Time-resolved spectroscopy in synchrotron radiation

Nuclear Instruments and Methods 177 (1980) 193-205 © North-Holland Publishing Company

TIME-RESOLVED SPECTROSCOPY IN SYNCHROTRON RADIATION Victor REHN * Michelson Laboratory, Naval Weapons Center, China Lake, CA 93555, USA

Synchrotron radiation (SR) from large-diameter storage rings has intrinsic time structure which facilitates time-resolved measurements from milliseconds to picoseconds and possibly below. The scientific importance of time-resolved measurements is steadily increasing as more and better techniques are discovered and applied to a wider variety of scientific problems. This paper presents a discussion of the importance of various parameters of the SR facility in providing for time-resolved spectroscopy experiments, including the role of beam-line optical design parameters. Special emphasis is placed on the requirements of extremely fast time-resolved experiments with which the effects of atomic vibrational or relaxational motion may be studied. Before discussing the state-of-the-art timing experiments, we review several types of time-resolved measurements which have now become routine: nanosecond-range fluorescence decay times, time-resolved emission and excitation spectroscopies, and various time-of-flight applications. These techniques all depend on a short SR pulse length and a long interpulse period, such as is provided by a large-diameter ring operating in a "single-bunch" mode. In most cases, the pulse shape and even the stability of the pulse shape is relatively unimportant as long as the pulse length is smaller than the risetime of the detection apparatus, typically 1 to 2 ns. For time resolution smaller than 1 ns, the requirements on the pulse shape become more stringent. Experiments requiring time resolution in the 10-100 ps range are conveniently done via observation of the phase shift of high harmonics of the storage-ring orbit frequency. Fast fluorescence decay time measurements and reflectance phase-change measurements have been done in this way. Because of the sensitivity of the harmonic amplitude to the pulse shape, these experiments require the SR pulse shape to be stable. With the same phase-shift technique, measurements attaining time resolutions as small as 0.1 ps could be feasible if the SR pulse shape were made sufficiently stable. Observation of the shape of individual pulses of SR from the Stanford Positron Electron Accelerator Ring has been made under a variety of conditions using an Imacon 600 streak camera. These data are not completely analyzed at this time, but preliminary results are presented and discussed. Evidence is given of various modes of electron bunch-shape oscillation. These are apparently sensitive to beam current, kinetic energy, and accelerator cavity voltage.

1. Introduction

their source requirements, we shall address the decisive questions, "What is the scientific importance of time-resolved experimentation?" and "Why is the use of SR important iin time-resolved experimentation?" These discussio,~s are given in section 2. In section 3, time-resolved experimentation is categorized into six classes and each class is discussed as it is represented in SR research. In section 4, parameters of the source relating to the time structure of SR available to the user, are discussed in an attempt to highlight those source parameters which are under the control of the facility designer only. Finally, in the concluding section, a speculative view is presented of the future for time-resolved spectroscopy in SR.

Since all storage ring.s utilize charged-particle bunching i n order to control particle energy, the synchrotron radiation (SR) emitted by storage-ring sources is alwa~ys pulsed. Uing the pulsed nature of the SR, an experimenter may observe time-dependent phenomena resulting from pulsed photo-excitation. An example is the decay of fluorescence intensity, and from this example it is evident that a large range of time scales may be involved: extranuclear fluorescence decay times range over at least 16 orders of magnitude. It is certainly evident that this range cannot be covered by a single excitation source. However, it is the present author's contention that large storage rings provide an ideal source for time-resolved experimentation in six to ten decades of time axis. Before considering time-resolved techniques and

2. The importance of time-resolved experimentation

in SR Natural phenomena in all fields of science contain marvelous kinetic and dynamic processes, some of which have practical implications. The growth rate of human hair has been shown to be controlled by

* Also at: Stanford Synchrotron Radiation Laboratory, Stanford, CA 94305, USA. 193

V. EXPERIMENTALTECHNIQUES

V. Rehn / Time-resolved spectroscopy

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Fig. 1. The time-resolved emission (TREM) spectrum o f /3-naphthol. Using variable delayed-coincidence techniques,

