Laser Photoacoustic Spectroscopy Markus W. Sigrist, Zurich (ΕΤΗ, Institute of Quantum Electronics, CH-8093 Zurich)

Photoacoustic spectroscopy (PAS), also known as optoacoustic spectro­ scopy, was pioneered by Alexander Graham Bell more than a century ago. The advent of the laser strongly stimu­ lated research and applications of this technique and the interest is such that international topical meetings on photo­ acoustic and photothermal phenomena are now held biennially. The photoacoustic (PA) effect is es­ Fig. 7 — Principle of the photoacoustic effect. sentially an energy-conversion process. When a sample (solid, liquid or gas) is Two types of modulation should be weakly absorbing gases. In these cases irradiated by a laser beam or some other distinguished: modulation of the inci­ acoustically resonant cells are often radiation source, part of the absorbed dent radiation and modulation of the used. The modulation frequency is then energy is converted into translation sample absorption. Schemes for the tuned to a resonance frequency of the cell energy of the molecules by radiation­ first include the widely-used amplitude preferably with pressure nodes at the less transitions. It is this de-excitation modulation by mechanical choppers, cell windows to reduce the background channel which is responsible for heat electro-optic and acousto-optic modu­ originating from window absorption. production within the sample although lators and the direct modulation of Finally, various different schemes secondary reactions may also play a the radiation source including notably exist for the detection of the thermal or role (see Fig. 1). If either the incident pulsed sources as well as frequency- acoustic disturbances caused by the radiation or the absorption by the sam­ or wavelength-modulation. These last absorbed radiation. The choice depends ple is modulated, the periodic heating offer the advantage of eliminating the on the sample, the sensitivity to be finally results in a pressure modulation. continuum background PA signal result­ achieved, ease of operation, rugged­ The PA signal thus originates from the ing from a wavelength-independent ab­ ness, and any requirement for non-con- thermal response of the sample to the sorption, e.g. of the entrance window of tact detection, e.g. in aggressive media. absorbed energy detected as pressure the PA cell containing the sample. On The methods which have been deve­ fluctuations acoustically. Owing to ther­ the other hand, modulation of the ab­ loped for solid samples are summarized mal diffusion processes the magnitude sorption characteristics of the sample in Fig. 2; they include the measurement of this signal depends not only on the can be achieved on the basis of the of the generated pressure wave either thermal and optical characteristics of Zeeman or Stark effect, by applying directly in the sample with a piezoelec­ the sample but also on the nature of the modulated magnetic or electric fields to tric sensor for the pulsed regime, or coupling mechanism between sample the sample. Thereby the absorption indirectly in the gas above the sample and detector. Consequently the cou­ wavelength is varied corresponding to a with a microphone (photoacoustics). pling medium plays an important role wavelength modulation and the conti­ Induced changes of the refractive index and one can differentiate between the nuum background is again suppressed. can be sensed by monitoring the deflec­ light-to-heat conversion and the heat- While the design of the PA cell is not tion of a probe beam, e.g. from a HeNe to-sound conversion efficiencies. critical for solid or liquid samples, it may laser, either within the (transparent) represent a crucial point for studies on sample or directly above the (plane) Experimental Arrangement sample surface (photothermal deflec­ Since the time of Bell who used the tion, mirage effect). Changes of the sur- Sun as radiation source, a foot-operated chopper for modulation and an ear­ phone as acoustic detector, numerous experimental arrangements have been developed which mainly differ in the radiation sources employed and the system of both modulation and detec­ tion. Since the PA signal amplitude is proportional to the radiation power, the high spectral brightness offered by lasers is advantageous for the measure­ ment of small absorptions, i.e. for weak­ ly absorbing media or trace detection. Fig. 2 — Photoacoustic and photothermal detection schemes. 