscopic methods, it is even of amatrix. The concentrationdetection possible to discuss the actual counting limit of 10’2M, for example, means that of atoms. Even with this ultimate in 10”’2grammolecularweights of a substance sensitivity, it is still.necessaryto put can be detected in 1 liter of material. limits on the selectivity, which can be either elemental or isotopic. For ENERGY UNITS : Energy levels of instance, one could speak of the atoms and molecules are frequently detection of one atom of cesium in the expressed in electron volts (eV) or, background of 10’gatoms of argon, or the alternatively, in wave numbers cm-’. A counting of 100 atoms of*’Kr in a mixture useful a~pproximateconversion is 1 eV = of 107 atoms of “Kr. In many cases the 8000 cm-’. The energy of light particles natural unit is based on the mass of an or photons is often expressed in terms of analyte in a matrix with a much larger wavelength. In this case, a useful mass. For high sensitivity and high conversion is 1 eV = 12345/A, where A is selectivity me,thods,notations such as the wavelength in Angstrom units and milligrams, mg (103 g); micrograms, 1 A= 104cmor 0.1 nm.

72 APPENDIX 111

PEER GROUP

The members of the expert panel are grateful for useful comments and suggestions from the following scientists. We are pleased that these many experts in laser science took time to review and comment on a draft of this report.

Prof. Isaac Abe17a Dr. Stephen L. Palfrey Department of Physics David Sarnoff Research Center University of Chicago CN 5300 5734 S. Ellis Avenue Princeton, New Jersey 08543-5300 Chicago, Illinois 60637 Dr. Marvin G. Payne Prof. Norman Dovichi MS 6378, Bldg. 5500 Department of Chemistry Oak Ridge National Laboratory University of Alberta P. O. BOX 2008 Edmonton, Alberta Oak Ridge, TN 37931-6378 CANADA T6G 2G2- Prof. Edward Piepmeir Dr. Jack Fassett 1-B Barnes, Escandido Village A21 Physics Stanford, CA 94305 National Inst. of Standards & Technology Dr. Richard L. Sheffield Gaithersburg, Maryland 20899 MS-H825 Los Alamos National Laboratory Prof. Robin M. Hochstrasser P. O. BOX 1663 Department of Chemistry Los Alamos, New Mexico 87545 University of Pennsylvania Philadelphia, Pennsylvania 19104 Dr. Gerald J. Small Ames Laboratory Prof. Bruce S. Hudson Iowa State University Department of Chemistry Ames, Iowa 50011 University of Oregon Eugene, Oregon 97403 Dr. Andrew C. Tam Dept. K07-803 E Dr. Ralph R. Jacobs IBM Almaden Research Center Director, Corporate Technical 650 Harry Road Development San Jose, California 95120 Spectra-Physics, Inc. 3333 North First Street Prof. J. D. Winefordner San Jose, California 95134-1995 Chemistry Division University of Florida Dr. John C. Miller Gainesville, Florida 32611 MS 6125, Bldg. 4500S Oak Ridge National Laboratory Dr. Edward A. Yeung P. O. BOX 2008 Department of Chemistry Oak Ridge, Tennessee 37831-6125 Iowa State University Ames, Iowa 50011 Prof. Dr. K. Niemax Institut fur Spektrochemie Postfach 778 4600 Dortmund 1 Federal Republic of Germany

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