the emission spectrum is recorded 3, 5 and 9 ns after excitation. The evolution of the time-integrated spectrum after the decay of a fast-decaying channel is shown (ref. [ 1]). the time necessary for the molecules to arrange themselves. Thus, molecular motions occurring in picoseconds impact on the economics of the barber shop! Time-dependent natural phenomena have been studied over forty-one decades of the logarithmic time axis, from the 10 -24 s required for a photon to cross a nucleon to the 101° y required for some nuclear excited states to decay. SR sources, with their intrinsic pulsed behavior, have been applied to the study of time-dependent phenomena in the 10 -1210 -1 s range. Recent analyses have suggested that SR sources may have the potential for time resolution approaching 10 -Is s. It is toward this goal that facility planners can fruitfully apply themselves. The results of time-resolved research in SR over the last six years should serve as motivation. Since the first reports of the use of SR for fluorescence lifetime measurements by Lindqvist et al. [1] and by Heaps et al. [2] in 1973, timeresolved spectroscopy has been applied in many areas. At the French SR source, ACO, fluorescence lifetime and time-resolved excitation and emission measurements were applied to the study of hemoglobin, NO, CO, fluorescein, naphthol and other organic and inorganic materials *. Fig. 1 illustrates a beautiful example of the early use of time-resolved emission (TREM) spectroscopy to show that two fluorescence * A partial s u m m a r y of m e a s u r e m e n t s at ACO is given in ref.

[31.

emission peaks (part a) represent different fluorescence lifetimes and, hence, different emitting states (part b) [1]. The peak at 4200 h is attributed to an anion created by dissociation of the photoexcited molecule, while the 3500 A peak represents the competitive fluorescence decay of the photoexcited molecule. Fluorescence lifetime measurements were made on long-lived states of organic molecules trapped in solid noble-gas films at cryogenic temperatures. Using SR from the NINA synchrotron at Daresbury, Hasnain et al. [4] reported lifetimes in the millisecond range in these systems. In addition to those at ACO, studies of fluorescence lifetimes of gaseous materials were reported by researchers working at the Stanford Synchrotron Radiation Laboratory (SSRL). Matthias and co-workers [5] used magnetic fields up to 40 G to observe quantum beats in the fluorescence decay associated with the Larmor precession of the Kr 5s (3/2)1 state, as shown in fig. 2. In addition to the lifetimes, these data yield spins and g factors of unresolved magnetic substates. Also on noble gases, fluorescence emission spectra studied as a function of pressure and excitation energy have been reported by Zimmerer [6]. From these studies the influence of the initial state on the branching ratio for emission in the first and second continua has been learned. Measurements of the spectrum and life105

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V. Rehn / Time-resolved spectroscopy

time of the second fluorescence continuum of Xe have been reported by Dutuit et al. [7]. Noble-gas excimers have been studied by time-resolved techniques at SSRL and DESY [8], the large synchrotron in Hamburg, FRG. Bonifleld and co-workers [9] obtained formation rates, vibrational-relaxation, and radiative-decay rates, as well as collisional mixing rates between two excimer species. These results were obtained from measurements of transmission and fluorescence excitation spectra of Kr~ and Xe~ excimers in Kr, Xe or Ar. Thornton and co-workers [10] reported fluorescence decay of the 0u and lu states of Xe~. Their measurements illustrate that photoexcitation selects specific initial states for which the decay dynamics may be studied individually. An ionic state of Xe was studied by Rosenberg and coworkers [11], who found that the measured lifetime (34.4 ns) is much longer than that calculated by multiconfigurational Hartree-Fock methods (9.5 ns) [12] due to the importance of relativistic effects neglected in the calculation. Noble-gas solids have been studied by time-resolved spectroscopy in SR. Monahan and co-workers [13] reported the photoluminescence decay times associated with excitation of both free and self-trapped excitons in solid Xe. Noble-gas solids have been used for host lattices for matrix-isolation studies of molecules. Both energy-transfer mechanisms between host lattice and dopant molecule [14], and excimer formation and dissociation [14,15] have been studied by time-resolved techniques by Monahan and Rehn and by Taylor and co-workers [16]. In fig. 3 is shown a time-resolved excitation (TREX) spectrum of the fluorescence of XeF2 (1%) dopant in solid Kr [15].. The fluorescence emission is from the XeF photofragment and shows a new band system in the absorbing XeF2. These results substantially support the recent theoretical model for dissociation of linear triatomic molecules [26]. Fluorescence-lifetime anisotropy has been utilized in the study of proteins and their amino acids [17,18]. Proteins containing only a single tryptophan amino-acid constituent were excited by linearly polarized SR near the long wavelength edge of the tryptophan absorption band, where the lifetime anisotropy is maximum. The time dependence was measured for emission polarized parallel and perpendicular to the excitation polarization in order to deduce the rotational correlation time of the single tryptophan in the fluidhke protein. Fig. 4 shows the lifetime anisotropy versus time for nuclease B and basic myelin protein,

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showing that the tryptophan exhibits only the motion of the protein in the former case but an additional rotational motion in the latter case. These examples are only a sample of the many contributions to science made by fluorescence studies using the time structure of SR. Others have been discussed in recent review articles [3,18,19]. Another important application of the time structure of SR to scientific research is the use of time-

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Fig. 4. Fluorescence-emission polarization anisotropy versus time for two proteins. An additional rotationM freedom for the emitting tryptophan within the protein molecule is responsible for the much shorter correlation time (0.09 ns) in myelin, compared with the 1.3 ns correlation time for the rigid protein molecule in nuclease B. V. EXPERIMENTAL TECHNIQUES

196

V. Rehn / Time-resolved spectroscopy

of-flight (TOF) techniques, which have been used with electrons, ions and photons emitted after excitation by SR. At SSRL operation in the single-bunch mode of SPEAR allowed Bachrach et al. [20] to develop an electron TOF spectrometer with angleresolving capabilities which they used in the study of A1 and other solids. The advantages of TOF detection of photoemitted electrons (as well as ions) lie in the collection of the entire kinetic-energy spectrum at one time, decreasing the data collection time by as much as a factor of 10 3. A corollary advantage of TOF detection is that changes which might occur in the sample while the electron energy distribution is being scanned are omitted from the data. The quicker data collection becomes essential when the entire Brillouin zone is being scanned with a small angular aperture, as in angle-resolved photoemission [21]. These same advantages have been utilized in the study of atomic Ba vapor by angle-resolved photoemission by Rosenberg and co-workers [22,23]. Their results indicate that the dominant channel for interaction of the auto-ionizing level with the continuum is the Auger decay. Sensitive TOF ion mass spectroscopy was utilized at SSRL by Knotek and co-workers in their recent demonstrations of photon-induced ion desorption [24,25]. Fig. 5 illustrates the spectrum of positive ions desorbed from an H20-doped BeO crystal under exposure to white SR radiation (central image) on the 4 ° beam line at SSRL. In these photoninduced desorption experiments, both TOF and TREX techniques were utilized and provided a remarkably sensitive and versatile surface-analysis tool. Fig. 6 illustrates the use of TREX spectra to study H + desorption induced by photons of energy near and above the Be K-core excitation energy. All these experiments utilize fast commercial pulsecounting equipment, for which the subnanosecond SR pulse width is fast enough to be negligible compared with the electronic risetime limitations. Recently, efforts have been made to improve the time resolution to the picosecond range or beyond, where the shape of the SR pulse is both broad and important. Much of the fluorescence of proteins and other biological materials has a lifetime in the range 10 - 1 1 10 -s s [3,17,18]. Photochemical reactions and surface chemical reactions are also dynamic processes which take place on the picosecond time scale. If, for example, photoabsorption changes the ionic state of an atom in a molecule or on a surface, then the resulting Coulomb force of one electronic charge at a

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Compared with commercial quadrupole mass analyzers, the TO[: ion mass analysis yields at least 102 greater count rate in this application (ref. [25]).

typical interatomic distance will cause an ion to move out of bonding range within a few picoseconds. Thus, ionic photodissociation either occurs or is quenched within a few picoseconds after photoabsorption. Most vibrational periods in molecules and crystals lie in the range 0.1-1 ps. The vibrational periods of molecular excited states have been shown to control the dissociation time in molecular photodissociation [26]. In crystals the vibrational period is related to the time constant for lattice-relaxation phenomena, which may be coupled into electronic space charge and screening phenomena. In biology, photosynthesis, nerve-impulse transmission, and other basic processes of life are controlled by molecular-vibrational kinetics on the picosecond time scale. It must be self-evident that picosecond-scale experimentation will have a vast impact on science if techniques and facilities are developed for convenient and efficient measurement. We may still ask whether an SR source offers

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197

v. Rehn / Time-resolved spectroscopy

enough advantage over competitive sources in the picosecond range of spectroscopy to justify a considerable funding allocation. This question has been addressed by Lopez-Delgado [3]. For h v > 10 eV, there are no competitive sources. For hv < 10 eV, a few laser sources have demonstrated picosecond-pulse performance. Dye lasers with frequency doubling may be usable and tunable below 6 eV, where a considerable amount of the life-sciences interest lies. However, most laser systems supply too many photons in too narrow a band and for too short a time, driving the sample into nonlinear optical or saturation behavior. There is a rationale for the study of intensely excited systems, and for such purposes laser sources offer the only choice at present. However, there are important reasons to study weakly excited systems, for which picosecond-pulse lasers are not well suited, due to their high noise level (2 to 5 orders of magnitude worse than storage rings) and low pulse-repetition frequency. Hence, below 6 eV picosecond-pulse lasers and SR are complementary sources in providing a means to study of intensely or weakly excited systems, while above 6 eV the SR source is without peer when tunability, high polarization, or low contamination rate are desirable. The conclusion seems apparent: SR sources must be developed for picosecond-resolved spectroscopy over the full spectral range available.

3. Typical time-resolved measurements and their SR source requirements If we are to seriously consider the needs of timeresolved experimentation in the development of SR facilities, it is incumbent upon us to study the experimental techniques in the light of facility and machine potentials. An attempt will be made to present the principles of currently used experimental techniques and to relate them to the more obvious machine and facility parameters. This discussion is meant to be a descriptive overview of the subject rather than a treatise. It is the author's hope that those responsible for planning, designing, funding and managing, as well as using SR facilities will find it useful. Six classes of time-resolved experimentation in SR may be distinguished: fluorescence or photoluminescence decay-rate measurements, TREM spectroscopy, TREX spectroscopy, TOF measurements, streak-camera measurements, and harmonic phase-shift spectroscopy. Table 1 shows features and

requirements of these general categories. The first four represent "stimulus-response" type experiments and share many of the same requirements and limitations, as more consideration will show. 3.1. Stimulus-response techniques

The fluorescence decay rate may be measured following SR excitation in either of two regimes. If the SR pulse width, w <~ 1 ns, and the fluorescence decay time, r >~ 1 ns, then commercial photon-counting electronics may be used with a time-to-amplitude converter (TAC) and a multichannel pulse-height analyzer (MCA) to record the fluorescence intensity versus time. A typical block diagram is shown in fig. 7 [28]. In this regime, the ns-/as regime, the experimental set-up is straightforward. Limiting factors are the rise-time jitter of the detector, the fast-pulse amplifiers, and the threshold discriminators. The SR source stability is usually much better than these electronic-jitter factors. Time synchronization is usually quite easily provided with either a fast logic pulse or a pulse from an SR beam monitor. The performance obtained is illustrated in fig. 8, where the count rate is plotted versus time for a scattered light pulse [28]. For these experiments, the most significant requirement for SR facility planning is the requirement for a long interpulse period. This should be several times the longest lifetime to be measured if accurate values of r are to be obtained *. Hence, fluorescence-decay measurements (and other stimulus-response experiments) in the ns-/as regime are facilitated most easily with a large-diameter storage ring using a high enough harmonic number to provide subnanosecond pulse width. Single-bunch operation facilitates measurements of longer lifetimes or more complex time-dependent decays. Slower processes which occur in biological systems, for example, might be studied effectively if the SR facility could provide a fast beam bump which could steer the SR pulse out of the acceptance cone of the beam-line optics for N consecutive pulses on a repetitive basis. In the picosecond regime, fluorescence decay-rate measurements are made by different techniques. This is necessary because the electronic jitter is too great * For single-exponential decays, a lifetime may be extracted from the first 10% of the decay, signal-to-noise permitting [29]. Hence, in some cases, lifetimes as long as ten interpulse periods may be measured with SR. V. EXPERIMENTALTECHNIQUES

198

V. Rehn /Time-resolved spectroscopy

Table 1 Techniques for time-resolved exPerimentation and their idealized SR source parameters Technique

Time range (s) (and potential)

Fluorescence decay TREX TREM TOF High harmonic phase shift

10 10-10 10-6_10-9 10-6_10-9 10-6_10-9 10 -9-10 -12

Streak camera

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Harmonic number

Bunch length/shape

No. of bunches

Large Large Large Large n.J. (a)

High High High High High

Short/8 Short/6 Short/6 Short/8 Various/ various Various/ Gaussian

One or few One or few One or few One or few Many or all

n.i.

(All)

(a) n.i. denotes "not important".

and the TAC time scales are too long for the apparatus o f fig. 7. A promising alternative is the harmonic phase-shift measurement [30], versions of which have been successfully used with amplitudemodulated lasers to provide sub-picosecond time resolution [31,32]. Further discussion o f . t h e har-

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Fig. 7. Typical block diagram for fluorescence decay, TREX, or TREM measurements. SR passes through the excitation monochromator (EMC) to the sample. Fluorescence radiation passes through the analyzing monochromator (AMC) to the photomultiplier (PM) detector. Amplified signal pulses pass through a pulse-height discriminator (DIS) before initiating the time-to-amplitude conversion (TAC). This conversion ends when the properly timed clock pulse arrives at the TAC "stop" input. For fluorescence decay rate measurements, the pulse-height spectrum output from the TAC is analyzed and stored in the multichannel pulse-height analyzer (MCA), displayed on the oscilloscope (OSC), or transferred to the computer (CPU) for further processing. TREX or TREM spectra may be obtained from the single-channel analyzer (SCA) output of the TAC, which selects only pulses in a predetermined height range (after ref. [28]).

monic phase-shift technique is presented below, but first other "stimulus-response" techniques for the ns-/as regime will be reviewed. The block diagram of fig. 7 shows TREX and TREM outputs available after the TAC via use of a single-channel analyzer (SCA). Of course, TREX and TREM techniques differ only in terms of which monochromator is scanned; the electronic system is the same. If the exciting photon energy is scanned, holding the detected energy constant, the output is a TREX spectrum; conversely for a TREM spectrum. The upper and lower thresholds of the SCA are set to permit counting only for a range of pulse heights from the TAC corresponding to the time range desired. Somewhat better performance is obtained by using a fast coincidence logic unit. The coincidence pulse is delayed from the excitation pulse and may be of any desired width. The output of the Coincidence logic unit then goes directly to a counter or scaler. The coincidence logic avoids the count-rate limitation imposed by the conversion time ( 1 - 5 0 ~us) of the TAC and offers a considerable improvement in those cases where another feature of high count rate must be rejected in the study of a weaker feature. TOF measurements in the ns-/as regime utilize somewhat different electronics from that used in TREX, TREM, and fluorescence decay measurements. Fig. 9(a) illustrates the TOF detector used for ions by Knotek et al. [25]. In this case, ions of various masses are emitted simultaneously (within a few picoseconds) following excitation by a pulse of SR. They are accelerated by a uniform field, gdrift/d, occurring in the space of width d between a planar acceleration screen and the sample. Then they drift

I1. Rehn / Time-resolved spectroscopy I

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Fig. 9. (a). Apparatus for TOF ion-mass analysis in photon-stimulated ion desorption. A uniform accelerating field, Vdrift/d, is applied in the space o f width d ~ 2 cm between the sample and the accelerating screen attached to the end of the drift tube. Ions with kinetic energy, e ~ 3 keV drift along the 5 cm drift tube to the microchannel-plate detector (after ref. [25]). (b) Typical ionmass resolution. Desorption of H*, O*, and OH* ions from a H20-doped surface of SrTiO3 illustrates the mass resolution (~-0.3 ainu) and the width of the mass peaks. The H+ peak represents about 106 counts in 4 min with hv = 30 eV excitation. The OH* peak represents about 2800 counts in 10 min with hv = 35 eV excitation on the 8° beam line at SSRL (after ref. [25]). V. EXPERIMENTAL TECHNIQUES

V. Rehn / Time-resolvedspectroscopy

200

through a field-free tube to a dual channel-electron multiplier array (CEMA). From the CEMA anode, a pulse of electrons is collected for each ion impinging on the CEMA. Under saturation conditions the output pulses are narrow (~<200 ps fwhm) and of constant height, independent of ion mass. For this detector geometry, the arrival time of an ion of mass M (ainu) and charge q (electron charges) is

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3.2. Harmonic phase-shift techniques + ( e + V d n ~ ) lt~

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where D is the length of the field-free drift space. Both d and D are in centimeters, Vdrift in volts, and e, the initial kinetic energy of the ion, is in electron volts. The approximation in eq. (2) is valid when e'~'~ Vdrift. The ion-mass resolution available is approximately independent of ion mass since both the separation of mass peaks and their width depends in the same way on mass. Fig. 9(b) illustrates the mass resolution obtained. For H ÷ ions the electronic jitter (1.5 ns) contributes appreciably to the peak width, but for all heavier masses the initial kineticenergy distribution dominates the pulse shape, pulse width, and mass resolution. The mass spectrum shown in fig. 5 was taken under these conditions and illustrates that, with SPEAR operating in the singlebunch mode, mass resolution of about ~ amu was obtained for massesM < 40 amu. Heavier masses may be studied by increasing Vdrift. The limitation occurs when the electronic jitter (or the SR pulse width, whichever is larger) dominates the width of the mass peak. For TOF measurements of photoelectron energy distributions, the experimental conditions are somewhat different. In order to maximize the collection efficiency, a potential Vdrirt is used, but a subsequent retarding potential may be used to allow greater dispersion of the electrons [second term of eq. (1)]. In the ideal case in which the first term is cancelled by the retarding field, the arrival time of electrons becomes t(ns) ~ 16.8 D/e v2 + constant ,

(3)

where the constant accounts for the time consumed in accelerating and retarding the photoelectrons. Eq.

At present there are no SR sources with pulse widths short enough to utilize such a short rise time in stimulus-response experimentation. Picosecond time resolution may be achieved, however, with much wider SR pulses using the high-harmonic phase-shift technique [27]. This technique deconvolves the response function from the excitation function by analysis of the change in pulse shape. It depends on nondispersive band-pass filters and high-frequency phase-sensitive detectors to select one harmonic of the SR source frequency. Using an rf mixer, other harmonics may be heterodyned to the filter frequency, one by one, allowing a Fourier-series study of the time dependence. A simple illustration of this technique is shown in fig. 10. The transmission delay experienced by a pulse of SR in passing through a plate of transparent material is illustrated. If the sample thickness is l and its refractive index is n, the SR pulse will be delayed by

a r = (I/c) (n - ] ) ,

(4)

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1I. Rehn / Time-resolved spectroscopy

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Fig. 12. Harmonic selection by the SAW filter. Using a Tektronix 7613/7L13 power spectrum analyzer, the power output of the SAW is displayed for harmonics 32 through 38 of the SPEAR orbit frequency. Note a 35 db rejection of other harmonics (after ref. [33]).

201

shift read directly when the sample is inserted. The surface acoustic wave (SAW) falters used in this apparatus were specially designed to select the 35th harmonic of the orbit frequency of SPEAR (44.817 MHz), and their performance is shown in fig. 12. Here the output power is plotted versus frequency as observed with SPEAR sync-pulse excitation on a power spectrum analyzer. Note that the rejection of adjacent harmonics is about 35 db. The band-pass response curve of the SAW filter is square so that the phase distortion near the center frequency is negligible. With this apparatus we have measured the transmission delay to a precision of-+2 ps; still better performance should be achievable if the SR pulse shape is adequately stable. The relationship between the phase shift, ~qs, and the delay time is given simply as Aq5 ~ co £xT,

(5)

where co is the harmonic frequency of the measurement. This relationship is correct only in the limit of zero dispersion of the sample, but it illustrates the value of selecting the highest feasible harmonic for measurement. In the case of fused SiO2, 10 mm thick, ~ T ~ 18 ps at X = 200 nm. If ~ = 27r (44.817 MHz), the expected phase shift 2xq5 = 0.29 °. The best precision offered by the PAR lock-in is 0.003 ° , allowing a measurement of Aq5 to 1% at best. It is clear that the use of a much higher harmonic would be beneficial if the SR pulse shape is adequately stable. Application of this technique to the measurement of fluorescence decay in the p s - n s time range has not yet been reported, but should present no difficulties in the case of a single pure-exponential decay. More complex time dependences require that phase-shift measurements be made at several appropriately spaced harmonic frequencies, which can be accomplished with heterodyning. The effect of fluorescence decay within the excitation pulse is to change the pulse shape with little delay of the pulse centroid. An analysis of the dependence of the harmonic phase on decay time will appear in a future publication [33]. Another promising application of SR harmonic phase-shift spectroscopy is the measurement of the spectral dependence of the optical-frequency phase shift upon reflection. Normally, this quantity is measured by ellipsometric techniques, which are unavailable at wavelengths shorter than about 120 nm due to the absence of suitable polarizers and quarter-wave plates. Considering the SR source as an amplitudeV. EXPERIMENTAL TECHNIQUES

202

V. Rehn / Time-resolved spectroscopy ~5 ROTATING

/

where

Hz SYNC MOTOR 2

N

E(t) = C Eio([ ) cos(~'~t + Oi) i=1

26 dB SPEAR C

~

is the resultant electric field of the superposition of the electric fields of N photons in the SR pulse, each with optical frequency ~2 and phase Oi. The monochromatic frequency ~2 is the same for all photons in the pulse, but the phases, {Oi}, comprize an arbitrary, fixed set for each SR pulse such that eq. (7) gives the correct envelope time dependence. The effect of a mirror reflection is

35 dg

PAR 5202 ~

~

G

50 MHz PHASE DET

39 dB 12 dB

REF ._r-LA2_ Fig. 13. High-harmonic phase-shift apparatus for reflectance phase-change spectroscopy. The rotating sample (fig. 14) alternates Ag and Au reflecting surfaces at a 5 Hz rate. The phase difference between the two surfaces is detected by the 5 Hz lock-in detector as the wavelength is scanned.

modulated source, a form of differential modulated ellipsometry may be envisioned, as stiown schematically in fig. 13 [33]. Here the basic phase-shift circuitry of fig. 11 is embellished with the addition of a 5 Hz rotating sample and associated lock-in detector. The sample, illustrated in fig. 14, is a mirror coated on one half with Ag and on the other half with Au. The signal received, then, represents the Ag surface for 100 ms and the Au surface for the next 100 ms. The information about the optical-frequency phase change upon reflection is carried into the rf phase as follows. We assume monochromatic, incoherent SR reflected by the sample mirror onto the photomultiplier-tube (PMT) detector. The PMT detects the intensity envelope, I(t)

=

eoE(t) "E ( t ) ,

(7)

(6)

ER(t ) = r e iq~ E l ( t ) ,

(8)

where the reflection coefficient r and the phase change q~ are functions of the optical constants (or dielectric function) of the mirror surface, and hence are spectral functions as well. If El(t) is given by eq. (7), then it is clear that each of the photon phases is shifted by q~ so that the envelope intensity pulse I(t) is not changed in shape (nondispersive reflection). At the PMT the harmonics are mixed so that output at harmonics of the SR orbit frequency contains the optical phase shift, ~5, diminished, however, by a function of the coherence length. The coherencelength function is a constant in our experiment, allowing us to obtain an output at 5 Hz proportional to the difference of optical phase changes between the Ag and Au reflections. Hence, by scanning the monochromator we obtain the spectral dependence of the difference of reflectance phase changes, Aq~ =-qSAg -- ~Au, between Ag and Au. Although we have observed Aq5 versus hu in the spectral range of the Ag surface-plasmon excitation (where large 2xq5variations occur), operating conditions at SPEAR prohibited us from obtaining reliable measurements. Nonetheless, the great promise of this experiment is the potential for measurements throughout the vacuum ultraviolet and soft X-ray spectral range and for thin-film or adsorbate-layer sensitivity. 3.3. Streak-camera measurements

200

nm Ag QUARTZ SUBSTRATE

200

nm

Au

Fig. 14. Ag-Au reflectance sample.

Streak cameras are devices for rapidly moving the image of an illuminated spot across a screen, allowing the image to be temporally dispersed. Several commercial brands are available with time-resolution specifications as small as 2 ps. The limited spectral range, high cost, and nonrepetitive characteristics of currently available units have minimized their applications in SR experimentation, although pico-

203

v. Rehn / Time-resolved spectroscopy

second-pulse laser studies have benefitted from use of streak cameras. Future developments may greatly increase their usefulness in SR research, but applications to date have been limited to beam diagnostic measurements. Using a Hadland Photomics Imacon 600 streak camera, instabilities in the pulse shape at SSRL have been studied by Monahan et al. [34]. These results show pulse profiles of individual SR pulses with a time resolution of about 5 ps. During a "machine physics" shift, SPEAR was operated with a variety of beam currents, electron energies, cavity voltages, and with either 458 or 860 MHz cavity frequencies. Their results show several features of the electron-bunch dynamics, both low and high frequency. At low frequencies (predicted to be in the 104 Hz range), both symmetric, "breathing-mode" oscillations and antisymmetric oscillations of the bunch have been observed, but only in random snapshots of the pulse shape. No systematic set of pulse-shape variations with time during the oscillation cycle could be obtained in these initial studies. More surprizing data on electron-bunch dynamics were shown in the high-frequency region. Most pulses showed amplitude modulation corresponding to frequencies in the 10-50 GHz range. These unpredicted pulse-shape instabilities, under further study at present [35], may be responsible for the erratic phase jumps and drifts observed in the highharmonic phase-shift measurements. The preliminary data show two resonant frequencies (~15 GHz and ~50 GHz) contributing to this modulation. These pulse-shape modulations average out in samplingscope measurements, as shown by Wilson et al. [36]. In the high-harmonic phase-shift measurements [33], it was noted that while SPEAR operated in highenergy, high-current, colliding beam mode, occasional large phase jumps occurred on a background of a fairly rapid, continuous, monotonic phase drift. Further experimentation is required to determine the relationships among these various forms of dynamic instability of the SR pulse shape in SPEAR and other SR sources. The time dependence of these pulseshape modulations must be obtained from future measurements before the limitations of SPEAR for the high-harmonic phase-shift technique are known. Streak-camera measurements, particularly if they can be approximately synchronized with one or another oscillation, may provide important information on the future of picosecond-resolved spectroscopy in SR.

The future of stimulus-response type measurements in SR has been brightened considerably by the streak-camera studies of Monahan et al. [34]. By reducing both the electron current and the electron energy in SPEAR, and increasing the cavity voltage to 2.5 MV, these authors were able to produce SR pulses of about 50 ps fwhm. This result lends credibility to the plans reported by the BESSY group in Berlin for providing 50 ps pulses for time-resolved spectroscopy. This result also suggests that both fast-fluorescence decay measurements and high-harmonic phase-shift measurements may be facilitated by idling all possible electron bunches in a storage ring with few electrons (-~2 × 109) in each. Such a small number of electrons per bunch accelerated to low energy and bunched by a high cavity voltage (possibly by SPEAR's 860 MHz cavity) may provide a stable set of 50 ps pulses spaced by 2.79 ns (or 0.91 ns in the 860 MHz case). This pulse train would provide an extremely high excitation rate for either experiment, with a low enough duty factor ( ~ or ~ , respectively) for many very fast phenomena to be studied. Note that a similar suggestion has been made recently and independently by Csonka [37] in his studies of time and coherence effects in storage rings.

4. Future potentialities

The experimental techniques and apparatus described above allow experimental measurement by conventional stimulus-response methods as short as the pulse length of existing storage rings will allow. Sampling electronics" is currently available with aperture time as small as 25 ps. Optically delayed probe-pulse techniques utilized in picosecond-pulsed laser experimentation can be applied in the time range 0.01-100 ps. Interferometer techniques can cover a similar time range. Phase-shift measurements of fluorescence-decay times as short as a few picoseconds should be reported soon. It seems possible to drive a streak camera synchronously with the SR source, which would allow its application in pulseshape averaging and data collection. Such a device, coupled with a computerized readout system, might have a time resolution of a few picoseconds. Hence, the limitations of time-resolved experimentation in SR are rapidly shifting from the experimental equipment and techniques to the storage ring. When super-high-speed electronics becomes a reality, conventional stimulus-response type meaV. EXPERIMENTALTECHNIQUES

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v. Rehn / Time.resolved spectroscopy

surements on the scale of 10 ps would be feasible in SR if source pulses could be shortened appropriately. We note that the pulse width of Cherenkov radiation obtained from the Stanford Linear Accelerator is about 5 ps [34], and other linear accelerators perform similarly. Even shorter pulses are envisioned by Csonka [37] and by Madey and co-workers [38]. Utilizing isochronous, coherent laser beams of modest power in a tuned laser cavity, Csonka predicts bunching of electrons on the scale of the optical wavelength. Such bunches would emit SR pulses separated by femtoseconds (10 -is s) or less. As the picosecond represents an interval typical of atomic or ionic motion, so the femtosecond represents an interval typical of electron motions. We may not see practical experiments on this time scale in the near future, but we surely foresee their eventual possibility and their potential as a source of basic information on dynamic electronic processes. Not only must storagering performance be understood and applied to the production of suitable SR time dependence before time-resolved experimentation in the picosecond-tofemtosecond range becomes a reality, but attention must be paid to beam-line design for low time dispersion as well. This work was supported by the Naval Weapons Center Independent Research F u n d and the Office of Naval Research, Contract Number NR 372-032. Experimental work in collaboration with K.M. Monahan and I.H. Munro was conducted at the Stanford Synchrotron Radiation Laboratory, which is supported by the National Science Foundation, Grant Number DMR 77-27489, in cooperation with the Department of Energy.

References [1] L. Lindqvist, R. Lopez-Delgado, M.M. Martin and A. Tramer, Proc. Int. Syrup. for Synchrotron Radiation Users, Daresbury (4-7 Jan., 1973) eds., G.V. Marr and I.H. Munro (Science Research Council, Daresbury Laboratory 1973) Report DNPL/R26, p. 257. [2] W.S. Heaps, D.S. Hamilton and W.M. Yen, Opt. Commun. 9 (1973) 304. [3] R. Lopez-Delgado, Proc. of the Course on Synchrotron Radiation Research, Int. Coll. on Appl. Phys., eds., A.N. Mancini and I.F. Quercia, Alghero, Italy (1976) p. 63. [4] S.S. Hasnain, T.D.S. Hamilton, I.H. Munro and E. Pantos, Daresbury Report DL/SRF/077 (1977). [5] E. Matthias, R.A. Rosenberg, E.D. Poliakoff, M.G. White, S.-T. Lee and D.A. Shirley, Chem. Phys. Lett. 52 (1977) 239. [6] G. Zimmerer, DESY Report SR-78/12 (1978).

[7] O. Dutuit, R.A. Gutchek, J. LeCalve and M.C. Castex, Proc. Fifth Annual SSRL Users' Meeting, (Oct., 1978) SSRL Report No. 78]09, p. 93. [8] U. Hahn and N. Schwentner, Nucl. Instr. and Meth. 152 (1978) 201. [9] T.D. Bonifield, F.H.K. Rambow, G.K. Walters, M.V. McCluster, D.C. Lorents and R.A. Gutchek, Proe. Fifth Annual SSRL Users' Meeting (Oct., 1978) SSRL Report No. 78]09, p. 85. [10] G. Thornton, E.D. