IEEE JOURNALOF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983 585 Picosecond Streak Camera Fluorometry-A Review

ANTHONY J. CAMPILLO AND STANLEY L. SHAPIRO

(Invited Paper)

Abstract-A general tutorial survey is presented describing the use of Several excellent reviews [ 1] - [4] have appeared describing the ultrafast streak cameras in picosecond fluorometry.Current instruments operation and history of the streak camera. This present paper exhibit time resolutions of 1-10 ps with detection sensitivitiesof a few goes beyond the instrumental aspects, and reviews the applica- photoelectrons. When linear photoelectric recording is employed,a real-time direct display of optical transients is provided. Representa- tion of laser fluorescence spectroscopy in some detail. We hope tive examples from the literature inphysics, chemistry, and biology this paper satisfies what appears to be a deficiency in this re- are given, as well as an extensive bibliography. gard in the open literature. We have attempted to present a reasonably self-contained primer in streak camera operation, including examples from the literature in physics, chemistry, INTRODUCTION and biology. We include an extensive bibliography, which is HE ultrafast streak camera is one of the most versatile representative of the field as a whole. Due to the continued Tinstruments used today in picosecondspectroscopy. Its growth of this subject, we are not able to include a complete usefulness arises from its excellent time resolution, its ability list of references here. to determine optical temporal profiles directly and in a single The key principles of electron optical streak camera opera- shot, if need be, and its straightforward integration into re- tion were established during the period from the late 1940’s markablysimple experimental configurations. Although the through the early 1960’s. In 1949, Courtney-Pratt employed a device itself is electronicallysophisticated, it resembles a deflectable image converter [5],[6] to measuretemporal/ photomultiplier/oscilloscope combination in convenience and spatial characteristics of light given off during explosive burn- operation. This is not coincidental, since the streak camera is ing in lead azide. This early device resembled currently used a photoelectric image converter camera possessing a time-base systems in most essential details, and exhibited a subnanosec- deflection system similar in principle to those used in cathode- ondresolution. In the mid-l950’s, Zavoiskii and Fanchenko ray oscilloscopes. However, the streak camera possesses two [7] , [8] calculated the conditions under which picosecond or significantadvantages over theoscilloscope. First, it yields subpicosecondresolutions might be obtainedfrom these thetemporal history of one spatialdimension. Second, devices, andenumerated the fundamental limitations. They demonstrated bandwidths far exceed that of any present or pointedout that picosecond resolution required the use of projected oscilloscope. high electric fields (tens of kV/cm) to extract photoelectrons The streak camera historically was first used as a diagnostic with sufficientlyuniform velocity from thecathode. These tool to characterize the output of mode-locked lasers, and for authors also coined the term “electron-optical chronography” monitoring laser fusionimplosion experiments, principally [9] for the new science that would employ these devices to atgovernment laboratories. However, the exceptional sensi- study rapidly varying luminous phenomena. However, experi- tivity ofthese devices makesthem especially attractive for mental progress was stalledbecause ofthe lack of suitable monitoringfluorescence. Recently, an explosive growth has opticaltransients to testpractical designs. Thesituation occurredin streak camera fluorometry, and this instrument changed dramatically in 1966 with the invention of the mode- is already regarded by many as a required laboratory device in locked Nd :glasslaser, which provided thenecessary short this application. That this is true, considering the significant optical pulse source. Because conventional detectors of light, expense associated with such systems, is fair testimony to the such as photodiodes, areincapable of measuring suchbrief power and reliability of the technique. pulses, these sources were first characterized with the aid of It seemed appropriate to includein this special issue devoted autocorrelationtechniques such as secondharmonic genera- to picosecond phenomena a general tutorial and review paper tion [lo]or two- [ll] orthree-photon fluorescence. How- describing the use of streak camerasin picosecond fluorometry. ever, these techniques do not unambiguously characterize the pulse shape. Streak cameras were subsequently used by several Manuscript received August 11, 1982; revised November 30, 1982. groups to examine individual pulses from thislaser [ 121-[ 161 , The work of S. L. Shapiro was supported in part by the U.S. Air Force allowing for the first time a direct measurement of the inten- Office of Scientific Research. sityprofile I(t). It was discovered [13]that photoelectron A. J. Campillo is with the Optical Sciences Division, U.S. Naval Re- search Laboratory, Washington, DC 20375. time dispersion could be minimized by locating a planar, fine- S. L. Shapiro was with the Molecular Spectroscopy Division, National mesh,high-potential electrode, called theextraction mesh, Bureau of Standards, Washington, DC 20234. He is now deceased. close to the tube photocathode, thereby achieving temporal

U.S. Government work not protected by U.S. copyright 586 JOURNALIEEE OF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983

MODE-LOCKED LASER resolutions of approximately 5 ps. Similar results were achieved M by applying a high-voltage pulse to the shutter grid element of a standard RCA C73435 image converter tube [14]. Shortly thereafter,streak cameras were used to monitor fastlaser- induced fluorescence [ 171 - [27] . STREAKCAMERA OPERATION Fig. 1 shows, in schematic form, some of the essential ele- ments in theoperation of astreak camera fluorometer. A mode-locked laser sourceprovides the necessary shortburst of light to excite the sample. A lens collects a portion of the DEFLECTION FILM resulting fluorescence and images the sample onto a slit, which PLATES is in turn imaged onto the photocathode of the streak tube. Fig. 1. Schematic of typical streak camera fluorometer. Electrons are promptly emitted by the cathode by means of the photoelectric effect, and are rapidly accelerated through a mesh and toward the anode. The extraction mesh is provided With deflection velocities approaching that of light and a spa- to minimize the velocity spread in the distribution of emitted tial resolution of about 7 lp/mm, the technical time limit is in photoelectrons.The resulting electron beam current, as a the subpicosecond range. As slower streak rates are used, this function of time, closely resembles the envelope function of quantity can become quite large (-ns). In general, the spatial the fluorescence I(t). Anaperture in the anode allows the resolution is determined by the electrostatic lensing character- electron beam to pass, ultimately impinging on a phosphores- istics of the image convertertube. During the streak, the cent screen. The focus cone provides an electrostatic lensing dynamic spatial resolution is somewhat poorer thanin the static field whichsharply images the slit onto the backphosphor mode. Equation (1) must be modified if the size of the mag- screen. Indeed, if the slit is removed, a well-formed image of nified/demagnified image of the slit on the phosphor screen is the illuminated sample cell will be projectedonto thephosphor. larger than 1/L. In this case, this quantity is substituted for By dispensing with one spatial dimension through the use of a ljt. Additionaldegradation of the technical time resolution slit, and by sweeping the electron beam across the phosphor may be caused by saturation effects in the streak system and screen by applying a linear voltage ramp to a set of deflection limitations due to the spatial resolution of the intensifier and/ plates, a measure of time can be achieved. This occurs because or recording medium. those electrons leaving the photocathode at earlier times will A second limiting factor is that due to photoelectron time arrive at the phosphor at one position, while those that leave dispersion AT,. This effect, which is alsocalled “chromatic atlater times will arrive at adifferent position, and time is time dispersion,” arises because electrons are emitted from the effectivelytransformed intospatial position. The resultant cathode with a distribution of velocities, and results incathode phosphorescence emitted by the screen at any spatial position to phosphor time-of-flight differences and subsequent loss of P(x) is proportional to the electron beam current, and conse- the observed signal is near the photocathode red cutoff, but quently, also to the fluorescence intensity Z(t). Typically, the becomes progressively more severe at shorter wavelengths. resulting phosphorescent spatial streak is intensified and sub- The electron time spread is given by the following relation: sequently photographed or electronically recorded. The deflection plates are driven by a ramp generatortriggered by a portion of the original excitation pulse. Typically, the deflectionplates are biased to initially image thebeam off where p is the electron charge/mass ratio, Av is the half-width screen. As the ramp is applied, the beam sweeps onto, across, of the distribution of initial electron velocities, and E is the and off the otherside. electric field strength at the cathode. It can be seen from (2) In assessing theperformance of a streakcamera, several that AT, can be reduced significantly by applying large elec- parameters must be considered. These include temporal reso- tricfields. This can be convenientlyaccomplished through lution, streak velocity, spatial resolution, timing jitter, optical the useof ahigh-transmission metallic meshplaced in close delay required, sensitivity, spectral characteristics, and dynamicproximity to the photocathode and the application of a high range. Thereare, of course, others, but theseare themost voltage [28] . When this field approaches 10 kV/cm, AT, is important, and we will discuss each in turn. reduced to subpicosecond values. The temporal resolution of a streak camera is determined by If one assumes that the functional form of the various con- several factors. The first is commonly called the technical time tributions to the time dispersion are Gaussian [ 141, then the resolution AT^ and is determined by the speed UD at which instrumental time resolution Aqnstris given by electrons are swept across the phosphor (on the order of lo9- 2 112 1O1O cm/s), and the spatial resolution L of the image converter Aqnstr X AT,)^ + (AT,) I . (3) tube (typically 5-25 line pairs/mm): Instrumental time resolutions of the best currently available streaktube are in the range of 0.7-2 ps [29], [30]. The 1 Photochron I1 tube claims the highestresoltuion at 0.7 ps ATT = -. (1) LVD [29] , [31] . Designsare currently under development which CAMPILLO ANDPICOSECOND SHAPIRO: STREAKFLUOROMETRY CAMERA 581 would extend the limit to below 200 fs [32],[33]. In addi- Anotherimportant quantity is thedynamic range. This tion, the time resolution of an experimental measurement will parameter is appropriately defined, for agiven time resolution, be degraded by group dispersion effects in the input collection as the range over which the measured light level can be in- optics and by the finite duration of the excitation pulse. If creased beyond a minimum required for an observable record this contribution Are is considered, then the resolution of a without introducing either spatial and/or temporal degradation measurement is approximately in the output record. This quantity is determined experimen- tallyby measuring ashort lightpulse ofknown duration, Arke,,, x AT&, AT:. t (4) determining the peak intensity at which the apparent half-width Several importantparameters (streak speed, optical delay increases approximately 20 percent, and dividing that intensity required, and timing jitter) are directly related to the streak by thebackground noise level of therecording medium. In camera deflection system. Typically, a 1-10 kV voltage ramp practice, the range of intensity over which the performance of is applied to the deflection plates, the rise time determining the the camera is acceptable becomes more restricted as temporal streak speed. In practice, the fastest rise time (1 ns) is limited resolution is imporved.Typical streak tube dynamic ranges by the intrinsic capacitance and induction of the deflection are 10-100 at 5 ps resolution and 2500 at ns resolution. system,which typically posseses anatural frequency in the Such degradation can arise from many sources. For instance, 100-200 MHz range. Use is normally made of a charge trans- arather gross spatialresolution deterioration occurs if the mission line switched by a krytron [29], spark gap [34], or photocathodecurrent density exceeds some critical value avalanche transistor string [35]. A portion of the excitation (typically 10 mA/cm2) for substantial periods of time (>ns). pulse isdirected to a photodiode to generate a fast voltage This current density would occur in practice only if 10 photo- pulse which triggers the ramp switch. Slower deflection speeds electrons/ps were emitted from a 100 pm photocathode spot are accomplished through the use of RLCintegrating networks, on average. A significant modification of the cathode surface the exact parametervalues determining the sweep rate.Because potentialthen occurs, which givesrise toaberration effects there is an inherent switch delay in the start of the ramp with [38] (e.g., a diverging microlens) in the electron lens system respect to the incomingtrigger signal, it is necessary to optically and subsequent defocusing (“blooming”) of the image at the delay the arrival of the excitation pulse with respect to the phosphor screen. This spatial image quality deterioration and trigger (as shown in Fig. l), typical delay times being tens of subsequent loss of time resolution has long been observed in nanoseconds, even at the fastest deflection speeds. In addition, nanosecondand microsecond experiments. When suchhigh the required delay will also vary with the streaksetting because photocathode current densities occur over shorter time inter- additionaltime willbe required to overcomethe off-screen vals, the spatialblooming is normallynot observed, but an bias voltage. Most researchers find it convenient to maintain intensity-dependent electron transit-time spread occurs, which bothan excessive fixedoptical delay and a variable trigger is due to space-charge effects occurring near the photocathode electrical delay. As the sweep rate is modified, the electrical [39], [40]. In most applications of picosecond streak camera trigger delay can be adjusted accordingly by adding or subtract- fluorometers, it is this mechanism that is primarily responsible ing lengths of cable. In addition to optical delay, the switch for loss of dynamic range. In effect, the Coulombic repulsion introduces jitter in the start of the streak. Trigger jitter can due to the local space charge in the electron pulse will cause range from a picosecond to a nanosecond, depending upon the electrons in the lead to be accelerated, while those in the rear type of switch used. Typical jitter in commercial cameras is ofthe packet willbe decelerated.Other causes of dynamic on the order of 100-350 ps. Since the temporal window on range limitations include saturation of subsequent intensifier manycameras can be less than a nanosecond at the fastest stages and the recording medium (e.g.,use of film). streak rates, such jitter can cause the camera tomiss the event underobservation. Certain applications, for example, low- RECORDING repetition ratesingle-shot experiments where thesample is There are twomain recording schemes commonly in use destroyed, require use of a low jitter switch. We discuss the today: photography and photoelectric recording. Photography subject of switches in somewhat more detail later, in the sec- was first employed to obtain streak records, and current com- tion on signal averaging. mercial streak cameras provide a film-back option to allow a In principle, streak cameras are capable of single photoelec- picture to be taken of the intensifier phosphor. Polaroid film tron detection, and several commercial cameras experimentally is typically used for timing, determining appropriate intensity approach this ideal to within an order of magnitude. It is also levels, and survey purposes, while amuch higher resolution important to employ high-speed light collection opticsto both emulsion may be used for recording data. Unfortunately, there collect fluorescence and image the slit onto the photocathode. are significant disadvantages associated with film recording that Typically, commercial systems supply f/2 to f/3 optics, while appear to outweight the advantages. First, processing the film someresearchers have onoccasion incorporated f/l optics. is oftentime consuming, and does not provided an instant The use of various photocathode material has allowed streak readout. Second, each piece of film is somewhat different and camera spectral sensitivity to span the broad range extending requires individual calibration, usually necessitating exposure fromthe X-ray [36], [37]and UV (gold photocathodes)to of a portion of the film to light which has passed through a thenear IR (5’1 photoresponse).Commercial systems are step-density wedge. Third, film readout is indirect, requiring available with the more commonly used photocathode spectral microdensitometry of the negatives. Fourth, the available dy- responses (Sl,S5, S20, GaAs, etc.). namic range is limited (about 4-30). The main advantages of 5 88 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983

RHODAMlNi SJETOH 530nm SAMPLE

FLUORESCENCE

OPTICAL STREAK IMAGE

I non LINEAR SWEEPRATE W i i VIGNETTING /x 6 [PSI /Ill, 0 50 100 150 200 250 300 3x) Pig. 2. By utilizing the streak camela input slit dimension to represent I lpsec) spatialextent, the complex time dependence of a beam's spatial profile can be obtained (fromDneprovskii et al. [41]). Fig. 3. A streak camera/data processing system is used to collect time- resolved fluorescencefrom a 5 X rhodamine-B solutionin etha- nol. The raw data shown in the upper right picture contains a 30 percentstreak time base nonlinearity as well as otherdistortions. photography are its low initial cost and clear two-dimensional After computer correction, a corrected scan is displayed (lower right). visualization of the streak. This latter point is important in In this figure, individual OMA channel spacing is 0.8 ps (from Barbara some applications as, for example, use of the streak camera et al. [46]). witha dispersive grating to displaywavelength changes in fluorescence along the second dimension. Indeed, despite the dynamic range limitations of film, the two-dimensional storage I TRYPTOPHAN In WATFR capability allows a greater numberof bits of information to be 3E-4M PH 7 5 Oeyries C recorded than available using a vidicon/multichannel analyzer, 310-430nm. (Bottom-Top Tracks) Sum of 28 Transients for example, Fig. 2, from Dneprovskii ef al. [41], illustrates m the ability of the streak camera to provide a two-dimensional A representation of space and time, as well as, of course, light intensity. Here, the dimension along the slit is used to moni- tor spatial beam profiles. As the light pulse traverses a sample of CdSoaSeo.6, its leadingedge is partially absorbedas coherent excitons are generated. This energy is eventually returned to the trailing edge by stimulatedreemission of theexcitons. Consequently, the spatial and temporalprofiles are modulated. Portions of the entering pulse (I) and transmitted pulse (11) are imaged onto the slit at different places.Successive scans parallel to the slit at various positions alongthe streak produce t the display shown. - I 1- i The replacement of photography by photoelectric recording i 0ffi 200 300 400 Channe 1 No. [42] -[45], for example, multichannel image detection, allows the fluorescence profile to be displayed instantly on an oscillo- Fig. 4. Time- and wavelength-resolved fluorescenceof tryptophan (3 X M) inwater at 5OC. Eachcurve is an average of 20 single scope and/or digitally stored for later computer analysis. This shots.This figure illustrates the ability of amultichannel image approach is linear, possesses adequate dynamic range (>loo), detector/computerto simultaneouslycollect and display several and affords a simplicity of operation. So great are these ad- tracks across a complex two-dimensional streak (from Robinson et ul. [47 1 ). vantages that this is currently the preferred mode ofrecording. Most streak camera fluorometers today are used with a vidicon/ OMA combinationor an equivalentvideo camera/computer For example, an effect known as target lag, arising from the analysis system. Figs. 3 and 4 are illustrative of the capability exponentialcharging characteristics of the vidicontarget ofcomputer-interfaced multichannel image detection.Com- diodes, causes the output to be extremely nonlinear unless a mercial systems allow processing of raw streak data and correc- sufficient number of targetscans (220) aregathered [44], tionof time base nonlinearities, aswell as other distortions Vidiconsare also often used in aone-dimensional mode, (Fig. 3 fromBarbara et al. [46]). In Fig. 4, Robinson et al. measuring and displaying the intensity of iight imaged onto [47] employed a spectrograph to disperse the various fluores- the detector head as a function of position (along the streak), cencewavelengths along the slitdirection. Successive time and integrating light in the second dimension (along the slit). profilesatdifferent wavelengths allow the researcher to Consequently, it is important that the slit of the streak camera determine the existence or absence of wavelength-dependent be aligned perpendicular to the streak direction and that the effects. light-sensitive channels of the OMA be set along the slit image Some care is necessary to ensure that linearity is maintained. or loss of spatial and temporal resolutionwill result. CAMPILLO AND SHAPIRO: PICOSECOND STREAKFLUOROMETRY CAMERA 589

I I

MLL

.w519 D 'SCOPE ~. 0 100 200 302 w 5w CHANNELM1.5 PS/CHI Fig. 6. Display of a train of calibrationpulses through two parallel mirrors. Spacing between pulses is approximately 100 ps.

exponential decays. A very simple yet accurate scheme [25] introduced by us in 1975 is now universally used. A test pulse is passed through an etalon consisting of two parallel flat mir- rors, each having a front surface reflectivity of 50-95 percent +J IJICT and an AR-coated rear surface. Emerging is a series of pulses, each separated by the roundtrip time in the etalon c/2L, and c each pulse reduced in intensity from the previous pulse by the ICT *Rd' product of the mirror reflectivities. The resulting train has a known exponentially decaying envelope function which can be Fig. 5. Diagram of experimentalpulse delay device (PDD) used to varied by insertion of density filters between the mirrors or by calibrate streak camera. See text for details (from Schelev et al. [34]). changing thespacing. Using thisscheme, bothintensity re- sponse and streak rate can be quickly calibrated. Fig. 6 shows CALIBRATIONTECHNIQUES a streak camera/OMA readout of a comb of calibration pulses The streak camera owes much of its present popularityto its generated by passing the doubled output of a Nd :YAG laser ability to provide an accurate representation of the time varia- (533 nm, -26 ps) through an etalon with a roundtrip time of tionof the luminescence under observation. Confidence in 100 ps. the performance of current cameras is due in large part to its proven linear behavior, supported through careful calibration SIGNAL AVERAGING data. Early cameras exhibited nonnegligible nonlinear response In their usual operationalformat, luminescence data are to both intensity variation and time. Most of the causes (non- obtainedin a single streak. We sawearlier that the level of linear deflection voltage, intensifier pincushion effects, dynamiclight viewed by a streak camera could not be increased aribi- curvature of the streaked slit image, etc.) for this have since trarily since saturation effectsoccur inthe photocathode, beenminimized by improved design. However, even today, intensifier,and recorder and subsequently distortthe data. nonlinearities resulting from improper operation (intense light When detecting fluorescence, however, a common problem is levels, nonlinear recording medium) or component failure can one of not having enough light, even after optimizing the input easily distortexperimental data.Consequently, it remains slit widthand collection optics. Usually, the solution is to prudentto calibrate instrumentson a regular basis.Early increase the intensity of the excitation laser. Unfortunately, attempts to calibrate streak cameras were often unnecessarily however,this often leads to saturation or nonlinear effects complicated.For example, step-density wedges placed over occurring in the sample when the photon flux exceeds a critical the streak camera slit were often used with a large-diameter value, usually in the range 10'3-1018 photons/cm2. Operating lightsource to calibrate the camera's response to intensity. at or belowthis criticalexcitation intensitylevel greatly restricts Streak rates were determined by viewing the reflections of a the ability of the streak system to adequately obtain data in a calibrating pulse off two or more glass slides. One such pulse single scan, and a tradeoff mustbe made between the signal-to- delay device (PDD) [34] using 12 glass slidesis shown in noise ratioand achievable timeresolution. This point has Fig. 5. Each reflection was focused by a lens at different points apparently not been appreciated by a number of researchers, along the length of the streak camera slit. The resulting comb so we include a discussion here. of pulses, each separated by 160 ps, could be used to verify Consider first the following simple instructive example and the magnitude and linearity of the streak rate. Arrangements derivation. An excitation impulse of magnitudeI, in photons/ such as this are somewhatcumbersome. Another approach cm2, is assumed incident on a thin absorbing sample of trans- consisted of viewing fluorescence from dyes exhibiting known mission I, which subsequently fluoresces with efficiency @ and 590 JOURNALIEEE OF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983 characteristic exponential lifetime TP A lens is used to image percent decay point is given by (1 : 1) the light emission with collection efficiency qOptonto a slit in contact with a photocathode. The slit has dimensions h (length) and w (width). Here it is also assumed that the reverse image of the slit onto the sample, also of dimensions h and w, isindeed smaller than the actual sample. The number of By using (6a), (6b) can be expressed instead in terms of the photons P, absorbed by the imaged sample region is simply desired time resolution. Mopt is often less than unity to en- Ihw (1 - T). The light P, emitted into all solid angles during a hance the instrumental time resolution. However, this usually short time interval At is given by P,@rfl (e-f17f)At where t' leads to a less efficient light collection system. Equation (6b) is the time elapsed after excitation. The number of photoelec- assumes thatthe recordingsystem spatial resolution corre- tronsemitted by the photocathode during At isgiven by sponds to the width of the slit image MoptMelee w. The quan- Peqoptqpcwhere qpc is the photocathode quantum efficiency. tity N is greater than 1 because of internal streak camera losses If we ask what is the corresponding minimum excitation im- and competition from noise sources. In the design shown in pulse intensity Imin required to generate one photoelectron at, Fig. 1, a fraction of photoelectrons are lost due to geometrical say, the 10 percent point in thefluorescence decay curve during aperturing of the extraction mesh, as well as coupling losses to atime interval corresponding to the desired timeresolution the intensifier stage, the latter being lost in either lens or fiber TR (i.e., At = TR), then the answer may be obtained by setting output coupling. In some designs, a multichannel plate inten- PeqOptqpcequal to 1, e-t7Tf= 0.1 and solving for I: sifier is incorporated into the streak tube, and there are asso- ciated aperturing losses in that case also. These losses cannot be compensated by the use of additional intensifier gain since the signal-to-noise of the system will usually be limited by the Poisson statistics of thesmall number of photoelectrons present in the streak tube at anygiven time. Examination of (5) leads to several important implications. First, detection of fluorescence in a single streakrequires a In practice, time resolutions of 10 ps typically require exci- tationintensity fluxes of1014 photons/cm2or greater for minimum excitation intensity which depends inversely upon signal-to-noise ratios of ten or better. A serious problem arises the desired time resolution. This follows logically since a suffi- in many systems as one attempts to obtain higher time resolu- cient number of photons must impinge on the streak camera tions via single-shot analysis: theintensity of theexcitation photocathode during the time-resolved interval to be detected. source becomes so great that nonlinear optical effects, excited As the time resolution is improved, a greater photon flux is state saturation, stimulatedemission, and other artifacts result. required,eventually exceeding a value which the molecular An excellent example of this was discussed in Campillo et al. systemcan tolerate. This molecular saturationultimately limitsthe time resolution achievable. In specifying camera 1431. These authorsstudied the emissionfrom chlorophyll sensitivity, manufacturers typically begin where (5) leaves off molecules within the light collection or antenna apparatus of by providing data on the minimum number of photoelectrons green plantchloroplasts. These moleculesform networks of N required to yield a detectable signal. For example, typically, several hundredchlorophylls that collect light andfunnel energy to a reaction center trap via a random walking Frenkel 10 photoelectrons (or approximately 100 photons impinging singlet exciton.The complex has arather large absorption on the photocathode) will yield an electrical signal (assuming cross section (-300 andintensities of photons/ photoelectric recording) which exceeds the system noise level. a'), cm2 result inmultiple excitations and subsequent singlet- However, from (5) we see that the slit parameters h and w and singlet exciton annihilation processes. This results in reduced the collection optics efficiency qopt are also critical. qOpt,h, fluorescence quantum efficiency and an anomolously shortened and w must be made as large as possible. In the case of a single observed lifetime. The resulting nonexponential decay profiles collection lens with 1-to-1 imaging, qOpt= F t/64 where F ' are shown as a function of excitation intensity in Fig. 7. While and t are theflnumber andtransmission ofthe lens. h is most samples can tolerate higher photon fluxes than can plant determined by the recording spatial resolution along the slit. chloroplasts,this drawback of single-shotanalysis places an w is usually constrainedby the desired timeresolution. In inherent limitation preventing useful application of improved most streak camera systems, light passing through the slit is time resolution in weakly fluorescing samples. actually relayed to the photocathode by a second lens system A logical solution to the above limitation is to accumulate a (seeFig. 1) matched to thecollection lens.The subsequent great number of streaks at lower excitation intensities, with yet magnification of the slit onto the photocathode is given by superior time resolution, and to sum these, the accumulated Mopt,and magnification of the resulting image after intensifi- datathereby beingeffectively signalaveraged. However, cation is given by Melee. If the slit width is increased to the achieving this requires that the electrical rampwhich drives point where the instrumental time resolution is approximately the deflection plates of the streak camera be accurately syn- equal to the technical time resolution, then rR will be given by chronized to the startof the fluorescence signal. Unfortunately, conventional techniques such as krytron or spark gap switches TR X Mopt Melec Rw (64 or avalanche transistor stacks commonly used for generating where R is the streak rate. In terms of slit width, an expression the sweepvoltage result in significantpulse-to-pulse jitter for the excitation intensity required for an SIN of 1 at the 10 (50-350 ps), as well as long-termdrift. This uncertainty in CAMPILLO AND SHAPIRO: PICOSECOND STREAK CAMERA FLU( >ROMETRY 591

65PS , A,B,C -1 I- I

+-- 500 PS -4 Fig. 8. Streak camerarecord of the rise of C2 Swanemission after 266 nm photolysis of acetylene at a few torr; average of 10 pulses. Also shown are a model curve with time constant 215ps and second- harmonic reference pulses at a pair separation of 500 ps (from Craig et ul. [53]).

jitter (2 ps) light-activated switch [51] to initiate the deflec- tionplate voltage, thereby allowing direct accumulation of nonrecurrent streaks. The first technique above attempts to compensate for the jitter, while the latter two attempt to eliminate it. The use of a fiducial pulse as a time marker, introduced by us [48J , [52] , has the advantage of using standard deflection circuits, and is currently used by many researchers. However, the technique requires either computer or humanprocessing, and the accumu- lation of more than a score of data shots is tedious. A recent modification [53] of the fiducial pulse technique is shown in Fig. 8, whichdepicts Cz swan-bandemission following W photolysis of gaseous acetylene at a pressure of several torr. Here a 532 nm fiducial pulse was split into two distinct marker TI ME pulses separated by 500 ps and directed via a second pathto an Fig. 7. OMA oscilloscope- display of 700 nm fluorescent streaks from adjacent part of the streak camera entrance slit. Dual traces, C.pyrenoidosa excited with a single 20 ps, 530 nm pulse. Excitation intensitiesare(a) lo", (b) 3 X 10'4,and(c)r3 X 10'' photon/cm2. fluorescence,and timing pulses wererecorded by aNuclear The variation in fluorescence lifetime with intensity is consistent with Datavidicon system interfaced to aNicolet minicomputer. singlet-singlet exciton processes occurring within the photosynthetic Becuase of the weak emission, single shot fluorescence curves antennaapparatus. (d) Train of calibration pulses (fromCampillo et al. [43]). were noise dominated. Each streak trace and its corresponding reference were stored individually and were then shifted to a commontiming mark and averaged. A rise timeof 21 5 ps the start of the streak effectively negates the beneficial effect could be deduced by averaging ten shots. of signal averaging since the temporal resolution of the accumu- Thesynchronously scanning streakcamera [49], [50], or lated scans would be significantly degraded. Several approaches "synchroscan" camera, has been developed for use with recur- to this problem have been tried with good success: 1) use of an rent lasers, that is, those producing a continuous train of ultra- intense pulse arriving at the streak camera priorto the onset of short pulses each separated by nanoseconds, as for example, fluorescence which serves as a temporal fiducial marker [48], CW passive [54] , [55] or synchronously pumped [56] -[58] and use of a computer to note the position of the prepulse, dye lasers. By modifyingthe deflection circuitry to enable establish the true zero time of each shot, and subsequently thecamera to streak repetitively to correspond to the time accumulatethe corrected streaks; 2) use ofa continuous between pulses in the train, it is possible to precisely superim- repetitively mode-locked laser and a synchronously scanning pose successiveimages ofthe streaked fluorescenceprofiles. streakcamera [49],[50] whosedeflection plate voltage is Thiseffectively allows the integrationof lo8 fluorescence phase locked to the recurring excitation source and results in records in a recording time of 1 s and eliminates the need for substantially less jitter (5 ps); and 3) use of a specialized low- image intensification. A steady image is formed, whichis 592 JOURNALIEEE OF QUANTUM ELECTRONICS,QE-19,VOL. NO. 4, APRIL 1983

- I LINE ...... UV EXCITATION PULSE

electrical sync I ------e-J Of avaliablej Fig. 9. Block diagram of the synchroscan deflection scheme(from Huston and Helbrough [50]). ;; ;; ...... clearly visible on the streak phosphor. This allows observation ...: .. of weakly emitting species and the avoidance of high-intensity .. .. effects in the sample. .. .. Fig. 9 shows a block diagram of the electrical layout of a .. .. synchronscan unit. An oscillator-power amplifier combination .. is coupled to the deflector plates of a streak tube to produce a .. sinusoidal deflection on the screen, the central linear portion _..... ofwhich is actuallyused. The resonant frequency of the Fig. 10. Time dependence of the Z line luminescence from CuCl at 8 K oscillator/amplifier circuits are tuned to match the repetition (solid line) and UV laser excitation (dashed line). Data obtained with frequency of the laser in the range 70-200 MHz and are com- synchroscan streak camera (from Fujimoto et al. [ 611 ). patiblewith the naturalfrequency rangeof thedeflection plates. The power amplifier was fed from a driver stage that quency-doubledrhodamine 6G laserpulses, Fujimoto et al. could be operated either as an oscillator or an amplifier, de- were able to demonstrate that the lifetimeof the bound exciton pending upon the characteristics of the excitation laser. For in CuCl at 8 K is 130 ps. Even with excitation pulses contain- example, when used with a CW synchronously pumped mode- ing only J, good signal-to-noiseratios were obtained. lockeddye laser, where a continuous stable high-frequency Operating at low-excitation intensities allowed these authors electrical signal was available from the argon laser mode-locked to avoid nonlineareffects such as the previously discussed RF source, this is directly amplified in the driver stage to a exciton-excitoninteractions that occur in many solid-state level sufficient to fully load the output stage. When a passively systems such as this. mode-locked system is used, a portion of the light pulse train A somewhatdifferent scheme [62], [63] hasbeen consid- is directed to a photodetector, which inturn produces the ered in whicha conventional camera is streakedat a low requiredelectrical signal. When a burstof optical pulses is subharmonic (1-40 kHz) of the train frequency of a mode- used, as forexample, from apulsed mode-locked Nd:YAG lockeddye laser. Thedeflection voltageof the camera is laser, the photodetector output is then used to pull into syn- derived from a step voltage generator built up with avalanche chronism the driver stage, which is now operated as an oscilla- transistors or a high-frequency electronic tube which charges tor and adjusted as closelyas possible to the correct frequency. the capacitance of the deflection plates and provides a sweep Resolutions of approximately 5 ps using synchronously rate of approximately 200 ps/cm. Basically, this is very similar pumped dye lasers at repetition rates of 140 MHz have been tothe method used in single-shot operation,but with the achieved using this technique [49], [50], [59]. In theory, this deflection system designedto accommodate kHz rates of input streaking method should yield subpicosecond jitter. Presently, pulses. Both synchroscan and this technique may be used with the jitter is believed to be due to causes other than in the cavity-dumped dye lasers. Cavity dumping reduces the excita- deflection system itself, for example, drift in the RF driving tion rate and allows investigation of fluorescence from samples electronics, jitter in the repetition rateof the mode-locked CW with long recovery times. lasers, and variation in the durations of the dye laser pulses. Recently, near jitter-free (-2 ps) streak camera observation Experimental dynamic range has also been improved from 30 of light signals generated by nonrecurrent laser pulses as out- for single-shot measurements to over 3 X IO3 in this mode of linedin 3) above has been achieved using alaser-activated operation [60] . The principle drawback to widespread use of switchemploying either cryogenically cooled Si [64], [65] the synchroscan approach lies in the availability of a number or ambient temperature GaAs doped with Cr [66] . The latter of straightforward sampling and nonlinear optical [57] tech- material is attractive because of its highdielectric strength and niques that may also be used with CW mode-locked lasers. By itshigh resistivity (10' cm),enabling dc biasing at room contrast, the streak camera, when used ina single sweep mode, temperaturewithout thermal runaway. Fig. 11, fromKnox appears superior to othertechniques. and Mourou [66], summarizes the manner in which the GaAs An example of the use of the synchroscan system is shown photoconductive switch is integrated into the sweep circuitry. in Fig. 10 from the work of Fujimoto et al. [61]. Using fre- These researchers employed a 3 mm gap biased at 0-5 kV dc CAMPILLO AND SHAPIRO: PICOSECOND STREAK CAMERA FLUOROMETRY 593

2w- 2w 3"

A- 2L 30 psec 150uJ OPTICAL 1.0611 DELAY I LINE

0-5kV DC SUPPLY

SAMPLE

&STAGE IMAGE INTENSIFIER CONVERTER TUBE Fig. 11. Schematic of optical experiment employing a low-jitter GaAs: Cr photoconductive switch and associated streakcamera deflection system (from Knox et aZ. [66]).

which,when illuminated by a portion of the 1.06 pm laser I 1 pulse, conductsand charges the deflection plates through a 0 250 500 current limiting resistor R,. As the deflection plates charge, the photoelectron beam is swept at nearly half the speed of TIME (psec) light in a linear manner from one side of the streak tube phos- Fig. 12. CdSeluminescence at 170' C. Upper trace is datataken phor, across, and eventually off screen at the other side. After during a single streak sweep. Lower trace is the average of 300 shots about a millisecond, the plate voltage returns to initialbias accumulated using the low-jitter deflection scheme shown in Fig. 11 level by discharging through the large bias resistor Rg. There (from Mourou etaZ. [67]). are several significant advantages associated with this scheme. First, and most importantly, is that the resultant short-term streak rates, the photoconductive switch is really required only jitter is a small fraction of the trigger laser pulse duration. For at the highest streak camera resolutions and can be profitably example, 30 ps pulses froma mode-locked Nd:YAG laser employed in alliance with commercially available streak units. result in jitter of about 2 ps. Consequently, signal averaging Finally, this scheme is not appropriate for rapid accumulation can be performed using this technique to further increase the of data generated by burstsof recurrent pulsesas, for example, timingprecision and signal-to-noise ratio. Fig. 12 illustrates from a mode-locked train generated with a pulsed Nd+3 :YAG the latter in a dramatic but typicalway. The upper trace shows laser. However, overall, the use of this switch greatly extends theluminescence from CdSe [67]obtained during a single the capability of existing streak cameras and providesan excit- streak. The signal-to-noise is too poor in this case to determine ing opportunity in future luminescence research. the decay time. However, after accumulating 300 streaks, an accurate estimate can be made of this parameter. Stavola et al. BIOLOGY [65] demonstrated that by averaging as few as 50 shots, a time Picosecondspectroscopy has played an important role in delay of 0.4 ps between 30 ps pulses is resolvable to within understanding the initialsteps in several photobiological k0.05 ps. They also showedthat the long-term timing drift processes [69], [70] . Interestingly, the early streakcamera present in other trigger/sweep circuits was negligible with this fluorescenceliterature (1974-1977) was almostentirely scheme.The second advantage is thatthe photoconductive devoted to studying the role of exciton migration [ 181 -[27] , switch can be triggered with fairly modest laser energies (400 [43], [71] -[79] in photosynthesis. The first steps in photo- pJ). This should be compared to the multi-mJ requirement of synthesis involve light absorption by specialized antenna pig- gas or mylar spark gap [68] switches. Third, the fact that the ments(chlorophylls, caroteins, etc.), followed by efficient photoelectron beam remains off screen for about a millisecond energy migration to a reaction center complex, whereupon the allows a long-lived (>,us) species to decay sufficiently before excitation is utilized to drive the photosynthetic process. The the retrace. Fourth, use of this scheme with signal averaging various pigment species have complementary overlapping ab- results in an enhanced dynamic range, There are some minor sorption bands that together span the visible spectrum. Once disadvantages associated with this technique. First, the switchesexcited, the accessory pigment molecules very quickly(-1 ps) are not commercially available and must be homemade. Sec- transfer their energyvia a nonradiative dipole-dipole interaction ond, the range of available streakrates is greatly restricted. to a Chl a molecule. The Chl a's first excited singlet state has Although the ramp speed can be varied somewhat by changing a lower energy than the singlet states of the antenna pigments, the dc bias voltage, it cannot be slowed to a value lower than so the reverse jump is highly improbable. Possible, however, permitted by the intrinsic carrier recombination time of the are further similar Forstertransitions between neighboring switch material. However, since the jitter associated with con- Chl a molecules, and consequently, the excited state, or Fren- ventionalschemes is primarilya problem only at the fastest kel exciton, begins a random walk through the Chl a antenna 5 94 JOURNALIEEE OF QUANTUM ELECTRONICS, VOL.NO. QE-19, 4, APRIL 1983 matrixuntil it wandersnear thereaction center complex, I whereuponit is quicklycaptured. The entire process,from I G (t) initial pigment absorption to reaction center capture, although c involving manyhundreds of separate energytransfer steps, occurs in less than one nanosecond and has quantumyields for photon utilization approaching 100 percent. The actual physico-chemical situation is fairly complicated, andthe exact organization of the antenna system was pre- viously unknown. Photosynthesis in green plants takes place on a thin (approximately 10 nm) membrane called the thyla- koid, which is shaped like a flattened hollow sac. The major I tI tI constituent of the thylakoid is chlorophyll, which exists in a p, variety of chlorophyll-protein complexes. The various protein F 685-695 F 735 complexes, each composed of a dozen or so chlorophyll mole- Fig. 13. On the basis of streakcamera measurements, this modified cules, accessory pigments, or other specialized photosynthetic tripartite fluorescence model of the antenna apparatus of green plants material(cytochromes, ferredoxin, etc.), are the building was proposed. The source term G(t)excites both the light-harvesting aggregate LH and the photosystem I1 (PS 11) chlorophylls.Fluores- blocksof thephotosynthetic apparatus. The dimensions of cence emission corresponding to LH, PS I1 and photosystem I (PS I) these building blocks are unfortunately too small to discern are at 685, 695, and 735 nm. The light-harvesting pigment system is their precise organization using electron microscopy. Thus, we presumed to consist of two fractions: in LH(1) the light harvesting pigments are closely associated with PS 1, and energy transfer occurs must rely on indirect information such as that obtained with with rate constant K;in LH(2) the pigments are not closely connected picosecondspectroscopic techniques. Repetitive structures, to PS I, and excitons decay mainly with the rate constant &I. The called quantasomes,measuring about 20 X 15 X 10 nm and decay rate of the735 nm fluorescence, which reflects theexciton arranged much likecobblestones, cover the thylakoid mem- density in PS I, is denoted by p~ (from CampiUo et al. [52]). branes.These quantasomes are oftenidentified with the photosynthetic unit (PSU), a hypothetical entity which con- the excitons and leadsto shorter decay times, as inFig. 7. The tains all the functional material necessary to perform photo- manner in which excitonsinteract in vivo shouldstrongly synthesis independently. reflect the existence or nonexistence of boundary conditions It is believed that in green plants there are at least two photo- (puddle model), provide information on the coupling of the systems, PS I and PS 11, which are in close proximity, each two photosystems, and assist in obtaining values for various having its own distinct cooperative photochemistry. PS I, the rate coefficients. smaller of the two units, uponred light absorption produces a Figs. 13and 14, from Campillo et al. [79] illustratehow strongreductant and a weak oxidant, whereas PS 11, upon fluorescence data from spinach chloroplasts at 77 K support a absorption of “bluer” photons, produces a strong oxidant and modifiedtripartite model of antenna organization. Fig. 13 a weak reductant. Electron flow from the weak reductant to depicts this model, in which there arebelieved to be two types the weak oxidant is coupled to the conversion of adenosine of light harvesting (LH) pigments; in one of these, denoted by diphosphate andinorganic phosphate to adenosine triphosphate LH(1), the chlorophyll molecules are tightly coupled to the (ATP).ATP assists the strong reductant to reduce C02 to a PS I pigment system with K > PII, i.e., the exciton lifetime is carbohydrate (CHzO), and the strong oxidant oxidizes H20 to mainly determined by energy transfer to PS I, with K-l x 140 02.PS I and PS I1 are thought to be located on the outer and ps. The fluorescence yield from this system is low. The second inner surfaces of the thylakoid, respectively. type of LH pigment, LH(2), consists of chlorophyll molecules Ultrafast streak camera measurements of fluorescence [ 181 - which are less tightly coupled to PS I (possibly because of a [27], [43], [71] -[91] from various molecular species can be large physical separation), and which decay mainly by the rate readilyemployed to deduceinformation about theantenna constant PII x (800 ps)-’. The fluorescenceyield from this apparatus of plants that is not presently obtainable by other system is relatively high. The735 nm emission isthus un- means. The observed fluorescence would accurately probe the affected by exciton annihilation within LH( 1) up to intensities excitedstate density of the species underobservation while of I- IO1’ photons . crn-’/pulse when the annihilation rate wavelength-dependent datatrack the variouspathways. In begins to compete with the rapid LH(1) -+ PS I transfer rate particular, in green plants, the fluorescence decay from Chl a characterized by the rate constant K. This model is capable of is predominantly determined by the exciton migration time to accountingfor all observationsshown in Fig. 14, as wellas thereaction center, whicheffectively quenches theChl a those presented in [75] and [79]. excited state population. This rate can, in turn, be utilized to Because of the complexity of thegreen plant apparatus, many test various models under consideration. Further information researchers have employed streak camera techniques in algae may be obtained by studying the additional quenching mech- [73], [81], [82],1881, in photosynthetic bacteria [74], [76], anisms thatcome into play at higherexcitation intensities. andin isolated light harvesting proteins [89]-[91]. This For example, by flooding the antenna network with additional greatly facilitates interpretation of the data, and has greatly excitons,Campillo et al. [43],[71] haveshown that the increased the understanding of thesesystems. excitons have a high probability of interacting with each other Streak cameras have also been applied to other photochern- via a singlet fusion scheme that effectively annihilates one of ically active biological systems. These include studies of the CAMPILLO AND SHAPIRO: PICOSECOND STREAK CAMERA FLUOROMETRY 595

pyrelene. Since the vapors were at 3OO0C, the collisionrate between molecules could have been no more than 108/s, about IOoo rlo4 times fewer collisions thanin the liquid phase. Since fluorescence is commonly observed from the lowest vibrational state of SI,the rapid rise time in the vapor was interpreted as direct evidence supportingrapid internal relaxation of the molecules, even inthe absence of collisions.The ability of large dye molecules to relax internally independent of their surroundings is a manifestation of the fact that such molecules have a large number of vibrational and rotational modes, so that the probability of mutual interactions among the modesis high. These interactions reduce the lifetime of any particular level becauseany excess energyprovided by the excitation wavelength can be rapidly redistributedover the large ensemble of densely packedlevels. Barbara et al. [46] studied the time and wavelength charac- Spinach teristics of s-tetrazine fluorescence in n-hexane excited by a 77O K t i! 530 nm pulse at the I." = 1level. Thedecay of the excited state vibration has been monitored by comparing the rise time 0 200 400 600 800 of the spectrally unresolved fluorescence to that of the (0,O) TIME (psec) fluorescence at 560 nm. Vibrational relaxation was found to Fig. 14. Time-resolved fluorescence from spinachchloroplasts at a occur within 8 ps. tem erature of 77 K and an incident intensity of 2 X 1014 photons. Several researchers have studied the resonant intermolecular crn-'. The 735 nrn fluorescence is signal-averaged data obtained by accumulating 180 shots using a temporal fiducial pulse arriving prior transfer process between two dissimilar molecules. This type to the arrival of the fluorescence (from Campillo et al. [52]). oftransfer was treatedby Forster [99], and resultsfrom dipole-dipole coupling between the two molecules. The rate visual pigmentsRhodopsin and Isorhodopsin [92],and the of energy transfer is proportional to l/r6 where r is the spacing pseudophotosynthetic protein Bacteriorhodopsin [93] . Fluo- between the'two molecules, and hence is a sensitive function rescence from these molecules is difficult to observe because of concentration. A Forster radius R, is defined as the inter- ofthe low quantum yieldsand because fluorescence from action distance where the probability of energy transfer just impurities in the sample' can mask the intrinsic fluorescence. equals the probability of the donor molecule losing its excita- Fluorescence has also been observed and used as a probe in tionthrough other mechanisms. R, is calculablefrom the studiesof dye molecules intercalated within DNA [24]or overlap of the acceptor absorption and donoremission spectra. incorporated in aqueous micellar dispersions [94] , [95]. Forster's equationdescribing the decay of the excited state molecules with timeis given by CHEMICALPHYSICS Picosecond spectroscopy has found numerous applications in - 0.846 fiN chemical physics. Molecular changes often occur in the sub- nanosecond time regime, and excitation by picosecond lasers where r is the lifetime andN is the number of molecules with- makes it possible to determine the evolution of a molecular in a sphere of radius R,. At high concentrations, the second system through its various energy states and transient chemicalterm dominates. Shapiro et al. [24]found that the fluores- species. Researchers have employed picosecond streak camera cence lifetime of chlorophyllsin chloroform showed the correct fluorometry to study phenomena such as internal conversion dependence of lifetime with concentration from 0.0038 to 0.1 [18] , vibrational decay [46] , intermolecular energy transfer M. Porter and Tredwell [loo] examined the transfer between [24],[loo]-[102], solventeffects [103]-[104],[106]- rhodamine 6G and malachite green in a low viscosity solvent, [ 1 101 , rotational diffusion [ 1 11 ]- [ 1 121 , conformational ethanol.The donor fluorescence decay function was also changes [ 171 , [63] , [ 1 131 , [ 1 141, chargetransfer phenomena found to be inagreement with (7) over atenfold range of [ 1 151- [ 1171 , photoejection, electron transfer and solvation acceptor concentrations ( 10-3-10-2). Adams et al. [ 1011 ob- [ 1 181 - [ 1191 , protonation and deprotonation [ 1201 - [ 1291 , served singlet-singlet resonance energy transfer from DODCI excited state hydrogen-bonded complexes [ 1301 -[ 1331 , ex- (donor) to malachite green and to DQOCI in ethanolic solu- cimer formation [ 1341- [ 1361 , and photochemistry [53] , tions. They obtained R, values of 45.6 A for malachite green 11371. and61.8 A for DQOCI fromthe fluorescence decay curves. Shapiro et al. [ 181 measured the upper limit for vibrational- Departurefrom Forster kinetics was observed atthe lowest stateinternal conversion in dimethyl POPOP andperylene acceptor concentrations employed. Sat0 et al. [ 1021 employed moleculesin the gaseous phase. The 353 nm pulseexcited Forster transfer as a probe of membrane formation. By moni- vibrational states high in the SI manifold, while the fluores- toring the energy transfer between rhodamine 6G and 3,3 '- cence was observed at 580 nm. The rise time was found to be diethylthiacarbocyanineiodide (DTC) inwater, anormal less than 20 ps for dimethyl POPOP and less than 30 ps for decay versus concentration was found, as expected. However, 596 IEEE 596 JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983 when 5 X M sodiumlauryl sulfate, substancea that Heisel et al. [63] measured the fluorescence decays of pina- stimulates micellar growth, was added, a departure from Forster cyano1 in glycerol (r = 280 ps) and a Sol50 mixture ofglycerol theory was observed at a critical concentration. At that point, andethanol (I-= 143 ps). Huppert et al. [110]studied the the local concentration of DTC appeared to increase about 90- dual-fluorescencekinetics of p-dimethylaminobenzonitrile in fold, suggesting that dye-enriched micelle formation occurred. several alcohols. The b” state emission was foundto rise Fluorescence from a dye molecule is greatly affected by the promptly and funnel into the a* state. Ths rate followed the environment in which it is placed. In solution, the yield and solvent viscosity. lifetime can be readily affected by parameters suchas viscosity, Inmost fluorescencemeasurements, the excitingpulse is polarity, pH, temperature, hydrogen bond donor, or acceptor laser derived and is polarized. However, because of this, when- strength. As a practical matter, understanding these processes ever the lifetime of the emission occurs on the same time scale can be important in a number of applications, for example, in as molecular rotation, as is often the case in liquids, then it is probingthe structure of biologicalmolecules. Researchers necessary to correctfor polarization effects of theemitted seeking to improve laser performance are searching for waysto light.Any rotationof the molecule between the time it increasedye fluorescence yield, while others seeking better absorbs and emits light affects the angle between the polariza- mode-lockingdyes are searching for ways to decreaseyield tionvectors ofabsorption and emission. Consequently, andthe accompanying fast decay. Severalresearchers have the emission Ill(t) polarizedparallel to the excitingpulse studied the effect of viscosity on lifetime as well as the effect polarizationdecays somewhat faster than the population of of atomic substitution. Fleming et a/. [ 1031 studied the photo- theemitting state. Conversely, the emission Ii(t) polarized physics of fluorescein and three of its halogenated derivatives, perpendicular tothe excitingpulse also hasa complicated eosin, erythrosin, and rose bengal in aqueous and simple alco- profile. Observing unpolarizedemission (i.e., Ill + ZL) after holicsolvents. In thesesystems, the fluorescencelifetime is having excitedit with polarizedlight will notcompletely primarily determined by the intersystem (S1-TI) crossing rate. eliminate the interference effects occurring between the two As the .solvent changesfrom i-Propanol to H20, or there is polarizations.However, itcan be shownthat the correct increased substituenthalogenation (“heavy atom effect”) of fluorescencedecay is given by Ill(t)+ 21L(t). Thus, if an the parent, fluorescein, the fluorescence maxima shift to the analyzer is used andset at a magicangle 01 = 54.7”(from blue and the lifetimes are observed to decrease markedly. This tan2 Q: = 2), then sucheffects are negligible. A numberof is consistent with both the increased SIS, spectral blue shift researchers,however, rely on fluorescencedepolarization to and a smaller Sl-Tl energy gap, which enhances the intersys- yield rotational relaxation data. A parameter, called the time- tem crossing rate. The solventeffect was further studied by dependent polarization anisotropy, is experimentally measur- Yu et al. [lo41 , who observed the lifetime of erythrosin in a able andcan be related to varioustheoretical models. It is water-acetonesolution as a functionof concentration. The defined by the quantity fluorescence lifetime was observed to decrease linearly as the concentration of water increased. This group also studied the effect of solvent viscosity q on the decayrate of malachite green and found that the lifetime varied as q3I2 in the range Of major interest is whether or not orientational relaxation 1 < q < 60 poise (p). This is consistent with the model of of molecules can be described by the Stokes-Einstein-Debye Forster and Hoffmann [105], who postulate that the increase relationship rOR = hV(V(kT)-’ , which assumes hydrodynamic in fluorescence is due to theinhibition of the rotational motion or “stick” boundary conditions. Fleming et aZ. [l 1 11 utilized of the phenyl rings of the dye molecules ina viscous medium. polarization spectroscopy to study eosin and rose bengal in a As a consequence, the rate of internal conversion and fluores- variety of solvents. Both the fluorescence decay curve and the cence lifetime should show a strong viscosity dependence. rotational correlation function derived from the experimental Mialocq et a/. [ 1061 studied the photobleaching and absorp- data decayed as pure exponentials. tion recovery, as wellas fluorescence of pinacyanol.In a Many moleculesundergo conformational changes after ab- viscous solvent, such as glycerol, the recovery time of 330 * 30 sorbinglight, including some having importance in ultrafast ps was approximately equal to the fluorescence lifetime, 302 * spectroscopy. For example, the photoisomer of DODCI plays 29 ps. In light alcohols, such as methanol or isopropanol, the an important role in the modelocking of rhodamine dye lasers, recovery time was less than10 ps, indicating very efficient especially at the longer wavelengths. This fact was verified by internalconversion between the two singlet states involved. Arthurs et a/. [17] , who studied DODCI under conditions of Sibbett et a/. [lo71-[lo91 investigateda number of poly- mode locking.They determined thatafter excitation, an methine dyes of the cyanine family in a variety of solvents equilibrium is establishedbetween thetwo forms and that and over a fair range of viscosities (0.5-250 cp). Although the both display fluorescence lifetimes of 330 L 40 ps. Sumitani cyaninedyes examined did not possess a structuralform et aZ. [ 1131 and Heisel et al. [63], [ 1141 studied trans-stilbene. necessary to be included in the Forster theoretical model of The trans-cis geometrical isomerization of stelbene is a rather viscosity-dependentfluorescence lifetime, efficient loss of fundamental form of photochemical reaction. Sumitani et a/. excitation energy within the dye should occur through partial studied the temperature dependence of the fluorescence life- rotation of the aromaticrings about thelinking aliphatic chain. times of trans-stilbene and observed a smooth sigmoidal rela- Themeasured lifetimes were found to followa or 4 power tion,quite similar to thetemperature dependence of the dependence of the viscosity. quantum yield. The resultsindicate that the relativerate is CAMPILLO AND SHAPIRO: PICOSECOND STREAK CAMERA FLUOROMETRY 591

I I

0 IO0 200 300 400 500 67 p,cosec/CHANNEL 0 Fig. 15. Fluorescence of the excited charge transfer (CT) complex in 0.95 picosec / CHANNEL acetonitrile (2.5 X M). Thesmooth curve is a theoreticalfit to Fig. 16. The dual fluorescence of DMABN in proponal is shown. The the experimental data (fromCrawford et al. [ 1161 ). solid curve is the theoretical calculation and the points areexperimen- tal data. The prepulse at channel 140 is a time marker (from Wang etul. [117]). constant from 77 to 295 K. Heisel et al. [ 1141 observed two components in the decay of the emission. The slow compo- existent molecular solvent clusters that form relatively shallow nent was attributedto a return to the trans-geometryafter potential wells. Once the electron is localized, the surround- photoisomerization. The rate constant for photoisomerization ing solventdipoles reorient themselves aboutthe electron, to excitedtwisted stilbene shows an Arrhenius temperature deepening the potential well and leading to the formation of dependence, with an activation energy of 2.6 kcal/mole and a thesolvated electron. Monitoring thesteps involved inthe frequency factor of 1.2 X 10l2 s-'. The reverse process has a localization of the quasi-freeelectron in the solventyields rate constant times smaller. 500 information on the structure and relaxation properties of the Another class of excited-state interactions involves the trans- liquid, aswell as thenature of the electron-solventcluster. fer of an electron between a donor moleculeD and an acceptor Thetrapped and solvated electrons possess distinctabsorp- A, one of which is in an electronically excited state, to produce tion bands, and typically, this process is followed through ab- a new species (A-D')", called an excited-state charge transfer sorptionspectroscopy. However, Auerbach et al. [119] have complex or exciplex. The excited molecule may be the elec- monitoredthe fluorescence fromtoluidenyl napthalene sul- trondonor or acceptor. Beforeelectron transfer can occur, fonate (TNS) inwater/ethanol solventmixtures. Their data the donor and acceptor must diffuse to within an appropriate suggest that in the excited state,TNS in waterhas apreexisting separationdistance. Consequently, environmental factors localsolvent structurethat promotes the electron transfer play an important role.Certain geometrical orientations are tothe solvent.Evidence based onthe dynamicsof solvent required to form the exciplex in nonpolar solvents, whereas in exchange in mixed solvent systems indicates that this structure highly polar solvents, there is direct electron transfer, resulting consists of approximately 12 water molecules. in an ion pair. Eisenthal et aZ. [ 1151 -[ 1171 studied several There has been considerable interest in using streak camera cases of exciplexes resulting from intramolecular conversion. fluorometry to study excited state protonation and deprotona- In the case of the intramolecular exciplex anthracene-(CH2)3- tion kinetics. This has been stimulated by a novel technique, N, N-dimethylaniline in acetonitrile [ 1161 ,two processes occur. the laser "pH jump," which enables the researcher to rapidly First is a very rapid (7 k 1 ps) electron transfer for molecules vary the hydrogen ion concentration of a solution using laser inextended conformations, producing solvated ion pairs excitation. This, in turn, can be used to initiate ground-state without passing throughthe exciplex state.Second, folded acid or base-catalyzedreactions and to followsubsequent conformers yield exciplexes within 2 ps, having a lifetime of reactions.It is useful to review several acid-baseconcepts 580 f 30 ps. This component emits in the 500-600 nm region here. Atypical reversible acidreaction is described bythe and is shown in Fig. 15. The dual fluorescence of p-(dimethy- equation lamino) benzonitrile (DMABN) in propanol solution was also studied [ 1171 . Two emitting states were observed to reach [AH] + [H20] 2 [A-] + [H30+]. (9) equilibrium with a rate of approximately 20 ps. The forward Here, AH is an acid that gives up a proton to water to form a reaction rate (20 ps) is the time required for the molecule to hydronium ion H30+and an anionA-. pH in this case is simply relax conformationally to a charge-transfer geometry. Fig. 16 -log,, [H,O'] where the bracket indicates the concentration shows the DMABN fluorescence at 350 nm and the rise of the in moles/liter. Thus, a pH of 2 indicates that the concentration CT complex at480 nm. of hydroniumions is mil.Equation (9) canbe described Besides electrontransfer from one molecular species to by a characteristic acid equilibrium constant K, defined as another, photoejection can occur, resulting in an excess elec- :ron remaining in the liquid [118] -[119]. The electrons are hought to be solvated by initially being trapped in already- 598 JOURNALIEEE OF QUANTUMELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983

If the amount of one of the constituents is varied, the others a pH jump in a space enclosed by liposomes. This has poten- will adjust to keep K constant. pK is defined as the -logloK, tial use in biological studies such as ATP synthesis where the and is a useful chemical shorthand for indicating the strength reaction is driven bya protongradient. If thepH jump is of an acid or base. For the reaction given (9), a pK of 8 would limited to one side ofa proton-impermeable membrane, a indicate a weak acid, whereas a pK of 1 would indicate a strong proton motive force will result. The resulting ApH' jump will acid. Physically, when the pH of a solution is adjusted so that beidentical to the magnitudeof the pH jump. Since their it equals the pK of the acid, then [AH] = [A-] . ApH was of the order of 3-4, they estimated that theresulting There is a strong link between the acid-base properties of a ApH' was 180-240 mV. Shizuka et al. [ 1281 studied proton molecule and its electronic structure. Thus, a change in elec- transferreactions in the excited state of I-aminopyrenein tronic structure, say by excitation after light absorption, shouldH20 (or D20)-acetonitrile mixtures. The proton dissociation produce a concomitant change in acid-base properties. Indeed, rates kH+ and kD+were measured to be 1.8 (i0.4) X lo9 and in many compounds the acid dissociation constant (pK, value) 1.2 (-10.3) X lo9 s-', respectively.Eisenthal et al. [129] varies betweenthe ground and electronically excited states. reported an excited state double-proton transfer on excitation Somewhat surprisingly, the changes in pK value can be many of 7-azaindole dimer. In nonpolar hydrocarbon solvents such orders of magnitude. For example, 2-naphthol-6-sulfonate is as hexane, high concentrations of 7-azaindole form dimers. a weak acid in its ground state, but in its first excited singlet Upon excitation with 266 nm radiation, there occursa double- state it is a strong acid, showing a pK change of 7. Fluorene, proton transfer within the dimer, yielding a tautomer forming an aromatic hydrocarbon, shows a pK change of 29 upon exci- within 5 ps. tation. Some compounds, such as phenanthrene, become less Shapiro and Winn [ 1301 , [ 13 I] studied the temporal be- acidic in the excited state. This phenomena is characteristic of haviorof thespectral emission ofcoumarin 102in several bases as well as acids. liquids as a function of temperature. In addition to proton- Intense picosecond pulses allow considerable concentrations ation,they also observed theformation of anexcited state of electronicallyexcited molecules to beprepared. Thus, complex (CA)" between the excited state coumarin molecules appreciable changes in the concentration of H+ (or H30'), and C" and solvent molecules A through the kinetics of the dis- by definition pH, can be brought about on a very rapid time appearance of a blue-edge emission and the formation of a red- scale. The changein solution pHallows otherground state edge emission. They also observed interesting spectral dynam- acid-base catalyzed reactions to be initiated. Thus, a reaction ical behavior in acridine [ 1311 - [ 1331 . Hereagain, this which is not photochemical in nature can be initiated using an behavior depended sensitively on the temperature and solvent ultrafast laser. Since the majority of solution phase reactions and was interpreted in terms of the hydrogen-bonding charac- are either acid or base catalyzed, including most reactions of teristics of the solvents. biological importance, this pH jump technique offersa general, Excited and ground state aromatic hydrocarbons in solution widely applicable method for rapid initiation of solution phase often form excimers, which can be identified by their charac- chemistry. teristic fluorescence. In most cases, the fluorescence is emitted Campillo et al. [120] , [121] were the first to demonstrate a fromsandwich-typea excimer, in which the orbitals of pH jump using the compounds 2-napthol and 2-naphthol-6- aromaticchromophores overlap, as inthe case ofpyrene. sulfonatein water solution. By excitingthese species with Tagawa et al. [ 1341 , however, observed two distinct excimers 266 nm radiation, they wereable to induce the pH to jump of poly(N-vinylcarbazole). The first is the sandwich type, from 7 to 4. The protonated form of each species emitted in a while the second (emitting at 375 nm) is formed within 10 ps different spectral region than its corresponding anion. Conse- of a 10 ps electron pulse. The sandwich-typeexcimer (emitting quently, by measuring the time history of both anionic and at 420 nm) takes several nanoseconds to form. Migita et al. acidic species under a variety of experimental conditions and [ 1361studied the intramolecular heteroexcimer formation using a kinetic scheme which includes terms for the fluores- processes in the excited state of p-(CH3),N-C6H4-(CH2)3- censelifetimes and quenching, the completekinetics of (9-anthryl), as well as P-(CH~)~N-C6H4-(CH2)3 -( 1-pyrenyl) excitedstate protonation were obtained.For 2-naphthol-6- inhexane and 2-propanol.Kobayashi [135] observed the sulfonate, kr. = (1.02 ri: 0.2) x 1O9 s-' and kb = (9 ri: 3) lo1' formation process of anthracene excimers in chloroform. L. mol-' . s-l. Here, kf is the deprotonation rate and kb is Somewhat surprisingly, only one group to date has applied thebackward protonation rate. Smith ef al. [122] also ob- streak camera fluorometry to the study of gas phase photolysis served anultrafast pH jump in2-naphthol-3, 6-disulfonate products. Craig et al. [53] followed the formation of the C2 and 8-hydroxy 1,3,6 pyrene trisulfonate. The rates of proton Swan system emission following 266 nm photolysis of acety- ejection were foundto be 3.1 X 10'' and 3.2 X lo1' s-', lene, and observed a 215 ps rise time (see Fig. 8). This experi- respectively.Thistlethwaite and Woolfe [ 1231studied the ment clearly demonstrates that the streak camera is capable fluorescence decay of aqueous salicylamide andalso interpreted of following the temporal development ofa variety of emissive thecomplex fluorescencespectrum andkinetics in terms of fragments. excited state proton transfer. Campillo et al. [ 1241 measured therate constant for protonation of electronicallyexcited SOLIDSTATE coumarin 102 inwater to be (1.4 rf: 0.4) X IO1' L . mol-' . s-'. The physics of solidsis rich in luminous phenomena occurring In this case, the solution becomes much more basic upon elec- on apicosecond time scale [97]. Researchershave studied tronicexcitation. Gutman and Huppert [125] demonstrated excitonmigration [48],[I381 andsubsequent interactions CAMPILLO AND SHAPIRO: PICOSECOND STREAK CAMERA FLUOROMETRY

[138] at low [61] , [140] and highexcitationintensities[ 1391, electron spin relaxation [ 1421 , [ 1431, excimer effects [ 1441 , and energy transfer in both bulk media [ 191 , and from molec- ular monolayers to a solid [ 1451 . Campillo et al. [ 1381 studied exciton-exciton interactions in tetracene at 170 K using high excitation intensities. From the variationof the decayrate of singlet exciton emission as a function of laser intensity, theywere able to measure a singlet- singlet annihilationrate X,, of cm3 . s-l, andan exciton diffusionrate D, of 4 X cm2 . s-'. Ina later study,they dopedtetracene with pentacene and studied the kinetics of electron energy transfer from host to guest. The guest fluores- cence rise time was observed to be the same as the decay time of the host, while the simple exponential character of the host fluorescence was consistent with a diffusion model for singlet exciton migration from host to guest. Kobayashi et al. [ 1391 '''kc.L' lime '7''''" studied the decay kinetics of free excitons, bound excitons, Fig. 17. Luminescent decay kinetics for p-type GaAs (Zn doped, NA = and excitonic molecules in CdS at 4.2 K. By varying the level 7 x 1017/cm3). 2(a) and 2(b) are for left-(utx) and 2(c) and 2(d) are of excitation, they were able to determine that the P band (at for right-circular-polarized excitation (ofX). 2(a) and2(c) are the 490.5 nm) is due to bimolecular emission from free excitons, right-circular-polarized luminescence (u!'~), and2(b) and 2(d) are that bound excitons are generated from free excitons through leftcircular-polarized luminescence (up"). The solid curve is the amonomolecular process, andthat excitonic moleculesare experimental data output from the streak camera. The broken curve is calculated from rate equations (from Seymour andAlfano [42] ). formed through bimolecular processes. Fujimoto et al. [61] , [ 1401 studied luminescent processes inCuCl using a low-power 20 ps. Nakashima et al. [ 1451 studied energy transfer processes synchronously pumped dye laser excitation source and a syn- betweenmonolayers of rhodamine B on surfaces of single chroscanstreak camera. This enabled these researchers to crystals of anthracene, phenanthrene and naphthalene. Fluo- perform time-resolved investigations of bound excitonlumines- rescencedecays were biexponential,indicating two types of cense in CuCl at 8 K in the absence of competing biexciton transfer process, one due to electron transfer from the crystal effects. The luminescence was filtered to allow observation of to excited rhodamine B, and the other due totwo-dimensional the I line at 390 nm, which is due to the radiative decay of an Forster-typeenergy transfer. Campillo et al. [19]monitored exciton bound to a neutral acceptor. Theluminescence profile, the fluorescence from components of commercial scintillator shown in Fig. 10, indicates that the bound exciton is formed materials. within 10 ps and decays within 130 ps. Pellegrino and Alfano [141] studied the 720-850 nm photo- OTHERAPPLICATIONS luminescence kinetics of GaAs at 100 K. Using 530 mn exci- Walden and Winefordner [146] have critically reviewed the tation, theluminescence kinetics reflect the intervalley transfer suggestion of employing streak camera fluorometry in quanti- of electrons from r to and from the upper valleys along the I, tativeanalysis of tracespecies in liquids, thecharacteristic and X direction. The emission profile is complex, displaying a molecularlifetimes yielding informationthat supplements rise of 5 ps and a decay time of approximately 25 ps. Seymour spectralidentification. However, their attempts proved un- and Alfano [142] studied the spin relaxation and recombina- satisfactory in many of themixtures they tried becauseof tion kineticsin GaAs using polarizationspectroscopy. A chemical, solvent, and optical interferences. partialspin alignment of theconduction-band electrons is In ths paper, we reviewed streak camera usage in laser fluo- created when the crystalis illuminated with circularly polarized rescence spectroscopy. However, the streak camera has proven light. As these spin-aligned electrons radiatively decay to the invaluable and is widely used in providing direct visualization valence-band and shallow-acceptor states, the resulting lumi- of mode-locked laser pulses [l], synchrotron radiation [50] , nescence is partially circularly polarized, reflecting the degree Cerenkovemission from linear electron accelerators [ 1471 , ofspin alignment of the recombiningelectrons. The experi- nonlinear optical effects [ 1481 , [ 1491, mode propagation in mentalresults are displayedin Fig. 17. Thespin relaxation optical fibers [150] , implosion of laser fusion microspheres, timeand initial spin alignment are separately determined in and explosive detonations [3]. Coverage of these subjects is, thesemeasurements. Gobel et al. [143] also observedlumi- unfortunately, beyond the scope of this paper. However, we nescence from GaAs and observed Mott transitions from the would like to draw the reader's attention to anexciting related electron-hole plasma to the excitonic state. This transition is application of streakcameras, ultrafast transient absorption smooth and does not show a phase separation. studies [ 1511 -[ 1551 . This is typically implemented in the Kobayashi [ 144J studied the formation of excimer excitons followingway. Fluorescence of anappropriate wavelength in pyrene and perylene crystals. Kobayashi found that exci- and having a lifetime of several nanoseconds is generated in a mers formed in both crystals in less than 20 ps, and concluded dye solution and serves as a quasi-CW probe light source in the that the sum of the time constants for conformational change picosecond time region. The probe light is passed through the of an energy-donating pairand excimer-energy transfer is about sampleand is observedwith a streak camera. Any transient 600 JOURNALIEEE OF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983

absorptions induced in the sample by a second intense actinic risetime of NE 102 scintillator,” Nucl. Instrum. Methods, vol. 120, pp. 533-534, 1974. lightpulse will beevident in the level of transmitted probe A. J. Campillo, V. H. Kollman, and S. L. Shapiro, “Ultrafast light. Use of multichannel image recorder of the streak camera streak camera,” Science (Letter to the Editor),vol. 189, p. 410, ,outputand subsequent computer interfacing allows adirect 1975. A. J. Campillo, R. C. Hyer, V. H. Kollman, S. L. Shapiro, and H. ‘scope display of the optical density of the sample versus time. D. Sutphin, “Fluorescence lifetimes of alphaand betacarotenes,” Biochem. Biophys. Acta, vol. 376, pp. 219-221, 1975. ACKNOWLEDGMENT V. Z. Pashchenko, A. B. Rubin, and L. B. Rubin, “Pulse flurom- eter measurement of the duration of the fluorescence of chloro- The authors of this paper were fortunate to have been at the phyll excited by a mode-locked laser,” Sou. J. Quantum Elec- LOS Alamos Scientific Laboratory during the earlyyears of tron., vol. 5, p. 730, 1975. streak camera development, where they had access to a variety V. H. Kollman, A. J. Campillo, and S. L. Shapiro, “Photosyn- of experimental and commercial streak cameras. The availabil- thetic studics with a 10 psec resolution streak camera,” Biophys. Bioclzem. Res. Commun., vol. 63, pp. 917-920, 1975. ity of these and associated equipment enabled them to carry S. L. Shapiro, A. J. Campillo,and V. H. Kollman, “Energy out the first extensive useof streak cameras of fluorescence transfer in photosynthesis: pigment concentrationeffects and fluorescent lifetimes,” FEBS Lett., vol. 54, pp. 358-361, 1975. spectroscopy. The authors thank K. Boyer for his encourage- S. L. Shapiro, A. J. Campillo, V. H. Kollman, and W. B. Goad, ment and support. Special thanks are also due to A. J. Lieber “Exciton-transfer in DNA,” Opt. Comnzun., vol. 15, pp.308- and H. D. Sutphin, both formerly of the LASL steak camera 310, 1975. V. Z. Pashchenko, S. P. Protasov, A. B. Rubin, K. N. Timofeev, developmentprogram, and to theirformer associates, R. C. L. M. Zamazova, and L.B. Rubin, “Probing kinetics of photo- Hyer and K. R. Winn. system 1 and photosystem I1 fluorescence in pea chloroplasts on a picosecond pulse fluorometer,” Biochim. Biophys. Acta., vol. REFERENCES 408, p. 143, 1975. G. S. Beddard, G. Porter, C. J. Tredwell, and J. Barber, “Fluo- [ 11 D. J. Bradley, Ultrashort Light Pulses, S. L. Shapiro, Ed., Berlin, rescence lifetimes in the photosynthetic unit,”Nature (London), New York: Springer-Verlag, 1977, ch. 2, p. 18. vol. 258, p. 166, 1975. [2] C.B. Johnson, “Photoelectronic streak-tube technology review,” E. H. Eberhardt, “The use of an accelerator mesh to increase the Proc. SPIE, vol. 94, p. 13, 1976. resolution and response speed of dissectors and multipler photo- [3] A. E. Huston, “High-speed photographyand photonic record- tubes,” ITTIndustr. Lab., Fort Wayne, IN, Res. Mcmo 437, ing,”J. Phys. E(GB),vol. ll,p.601, 1978. 1966. [4] N. H. Schiller, Y. Tsuchiya, E. Inuzuka, Y. Suzuki, K. Kinoshita, D. J. Bradley and W. Sibbett, “Subpicosecond chronoscopy,” K. Kamiya, H. Iida, and R.R. Alfano, “An ultrafaststreak Appl. Phys. Lett., vol. 27, p. 382, 1975. camera system: Temporal disperser and analyzcr,” Opt. Spectra, Y. Tsuchiya, E. Inuzuka, M. Koishi, and M. Miwa, “Performancc p. 55, June 1980. andapplication of a 2 picosecondstreak camera system,” in [5] J. S. Courtney-Pratt, “A new methodfor the photographic Proc. Conf: Electro-Opt.lLaser Int. 82,Brighton, England, Mar. study of fast transientphenomena,” Research, vol. 2, p. 287, 1982. 1949. Certain commercial equipment,instruments, or materialsare [6] -, “A new photographicmethod for studying fast transient idcntified in this paper in order to adequately specify the ex- phenomena,”Proc. Roy. Soc., vol. A204, p. 27, 1950. perimentalprocedure or to provideexamples of current tech- [7] E. K. Zavoiskii and S. D. I+’anchenko, “Studiesof very fast nology. In no case does such identification imply recommenda- light processes,” Dokl. Akad.Nauk SSSR, vol. 100, no. 4, p. tion or endorsement by either the National Bureau of Standards 661, 1955. or the U.S. Naval Research Laboratory, nor does it imply that [ 8) -, “Physical fundamentals of electron-optical chronography,” the materials orequipment identified are necessarily the best Sov. PhJx Dokl., vol. 1, p. 285, 1956. available for the purpose. [9] Some authors employ the term “electron-optical chronoscopy.” D. J. Bradley, K. W. Jones,and W. Sibbett, “Picosecond and [lo] J. A. Armstrong,“Measurement of picosecond laser pulse femtosecond streakcameras: present andfuture designs,” widths,” AppZ. Phys. Lett., vol. 10, p. 16, 1967. Phil. Trans. Roy. Soc. London, vol. A298, pp. 281-285,1980. [ 11] J. A. Giordmainc, P. M. Rentzepis, S. L. Shapiro,and K. W. H. Niu, W. Sibbett, and M. R. Baggs, “Theoretical evaluation of Wecht, “Two-photon excitation of fluorescence by picosecond the temporal and spatial resolutions of photochron streak image light pulses,” AppZ. Phys. Lett., vol. 11, p. 216, 1967. tubes,”Kev. Sci. Instrum., vol. 53, p. 563, 1982.

, [ 121 A. A. Malyutin and M. Ya Schelev, “Investigation of thetem- M. Ya Schelev, M. C. Richardson, and A. J. Alcock, “Operation poral structure of neodymium-laser emission in the mode self- of a grid-shuttered image converter tubein the picosecond locking regime,” JETP Lett., vol. 9, p. 266, 1969. region,” Rev. Sci. Instrum., vol. 43, p. 1819, 1972. [13] D. J. Bradley, B. Liddy, and W. E. Sleat, “Direct measurement S. W. Thomas and L. W. Coleman, “Laser-triggered avalanche- of ultrashort light pulses with a picosecond streak camera,” Opt. transistor voltage generator fora picosecondstreak camera,” Commun., vol. 2, p. 39, 1971. Appl. Phys. Lett., vol. 20, p. 83, 1972. [14] M. Ya Schelev, M. C. Richardson,and A. J. Alcock, “Image 361 C. F. McConaghy and L. W. Coleman, “Picosecond X-ray streak converter streakcamera with picosecond resolution,” Appl. camera,” Appl. Phys. Lett., vol. 25, p. 268, 1974. Phys. Lett., vol. 19, p. 354, 1971. 371 D. J. Bradley, A. G. Roddie, W. Sibbett, M. H. Key, M. J. Lamb, [ 15 J S. D. Fanchenkoand B. A. Frolov,“Picosecond structure of C.L.S. Lewis, and P. Sachsenmair,“Picosecond X-ray chrono- the emission of a laser with a nonlinear absorber,” Sov. Phys. scopy,” Opt. Commun., vol. 15, p. 231, 1975. JETP Lett., vol. 16, p. 101, 1972. 381 R. Kalibijian, “High-intensity laser-beam effectson the spatial [16] M. M. Butslov, B.A. Demidov, S. D. Fanchenko, V. A. Frolov, resolution of a streak camera tube,” .I. Appl. Phys., vol. 46, pp. and R. V. Chikin,“Observation of processes ofpicosecond 4875-86.1975. duration by electron-optical chronography,” Dokl. Akad. Nauk [39] D. Bradley, S. F. Bryant, J. R. Taylor, and W. Sibbett, “lnten- SSSR, vol. 209, p. 1060, 1973. sity dependent time-resolution and dynamic range of photochron [17] E. G. Arthurs, D. J. Bradley, and A. G. Roddie, “Picosecond streak-cameras,” Rev. Sci. Instrum., vol. 49, pp. 215-219, 1978. measurements of 3,3’-diethyloxadicarbocyanine iodideand [40] H. Niu and W. Sibbett, “Theoretical analysis of space-charge photoisomer fluorescence,” Chem. Phys. Lett., vol. 22, p. 230, effects in photocron streak cameras,” Rev. Sei. Instrum., vol. 1973. 52, pp. 1830-1836,1981. [18] S. L. Shapiro, R. C. Hyer,and A. J. Campillo,“Polyatomic [41] V. S. Dneprovskii, V. N. Chumash, and E. V. Zimenko, “Co- molecularrelaxation in the absence of collisions,” Phys. Rev. herent interaction of picosecond frequency-tunable light pulses Lett., vol. 33, pp. 516-518, 1974. with excitons in a semiconductor,” Appl. Phys., vol. 25, p. 157, [19] A. J. Campillo, S. L. Shapiro,and R. C. Hyer,“Fluorescence 1981. CAMPILLO ANDPICOSECOND SHAPIRO: STREAKFLUOROMETRY CAMERA 601

[42j L. A. Lompre, G. Mainfray, and J. Thebault, “Instant recording G. Mourou, W. Knox, and S. Williamson, “Advances in picosec- of the duration of a single mode-locked Nd: YAG laser pulse,” ond optoelectronics,”Proc. SPIE, vol. 322, p. 107, 1982. Appl. Phys. Lett., vol. 26, pp. 501-503, 1975. A. J. Lieber, H. D. Sutphin, and C.B. Webb, “Subpicosecond [43] A. J. Campillo, V. H. Kollman, and S. L. Shapiro, “Intensity proximity-focusedstreak camera for X-ray and visible light,” dependence of the fluorescence lifetime of in vivo chlorophyll Proc. SPZE,vol. 94, p. 7,1976. excited by a picosecond light pulse,” Science, vol. 193, pp. 227- A. J. Campillo and S. L. Shapiro, “Picosecond relaxation mea- 229, 1976. surements in biology,” in Ultrashort Light Pulses, S. L. Shapiro, [44] D. J. Bradley, S. F. Bryant, and W. Sibbett,“Intensity depen- Ed. Berlin, Germany: Springer, 1977, ch. 7. dent time resolution and dynamic range of photochron pico- R. R. Alfano, Ed., Biological Events Probed by Ultrafast Laser second steak cameras, I1 linear photoelectric recording,” Rev. Spectroscopy. New York:Academic, 1982. Sci. Instrum., vol. 5 1, pp. 824-83 1, 1980. A. J. Campillo, S. L. Shapiro, V. H. Kollman, K. R. Winn, and [45] Y. Takagi, M. Sumitani,and K. Yoshihara, “High-speed image R. C. Hyer, “Picosecond exciton annihilation in photosynthetic processor for picosecond time resolved spectroscopy,” Rev. systems,” Biophys. J., vol. 16, p. 93, 1976. Sci. Instrum., vol. 52, pp. 1003-1009, 1981. A. J. Campillo and S. L. Shapiro, “Use of picosecond lasers for [46] P. F. Barbara, E. E. Brus, and P. M. Rentzepis, “A picosecond studying photosynthesis,”Proc. SPIE, vol. 94, p. 89,1976. time-resolved fluorescence study ofs-tetrazine vibrational L. Harris, G. Porter, J. A. Synowiec, C. J. Tredwell, and J. relaxation in solution,” Chem. Phys. Lett., vol. 69, p. 447, 1980. Barber,“Fluorescence lifetimes in chlorellapyrenoidosa,” [47j G. W. Robinson, T. A. Caughey, R. A. Auerbach, and P. J. Biochim. Biophys. Acta, vol. 449, p. 329, 1976. Harman, “Coupling anultraviolet spectrograph to a SC/OMA A. J. Campillo, R. C. Hyer, T. G. Monger, W. W. Parson, and forthree dimensional (A, I, t) picosecondfluorescence mea- S. L. Shapiro, “Light collection and harvestingprocesses in surements,” inMultichannel Image Detectors, Y. Talmi, Ed. bacterial photosynthesis investigated on a Picosecondtime Amer. Chem. SOC.,ACS Symp. Ser. 102, 1979, ch. 9. Scale,”Proc. Natl. Acad. Sci. USA., vol. 74, p. 1997, 1977. [48] A. J. Campillo, S. L. Shapiro, and C. E. Swenberg, “Picosecond N. E. Geacintov, J. Breton, C. Swenberg, A. J. Campillo, R. C. measurements of exciton migration in tetracene crystals doped Hyer, and S. L. Shapiro, “Picosecond and microsecond pulse with pentacene,” Chem. Phys. Lett., vol. 52, p. 11, 1977. laser studies of exciton quenching and exciton distribution in [49] M. C. Adams, W. Sibbett, and D. J. Bradley, “Linear picosecond spinachchloroplasts at lowtemperatures,” Biochim. Biophys. electron-optical chronoscopy at a repetition rate of 140 MHz,” Acta, vol. 461, p. 306, 1977. Opt. Commun., vol. 26, p. 273,1978. 761 V. Z. Paschenko, A. A. Kononenko, S. P. Protasov, A. B. Rubin, [50] A. E. Huston and K. Helbrough, “The synchroscan picosecond L. B. Rubin, and N. Ya Uspenskaya, “Probing the fluorescence streak camera system,” Phil. Trans. Roy. Soc. Lond., vol. A298, emission kinetics of the photosynthetic apparatus of rhodopseu- p. 287, 1980. domonas sphaeroides strain 760-1,on apicosecond fluorometer,” [51] G. Mourou and W. Knox, “A picosecond jitter streak camera,” Biochim. Biophys. Acta, vol. 461, p. 403, 1977. Appl. Phys. Lett., vol. 36, p. 623, 1980. 771 G. Porter, J. A. Synowiec, and C. J. Tredwell, “Intensity effects [52] A. J. Campillo, S. L. Shapiro, N. E. Geacentov, and C. E. Swen- on the fluorescence of in vivo chlorophyll,” Biochim. Biophys. berg, “Singlepulse picosecond determination of 735 nm fluores- Acta, vol. 459, p. 329,1977. cence risetime in spinach chloroplasts,” FEBS Lett., vol. 83, p. 781 G. F. Searle, W. J. Barber, L. Harris, G. Porter, and C. J. Tred- 316,1977. well, “Picosecond laser study of fluorescence lifetimesin spinach [53] B.B. Craig, W. L. Faust, L. S. Goldberg, and R. G. Weiss, “UV chloroplast photosystem I andphotosystem I1 preparations,” short-pulse fragmentation ofisotopically labled acetylene: Biochim. Biophys. Acta, vol. 459, p. 390, 1977, Studies of emission with subnanosecond resolution,” J. Chem. 791 A. J. Campillo, S. L. Shapiro, N. E. Geacentov, and C. E. Swen- Phys., vol. 76, p. 5014,1982. berg, “Single-pulse picosecond determination of 735 nm fluores- [54] E. P. Ippen, C. V. Shank, and D. Dienes, “Passive mode locking cence risetime in spinach chloroplasts,” FEBS Lett., vol. 83, p. of the CW dye laser,” Appl. Phys. Lett., vol. 21, p. 348, 1972. 316, 1977. [551 F. O’Neill, “Picosecond pulses from a passively mode-locked 801 A. J. Campilloand S. L. Shapiro, “Picosecondfluorescence CW dye laser,’’ Opt. Commun., vol. 6, p. 360, 1972. studies of exciton migration and annihilation in photosynthetic 1 C. K. Chan and S. 0. Sari, “Tunable dye laser for production of systems.A review,” Photochem.Photobiol., vol. 28, p. 975, picosecond pulses,” Appl. Phys. Lett., vol. 25, p. 403, 1974. 1978. ‘I H. Mahr and M. D. Hirsch, “An optical up-conversion light gate G. Porter, C. J. Tredwell, G.F.W. Searle, and J. Barber, “Pico- withpicosecond resolution,” Opt. Commun., vol. 13, p. 96, second time-resolved energy transfer in porphyridium cruetum, 1975. part I. In the intact alga,” Biochim. Biophys. Acta, vol. 501, p. J. M. Harris, R. M. Chrisman, and R. E. Ltyle, “Pulse generation 232,1978. in a CW dye laser by mode-locked synchronous pumping,” Appl. G.F.W. Searle, J. Barber, G. Porter, and C. J. Tredwell, “Pico- Phys. Lett., vol. 26, p. 16, 1975. second time-resolved energy transfer in porphyridium cruetum, M. C. Adams, “Repetitive picosecond chronoscopy with mode- part 11. Inthe isolatedlight harvesting complex (phycobili- locked CW dye lasers,” Ph.D. dissertation, Imperial College, somes),” Biochim. Biophys. Acta, vol. 501, p. 246, 1978. London, England, 1979. J. Barber, G.F.W. Searle, and C. J. Tredwell, “Picosecond time [60] J. R. Taylo;, M.’C. Adams,and W. Sibbett, “Investigation of resolved study of MgClz-induced chlorophyll fluorescence yield viscosity dependent flurorescence lifetime using synchronously changes from chloroplasts,” Biochim. Biophys. Acta, vol. 501, operated picosecond streak camera,” Appl. Phys., vol. 21, p. 13, P. 174,1978. 1980. [84 j k. J. Campillo and S. L. Shapiro, “Light collection and exciton [61] J. G. Fujimoto, T. K. Yee, and M. M. Salour, “Picosecond spec- dynamics in photosynthetic membranes,” Picosecond Phenom- operatingstreak camera,” Appl. Phys. Lett., vol. 39, p. 12, ena, C. V. Shank, E. P. Ippen, and S. L. Shapiro, Eds. Berlin, 1981. Germany: Springer, 1978, pp. 140-148. [62j F. Heisel, J. A. Miehe, and B. Sipp,“Applications of synchro- [85] C. J. Tredwell, J. Synowiec, G.F.W. Searle, G. Porter,and J. nouslypumped and cavity-dumped dye-laser associatedwith Barber, “Picosecond time resolved fluorescence of chlorophyll streak camera operated at the repetitive mode to time-resolved in vivo,” Photochim. Photobiol., vol. 28, p. 1013,1978. spectroscopy,” II Nuovo Cimento, vol. 63B, p. 221, 1981. [86] G.F.W. Searle, C. J. Tredwell, J. Barber, and G. Porter, “Pico- [63] -, “Characteristicsand performances of a streak camera second time-resolved fluorescence study of chlorophyll organi- operating in repetitivemode,” Rev. Sci. Instrum., vol. 52, p. zation and excitation energy distribution in chloroplasts from 992,1981. wild-type barley and a mutant lacking chlorophyll b,” Biochim. [64 j G. Mourou and W. Knox, “High-power switching with picosecond Biophys. Acta., vol. 545, p. 496,1979. precision,” Appl. Phys. Lett., vol. 35, pp. 492-495, 1979. [87] S. S. Brody, G. Porter, C. J. Tredwell, and J. Barber, “Picosecond [65 j M. Stavola, G. Mourou, and W. Knox, “Picosecond time delay energytransfer in anacystisnidulans,” Photobiochem.Photo- fluorimetry using a jitter-free streak camera,” Opt. Commun., biophys., vol. 2, p. 11, 1981. VO~.34, pp. 404-408,1980. [88j S. S. Brody, C. Tredwell,and J. Barber,“Picosecond energy [66] W. Knox and G. Mourou, “A simple jitter-free picosecond streak transfer inporphyridium cruentum and anacystisnidulans,” camera,” Opt. Commun., vol. 37, pp. 203-206, 1981. Biophys. J., vol. 34, p. 439, 1981. 602 JOURNALIEEE OF QUANTUMELECTRONICS, VOL.NO. QE-19, 4, APRIL 1983

[89] D. Wong, F. Pellegrino, R.R. Alfano,and B. A. Zilinskas, [lll] G. R. Fleming, J. M. Morris, and G. W. Robinson, “Direct ob- “Fluorescence relaxation kinetics and quantum yield from the servation of rotational diffusion by picosecond spectroscopy,” isolated phycobiliproteins of the blue-green alga nostoc sp. Chem. Pkys., vol. 17,p. 91,1976. measured as a function of single picosecond pulse intensity, I,” [112] R. J. Seymour, P. Y. Lu, and R. R. Alfano, “Picosecond polar- Photochem. Photobiol., vol. 33, p. 691, 1981. ization kinetics of photoluminiscenceof semiconductorsand [90] F. Pellegrino, D. Wong, R.R. Alfano,and B.A. Zilinskas, dyes,” in Picosecond Phenomena II, R. M. Hochstrasser, W. “Fluorescence relaxation kinetics and quantum yield from the Kaiser, and C. V. Shank, Eds. Berlin, Germany: Springer- isolated phycobiliproteinsof the blue-green alga nostoc sp. Verlag, 1980,~.111. measured as a function of single picosecond pulse intensity, I,” [113] M. Sumitani, N. Nakashima, and K. Yoshihara, “Temperature Photochem. Photobiol., vol. 34, p. 691,1981. dependence of fluorescence lifetimes of trans-stilbine,” Chem. [91] T. M. Nordlundand W. H. Knox, “Lifetimeof fluorescence Phys. Lett.,vol. 51, p. 183, 1977. from light-harvesting chlorophylla/b proteins,” Biophys. J., [114] F. Heisel, J. A. Miehe, and B. Sipp, “Picosecond analysis of vol. 3,p. 193,1981. trans-stilbine fluorescence,” Chem. Pkys. Lett., vol. 61, p. 115, [92] A. G. Doukas, P. Y. Lu, and R. R. Alfano, “1:luorescence relaxa- 1979. tion kineticsfrom rhodopsin and isorhodopsin,” Biophys. J., [115] K. B. Eisenthal, M. K. Crawford, C. Dupuy, W. Hetherington, vol. 35, p. 547, 1981. G. Korenowski, M. J. McAuliffe, and Y. Wang, “Photodissocia- [93] S. L. Shapiro, A. J. Campillo, A. Lewis, G. J. Perreault, J. P. tion, short-lived intermediates and chargetransfer phenomena Spoonhower, R.K. Clayton, and W. Stoeckenius, “Picosecond in liquids,” in Picosecond Phenomena IZ, R. M. Hochstrasser, andsteady state, variable intensity and variable temperature W. Kaiser, and C.V. Shank, Eds. Berlin, Germany: Springer- emission spectroscopy of bacteriorhodopsin,” Biophys. J., vol. Verlag, 1980, p. 220. 23, p. 384,1978. [ 1161 M. K. Crawford, Y. Wang, and K.B. Eisenthal, “Effects of con- [94] M.A.J. Rogers, “Picosecond fluorescence studies of rose bengal formation and solvent polarity on intramolecular charge transfer: in aqueous micellar dispersions,” Chem. Phys. Lett., vol. 78, p. A picosecond laser study,” Chem. Phys. Lett., vol. 79, p. 529, 509,1981. 1981. [95] -, “Picosecondfluorescence studies of xanthenedyes in [117] Y. Wang, M. McAuliffe, F. Novak,and K. B. Eisenthal, “Pico- anionic micelles in waterand reverse micelles in heptane,” J. second dynamics of twistedinternal charge-transfer phenomena,” Phys. Chem., vol. 85, p. 3372,1981. J. Phys. Chem., vol. 85, p. 3736,1981. [96] K.B. Eisenthal, “Picosecond relaxation processes in chemistry,” [ 1181 G. A. Kenney-Wallace, L. A. Hunt, and K. L. Sala, “Picosecond in Utraskort Light Pulses, S. L. Shapiro, Ed. Berlin, Germany: electron relaxation and photochemicaldynamics,” inPicosecond Springer-Verlag, 1977, ch. 6. Phenomena II, R. M. Hochstrasser, W. Kaiser, and C. V. Shank, [97] D. von der Linde, “Picosecond interactions inliquids and solids,” Eds. Berlin, Germany: Springer-Verlag, 1980, p. 203. in Ultrashort Light Pulses, S. L. Shapiro, Ed. Berlin, Germany: [119] R. A. Auerbach, J. A. Synowiec, and G. W. Robinson, “Dynam- Springer-Verlag, 1977, ch. 5. ical evidence for preferential structure in electron photoejection [98] G. A. Kenney-Wallace, “Picosecond relaxation processes in from arylaminonapthalene sulfonates,” in Picosecond Phenom- liquids,” Phil.Trans. Roy. Soc. London, vol. A299, p. 309, ena II, R. M. Hochstrasser, W. Kaiser, and C. V. Shank, Eds. 1980. Berlin, Germany: Springer-Verlag, 1980, p. 215. [99] Th. Forster, “Die Viskositatsabhangigkeit der Fluoreszenzquan- [120] A. J. Campillo, J. H. Clark, S. L. Shapiro, and K. R. Winn, tenausbevten einiger Farbstoffsysteme,” Z. Naturforsh. A, vol. “Picosecond studies of excited state proton transfer reactions: 4a, p. 321, 1949. The laser pH jump,” in Picosecond Phenomena, C. V. Shank, 100 G. Porter and C. V. Tredweil, “Picosecond time resolved energy E. P. Ippen, and S. L. Shapiro, Eds. Berlin, Germany: Springer- transfer between Rhodamine 6G and malachite green,” Chem. Verlag, 1978, p. 319. Phys. Lett., vol. 56, p. 278, 1978. [121] J. H. Clark, S. L. Shapiro, A. J. Campillo, and K. R. Winn, 101 M. C. Adams, D. J. Bradley, W. Sibbett, and J. R. Taylor, “Real- “Picosecond studiesof excited-state protonation and depro- time picosecond measurements of electronic energy transfer tonation kinetics. The laser pH pump,” J. Amer. Chem. Soc., from DODCl to malachite green and DQOCI,” Chem. Pkys. vol. 101, p. 746,1979. Lett., vol. 66, p. 428, 1979. [ 122 ] K. K. Smith, K. J. Kaufmann, D. Huppert, and M. Gutman, 102 H. Sato, Y. Kusumoto, N. Nakashima, and K. Yoshihara, “Pico- “Picosecond protonejection: An ultrafast pH jump,” Chem. second study of energy transferbetween Rhodamine 6G and Phys. Lett., vol. 64, p. 522, 1979. 3,3‘-diethylthiacarbocyanineiodide in the premicellar region: [123] P. J. Thistlethwaiteand G. F. Woolfe, “Kinetic evidence for Forster mechanism with increased local concentration,” Ckem. excited stateproton transfer in salicylamide,” Chem. Phys. Phys. Lett., vol. 71,p. 326, 1980. Lett.,vol. 63, p. 401, 1979. [lo31 G. R. Fleming, A.W.E. Knight, J. M. Morris, R.J.S. Morrison, [124] A. J. Campillo, J. H. Clark, S. L. Shapiro, K. R. Winn and P. K. and G. W. Robinson, “Picosecondfluorescence studies of Woodbridge, “Excited-state protonation kinetics of coumarin xanthene dyes,” J. Amer. Chem. SOC.,vol. 99, p. 4306,1977. 102,” Chem. Pkys. Lett., vol. 67, p. 218, 1979. [lo41 W. Yu, F. Pellegrino, M. Grant,and R. R. Alfano,“Subnano- [125] M. Gutmanand D. Huppert, “Rapid pH and ApH’ jumpby second fluorescence quenching of dye molecules in solution,” short laser pulse,’’ J. Biochem. Biophys. Methods, vol. 1, p. 9, J. Chem. Pkys., vol. 67, p. 1766, 1977. 1979.__ [ 1051 Th. Forster and G. Hoffmann, “Experimentelle und Theoretische 126) S. L. Shapiro, K. R. Winn, and J. H. Clark, “Excited-state pro- Untersuchung des Zwischenmolekularen Ubergangs von Elektro- ton-transfer kinetics in 1-Naphthol, 1-Naphthol sulfonates, and nenanrcgungsenergie,” 2. Phys. Chem. NF, vol. 75, p. 63, 1971. organometallic complexes,” in Picosecond Phenomena II, R. M. [lo61 J. C. Mialocq, J. Jaraudias, and P. Goujon, “Picosecond spectro- Hochstrasser, W. Kaiser, and C.V. Shank, Eds. Berlin, Ger- scopy of pinacyanol (1,1’-diethyl-2,2’-monocarbocyaninechlo- many: Springer-Verlag, 1980, p. 227 ride),” Chem. Phys. Lett., vol. 47, p. 123, 1977. 1271 D. Huppert, E. Kolodny, and M. Gutman, “Picosecond proton [lo71 J. R. Taylor, M.C. Adams, and W. Sibbett, “lnvestigationof transfer,” in Picosecond Phenomena II, R. M. Hochstrasser, viscosity dependent fluorescence lifetime using a synchronously W. Kaiser, and C. V. Shank, Eds. Berlin, Germany: Springer- operated picosecond streak camera,’’ Appl.Phys., vol. 21, p. Verlag, 1980, p. 242. 13, 1980. 1281 H. Shizuka, K. Tsutsumi, H. Takeuchi, and I. Tanaka, “Proton [log] D. Welford, W. Sibbett, and J. R. Taylor, “Dual component transferreactions in theexcited state of 1-aminopyrene by fluorescence lifetime of some polymethine saturable absorbing picosecond/streakcamera andnanosecond spectroscopy,” dyes,” Opt. Commun., vol. 34, p. 175, 1980. Chem. Phys., vol. 59, p. 183, 1981. [lo91 W. Sibbett, J. R. Taylor, and D. Welford, “Substituent and en- 1291 K.B. Eisenthal, K. Gnadig, W. Hetherington, M. Crawford, and vironmentaleffects onthe picosecondlifetimes of thepoly- R. Micheels, “Picosecond laser studies of electron-hole interac- methine cyanine dyes,” IEEE J. Quantum Electron., vol. QE- tions and double proton transfer,” in Picosecond Phenomena, 17, p. 500, 1981. C.V. Shank, E.P. Ippen, and S. L. Shapiro, Eds. Berlin, Ger- [llO] D. Huppert, S. D. Rand, P. M. Rentzepis, P. F. Barbara, W. s. many: Springer-Verlag, 1978, p. 34. Struve, and Z. R. Grabowski,“Dynamics of thedual fluores- 1301 S. L. Shapiroand K. R. Winn, “Picosecond time-resolved spec- censes of p-dimethylaminobenzonitrile,” J. Photochem., vol. tral shifts in emission: dynamics of excited state interactions in 18, p. 89, 1982. coumarin 102,” Gkem. Phys. Lett., vol. 71, p. 440, 1980. CAMPILLO AND SHAPIRO: PICOSECOND STREAK CAMERA FLUOROMETRY 603

[131] -, “Picosecond studies of temperature and solvent effects on tion of an intense intramolecular charge-resonance transition,” the fluorescence from coumarin 102 andacridine,” inPicosecond J. Chem. Phys., vol. 71,p. 2892,1979. Phenomena ZZ, R. M. Hochstrasser, W. Kaiser, and C. V. Shank, [154] Y. Wang, M. K. Crawford, M. J. McAuliffe, and K. B. Eisenthal, Eds. Berlin, Germany: Springer-Verlag, 1980, p. 237. “Picosecond laser studies of electron solvation in alchohols,” [132] -, “Temperaturedependence of the fluorescencedecay of Chem. Phys. Lett., vol. 74, p. 160, 1980. acridine measured with picosecond techniques,” J. Chem. Phys., [155] Y. Liang, D. K. Negus, R. M. Hochstrasser, M. Gunner, and P. L. vol. 73, p. 1469,1980. Dulton, “Picosecond kineticabsorption studies of aniron [133] -, “Picosecondkinetics of, acridine insolution,” J. Chern. porphyrinand bacteriochlorophyll using a streak camera,’’ Phys.,vol.73, p. 5958,1980. Chem. Phys. Lett., vol. 84, p. 236,1981. [ 1341 S. Tagawa, M. Washio, and Y. Tabata, “Picosecond time-resolved fluorescence studies of poly (N-vinylcarbazole) using a pulse- radiolysis technique,” Chem. Phys. Lett., vol. 68, p. 276, 1979. [ 1351 T. Kobayashi,“Picosecond spectroscopic study onexcimer formation in solution and in crystalline phase,” in Picosecond Phenomena ZI, R. M. Hochstrasser, W. Kaiser, and C. V. Shank, Eds. Berlin, Germany: Springer-Verlag, 1980, p. 339. [136] M. Migita, T. Okada, N. Matoga, N. Nakashima, K. Yoshihara, Y. Sakata,and S. Misumi, “Picosecond time-resolved fluores- cencestudies of intramolecularheteroexcimers,” Chem. Phys. Lett., vol. 72, p. 229,1980. I1371 G.R. Fleming, J. M. Morris, R. J. Robbins, G. J. Woolfe, P. J. Thistlethwaite,and G. W. Robinson, “Nonexponential fluo- rescence decay of aqueous tryptophan and two related peptides by picosecond spectroscopy,” €’roc. Nut. Acad. Sei. U.S.A.,vol. 75, p. 4652, 1978. 11381 A. 1. Campillo, R. C. Hyer, S. L. Shapiro, and C. E. Swenberg, “Exciton interactions in crystalline tetracene studied by single picosecondpulse excitation,” Chem.Phys. Lett., vol. 48,p. 495,1977. [I391 T.Kobayashi, Y. Segawa, and S. Namba, “Picosecond decay kineticsof CdS luminescence studied by astreak camera,” Solid State Commun., vol. 31, p. 253, 1979. [ 1401 J. G. Fujimoto and M. M. Salour, “Ultrafast picosecond chronog- raphy,”Boc. SPZE, vol. 322, p. 137,1982. [I411 P. Pellegrino and R. R. Alfano, “Video imaging systems in picosecond laser spectroscopy,” in Multichannel Image Detec- tors, Y. Talmi, Ed. Amer. Chem. SOC.,1979, p. 231. [142] R. J. Seymour and R.R. Alfano, ‘‘Timeresolved measurement of the electron-spin relaxation kinetics in GaAs,” Appl. Phys. Lett., vol. 37,p. 231, 1980. [143] E. 0. Gobel, P.H. Liang, and D. vonder Linde, “Picosecond luminescence spectroscopy of highly excited GaAs,” Solid State Commun., vol. 37,p. 609,1981. [144] T. Kobayashi, “The observationof the excimer formation process in pyrene and perylene c,rystals using a picosecond ruby laser andstreak camera,’. J. Chem. Phys., vol. 69,p. 3570, 1978. [ 1451 N. Nakashima, K. Yoshihara, and F. Willig, “Time-resolved measurements of electron and energy transfer of rhodamine B monolayer on the surface of organic crystals,” J. Chem. Phys., vol. 73, p. 3553,1980. [146] G. L. Walden and J. D. Winefordner, “A streak camera system for time resolved fluorimetry,” Spectrosc. Lett., vol. 13, p. 793, 1980. [147] J. C. Sheppard, R. H. Pantell, R. A. Gearhart, R. H. Miller, L. F. Chase, K. M. Monahan, and B. A. Watson, “Picosecond resolu- tion of a Cerenkov cell-streak camera arrangement for monitor- ing charged particle bunches,” Rev. Sci. Instrum., vol. 51,p. 1634, 1980. [148] A. J. Campillo, R. A. Fisher, R. C. Hyer, and S. L. Shapiro, “Streak camerainvestigation of the self-focusing onset in glass,” Appl. Phys. Lett., vol. 25, p. 408, 1974. [149] R. S. Adrain, E. G. Arthurs,and W. Sibbett, “Tunablepico- second transient stimulated raman scattering in ethanol,” Opt. Commun., vol. 15, p. 290,1975. [l50] J. P. Wilson, W. Sibbett,and P. G. May, “Synchroscan streak camerameasurements of mode-propagation in optical fibers,” in Picosecond Phenomena ZIZ, K; B.-Eisenthal, R. M. Hochstras- ser, W. Kaiser, and A. Laubereau, Eds. Berlin, Germany: Springer-Verlag, 1982. 511 A. Muller, J. Schulz-Hennig, and H. Tashiro, “Ultrafast absorp- tionspectroscopy of polymethine laser dyes using astreak camera,” Z. Phys. Chem. NF, vol. 101, p. 316, 1976. 521 -, “Excited state absorption of 1,3,3,1’, 3,3’-hexamethylindo- tricarbocyanine iodide: A quantitative study by ultrafast aborp- tion spectroscopy,” Appl. Phys.,vol. 12, p. 333, 1977. 531 K. Yoshihara, A. Namiki, M. S. Sumitani, and N. Nakashina, “Picosecond flash photolysis of cis- and trans-stilbine observa-