167 Studies on Liquids In principle there are five important interaction mechanisms which can be responsible for the excitation of acous­ tic waves in liquids by the impact of pulsed laser radiation: dielectric break­ down, vaporization, thermoelasticity, electrostriction and radiation pressure. Their contributions depend on the para­ meters of the incident laser beam as well as on the optical and thermal properties of the medium. PAS studies on liquids are essentially based on the Fig. 3 — Experimental set-up for NORMALIZED pure thermoelastic effect where the photoacoustic studies on gases. ABSORPTION SIGNAL thermal expansion of the laser-heated volume generates the acoustic wave. Experimental and theoretical studies face reflectivity or slight deformation Studies on Solids have been performed for a wide absorp­ of the surface (photothermal displace­ A main advantage of PAS applied to ment) can also be detected without solids is the fact that no elaborate sam­ tion range including "transparent" and contact by a probe beam. Finally, varia­ ple preparation is required and unpo­ opaque liquids. tions of the thermal radiation from the lished sample surfaces pose no pro­ In the first case, minimum absorption surface can be monitored with an in­ blems. Since the PA signal is proportio­ coefficients of 10-6 cm-1 have been de­ frared detector (photothermal radiome- nal to the absorbed energy, even spec­ tected corresponding to absorbed laser try). Other techniques include pyroelec­ tra of strongly scattering samples, like pulse energies of only 10- 9 J. Owing to tric detection in thin films, thermal e.g. powders, can easily be measured. the high sensitivity, the method has lensing and interferometric methods. Another advantage is the high sensivity been successfully applied e.g. to the Studies on liquids are usually per­ that can be achieved because the PA measurement of the 8th harmonic of formed with pulsed lasers and piezo­ detection is a null method for measuring the C-H fundamental stretching vibra­ electric or probe beam deflection moni­ absorption. To date, PAS performed tion of benzene and to the detection of toring of the generated acoustic signal. with pulsed lasers and direct piezoelec­ impurities in transparent liquids. The most widely used arrangement for tric detection of the generated acoustic Depending on the penetration depth gases employs a chopped laser beam waves permit the detection of absor­ and the actual laser beam diameter, directed through an appropriate gas cell bances below 10-7. In the pulsed re­ plane or hemispherical sound waves are where the pressure modulations are de­ gime noise is easily rejected by time excited in opaque liquids for most media tected either by commercial condenser gating. Furthermore, the high laser within the IR wavelength range. In this microphones with sensitivities up to intensities enable the use of non-linear case, different boundaries, is. surface 100 mV/Pa or by miniature electret mi­ optical techniques to generate intense conditions as e.g. free and confined sur­ crophones with sensitivities of typically tunable infrared and ultraviolet radiation faces or optically thin and thick surface 10 mV/Pa. These acoustic detectors are for spectroscopy. films, can be investigated. PAS allows easy to use and their response is appro­ Recent work has been aimed at us to analyze and monitor surface films priate for most PA studies. The schema­ studying adsorbates on the surfaces of on liquids in situ in a non-contact and tic set-up is shown in Fig. 3. The phase- solids. PAS is expected to be rather sen­ rather simple manner. sensitive detection of the microphone sitive to surface adsorption, especially if A recent example concerns the moni­ signals by the lock-in amplifier sup­ the substrate is transparent or highly toring of polymerization in a surface presses any broadband background reflective in the wavelength region in film. The sample consisted of an opti­ noise. The transmitted laser power, indi­ which the adsorbate absorbs. Both cally thick layer (100 µm) of octadecyl cated by B, is used for normalizing the sinusoidal modulation of the incident methacrylate (ODMA, C22 H42 O2) PA signal so that the B/A signal is direct­ laser beam and pulsed lasers have been floating on a water surface. This layer ly proportional to the absorption coeffi­ used for this purpose. For example, can be polymerized in a nitrogen at­ cient of the sample at the laser wave­ ammonia (NH3) adsorbed on a silver mosphere by UV irradiation at room length. substrate, cooled to 90 K in ultrahigh temperature. During the polymerization vacuum, has been detected with sub­ process the absorption band of ODMA Applications of PAS monolayer sensitivity. centred at 940 cm-1 diminishes and Photoacoustic and photothermal In another PAS study the thermal finally vanishes completely. We used phenomena have been widely used for disordering of Langmuir-Blodgett (LB) this fact for PA monitoring of the poly­ numerous non-spectroscopic applica­ monolayers of cadmium stearate on merization by measuring the PA signals tions like the determination of thermal sapphire has been investigated. A which are excited by the impact of CO2 diffusivity, non-destructive testing of pulsed laser with piezoelectric detec­ laser pulses at different wavelengths as materials (in particular the probing of tion followed by boxcar integration a function of the UV irradiation time sub-surface defects) by thermal wave gave a detection limit of ca 5 x 10-10 J tuv . An example is shown in Fig. 4 imaging, monitoring de-excitation pro­ absorbed per laser pulse for an ave­ where the time dependence is plotted cesses, studies of phase transitions, rage of 100 pulses. The technique has for the 10P(28) laser transition located etc. Fiere only the spectroscopic appli­ also been applied to photochemical at the centre of the ODMA absorption cations are considered as they demon­ studies of adsorbed organometallics as band and the 10P(20) transition situat­ strate the main characteristics and the well as to in situ diagnostics of laser ed on the edge of the band. At the potential of PA spectroscopy. chemical vapour deposition (CVD). time tuv = O, i.e. before polymerization 168 with respect to concentration. The me­ centrations. It should be emphasized thod is thus suitable for both immission that even the isomers of xylene can be and emission studies. determined separately. This separation (v) Detection of numerous components is difficult, if not impossible, to achieve with one single apparatus, equipped e.g. with conventional methods, e.g. with with a tunable IR laser, is possible. gas chromatography. (vi) Only a short absorption path is re­ Our second PA apparatus is equipped quired, i.e. wavelengths beyond atmo­ with a sealed-off CO2 laser, line-tuna­ spheric windows can be applied. ble between 9.2 pm and 10.8 pm. A (vii) Continuous in situ measurements special design of the acoustically reso­ on flowing air with suitable PA cells are nant PA chamber permits continuous possible. measurements on air samples with a Since most molecules exhibit charac­ flow rate of ca 1 I/min without loss in Fig. 4 — Laser-photoacoustic monitoring of teristic absorption features within the sensitivity. The minimum detectable ab­ photopolymerization of ODMA layer. Mea­ mid IR range, the so-called fingerprint sorption coefficient is 10-8 cm-1 which sured amplitudes vs UV irradiation time tuv. region, tunable lasers emitting in this corresponds to trace gas concentra­ wavelength range are most appropriate tions of ppbV (µg/m3) under interfe­ starts, the PA amplitude is considerably for trace gas detection. In the course of rence-free conditions. For the in situ larger for the 10P(20) than for the our own studies in this field we have monitoring of air pollutants, the entire 10P(28) laser transition. built two computer-controlled PA sys­ PA apparatus is installed in a mobile air- Studies on Gases tems: a stationary apparatus with a CO conditioned trailer of 4 m length. Numerous laser-PA studies have been laser and a mobile system with a CO2 Hitherto, our mobile system has been performed in gaseous media including laser. Both lasers are only line-tunable, applied at various locations for the in studies on the absorption by molecules not continuous, which poses special re­ situ recording of specific pollutants like in excited states, and investigations on quirements on the analysis of measured ethylene (C2H4) in outdoor air. Since chemically reactive gases which may PA spectra of multicomponent gas mix­ today considerable amounts of C2H4 produce transient intermediate chemi­ tures with interfering absorptions. The are emitted in the atmosphere by road cal species. Most applications, how­ analysis is performed on the basis of traffic and various industries, the en­ ever, concern trace gas monitoring. In calibration spectra of single trace con­ hanced C2H4 concentrations are a con­ addition to common PAS, various spec­ stituents by using a new iterative for­ tinual source of stress for plant growth troscopic schemes with PA detection malism. This procedure permits the de­ in urban areas. Furthermore, C2H4 acts have been applied. These include termination of individual concentrations as a precursor substance for photo­ Doppler-free spectroscopy, vibrational as well as the evaluation of the best chemical smog. The 10P(14) CO2 laser overtone spectroscopy, PA Raman laser transitions for the detection of transition at 949.49 cm-1 is ideally sui­ spectroscopy, harmonic saturated specific components, yielding a mini­ ted for detecting C2H4 because of the spectroscopy as well as non-linear mum of interference with other compo­ proximity of the C2H4 vibrational band spectroscopy, i.e. two-photon and in­ nents at optimum sensitivity. v7 located at 948.77 cm-1. With the frared multiphoton absorption. A specific application concerns the resulting large absorption cross section analysis of motor vehicle exhaust gases of 1.3 x 10-18 cm2 we achieve a detec­ Today, atmospheric pollution by nu­ performed with the CO laser PA system. tion limit for C2H4 in ambient air of merous trace constituents at ppmV Since water vapour exhibits a consi­ below 5 ppbV if attention is paid to the (10--6) or ppbV (10-9) concentrations is derable absorption in the wavelength interfering absorption by CO2. Diurnal of major concern. Different techniques range between 5.15 µ m and 6.35 pm of concentration profiles are correlated have been introduced for the detection our CO laser, we have used a dual-beam with the strength of traffic, wind data of trace gases which is a prerequisite arrangement with specially designed and, on sunny days, with the local for understanding the complex chemis­ sample and reference PA cells. In spite ozone production. try of the atmosphere, particularly pho­ of the water-vapour interference we Recent studies which demonstrate tochemical smog formation. However, have been able to derive the concentra­ the versatility of PAS concern in situ since conventional methods like chemi­ tions of 12 of the most important com­ measurements on industrial exhaust air luminescence, common IR spectrosco­ ponents like nitric oxide, ethylene, which represents a mixture of nume­ py, flame ionization, gas chromatogra­ propylene, aromatic hydrocarbons and rous components with concentrations phy etc. do not meet all the require­ aldehydes at ppmV and sub-ppmV con­ ments, novel schemes have to be stu­ varying in time. The continuous record- died for the continuous, sensitive and selective monitoring of numerous pollu­ tants either in situ or remotely. For in situ applications PAS can be characterized by the following features: (i) High sensitivity which permits trace gas detection at the ppbV-level under interference-free conditions. Fig. 5 — Concentration pro­ (ii) Simple set-up, e.g. no cryogenic cool­ file of ethanol-vapour in in­ ing for a radiation detector is required. dustrial exhaust air. (iii) Easy calibration with certified gas •— ; Laser-photoacoustic mixtures. monitoring, (iv) Wide dynamic range, i.e. linear res­ x ; Gas chromatography ponse over several orders of magnitude data. 169 ing of the main constituents is thus of Perspectives FURTHER READING great importance. As an example Fig. 5 Since photoacoustics involves opti­ Zharov V.P. and Letokhov V.S., Laser Opto- shows the PA monitoring of ethanol cal, thermal and acoustical phenome­ acoustic Spectroscopy, Springer Ser. Opt. (C2H5OH)-vapour present in the ex­ na, photoacoustic and photothermal Sciences, Vol. 37 (Springer Berlin, Heidel­ haust stream of a pharmaceutical pro­ sensing schemes are ideally suited for berg) 1986. duction plant besides numerous other studying various material properties. Tam A.C., Rev. Mod. phys. 58 (1986) 381- Apart from non-spectroscopic applica­ 431. volatile organic compounds. The PA Sigrist M.W., J. Appl. phys. 60 (1986) R83- measurements were performed periodi­ tions like e.g. thermal wave imaging R121. cally at selected CO2 laser transitions which today represents an important Rothberg L., J. phys. Chem. 91 (1987) yielding ethanol and methanol con­ technique for the non-destructive eva­ 3467-3474. centrations simultaneously. In general, luation (NDE) of materials, laser-PA Photoacoustic and Photothermal Phenome­ good agreement between the photo- spectroscopy offers great potential for na, Eds. P. Hess and J. Pelzl, Springer Ser. acoustically derived concentrations and applications ranging from surface phy­ Opt. Sciences, Vol. 58 (Springer Berlin, Hei­ independent gaschromatographic (GC) sics and chemistry to environmental delberg) 1988. measurements (represented by x in Fig. sciences. The few examples discussed Sigrist M.W., Bernegger S. and Meyer P.L., indicate the wide gamut of PAS applica­ Infrared Phys. 29 (1989) 805-814. 5) is obtained under steady state condi­ Photoacoustic, Photothermal and Photo­ tions. The PA data are not systemati­ tions that can be expected. chemical Processes in Gases, Ed. P. Hess, cally higher than the GC data as could Acknowledgement Top. Curr. Phys., Vol. 46 (Springer Berlin, be supposed from the results in Fig. 5, The author is grateful to S. Bernegger and Heidelberg) 1989. but the time resolution offered by PAS is P.L. Meyer for their collaboration. Our pho­ Photoacoustic, Photothermal and Photo­ better and can be exploited for the toacoustic studies were funded by the chemical Processes at Surfaces and in Thin monitoring of even short-term concen­ Swiss National Science Foundation (Natio­ Films, Ed. P. Hess, Top. Curr. Phys. Vol. 47 tration fluctuations. nal Research Program 14) and ETH Zurich. (Springer Berlin, Heidelberg) 1989.

transmission spectrum of a polymer EQEC: 2nd European Quantum layer sandwiched between two trans­ Electronics Conference parent electrodes. Stockman (Novo­ sibirsk) gave a very interesting presen­ Dresden, 28 August - 1 September 1989 tation of the giant optical non-linearities Following EQEC-1, the first Divisional the first relaxation oscillation spike of fractal clusters such as aggregates of Conference of the Quantum Electronics occurs. Dunn (St. Andrews) discussed colloidal gold. Division held in Hannover in 1988, the use of excimer lasers to pump opti­ Walther (Max-Planck-lnstitut, Garch- EQEC-2 took place in Dresden's Town cal parametric oscillators (OPO's) in the ing) reported on recent single-atom Hall under the auspices of the GDR visible. Generally speaking OPO's have experiments which allow many early Academy of Sciences, the Friedrich been of marginal importance during the "Gedanken" experiments to be per­ Schiller University, Jena, and the Physi­ past 25 years but they are now rapidly formed in the laboratory. Such experi­ cal Society of the GDR. gaining popularity owing to new non­ ments may resolve some basic issues in The four plenary papers highlighted linear materials and new pump sources quantum physics since ensemble avera­ present mainstreams in quantum elec­ becoming available. Prior (Rehovot) ad­ ging is no longer present. Another signi­ tronics. Akhmanov (Moscow) reported dressed current models used to des­ ficant contribution in a related area in­ on non-linear optics in 2-dimensional cribe laser fluctuations, namely phase cluded the report by Chebotaev (Novo­ systems, discussing non-linear wave diffusion, random phase jumps and tele­ sibirsk) on synchronisation in quantum dynamics, large scale transverse wave graph phase noise, generalizing these transitions, making connections with interaction, optical turbulence and spa­ into a non-Markovian stochastic jump Ramsey fringes, photon echoes and tial squeezing. These abstract concepts model. Distributed feedback gas lasers free induction decay. Possible applica­ were beautifully illustrated by experi­ were analysed in their own right, and as tions would be super high resolution mental results shown in an impressive a model system for DFB solid state spectroscopy and high-speed atomic movie. Welling (Hannover) reviewed the lasers, by Kneubühl (Zurich). DFB gas memory systems. In a theoretical paper many routes towards very-short-wave- lasers allow easy tests of theory since Stenholm (Helsinki) touched upon the length lasers and their potential applica­ their parameters are much better con­ issue of Landau-Zener crossing during a tions. Glauber (Cambridge) discussed trolled than in the case of a DFB semi­ laser assisted collision. For reasons not the theory of radiating atoms in reso­ conductor laser. Lasers were also ad­ yet understood the range of validity of nant cavities, while Paul (Berlin) addres­ dressed by Ovaevski (Moscow) in the the "classical" Landau-Zener result is sed a basic issue in quantum optics, context of the intriguing analogy bet­ apparently much larger than expected. namely interference between indepen­ ween lasing action and superconduc­ Finally, Noordam (Amsterdam) reported dent single photons, touching upon tivity that can be made by comparing recent results obtained in producing basic issues in quantum measurement Cooper pairs with photons. electronic wave packets in Rydberg theory. Regarding non-linear optical mate­ atoms using picosecond laser excita­ An important contribution in the area rials, Kobayashi (Tokyo) considered the tion. A full set of abstracts of all invited of laser physics was made by Bimberg problems which arise when one has to and contributed papers has been pu­ (W. Berlin) who reported on gain swit­ assess the many new materials (parti­ blished in Volume 13D of the Europhy­ ching of semiconductor lasers where cularly polymers) which are presently sics Conference Abstracts series which picosecond pulses can be produced by being developed for fast optical swit­ is available from the EPS Secretariat. driving the laser with a much longer ches and optical logic. He introduced a electrical pulse in such a way that only simple method based on measuring the J.P. Woerdman, Leiden 170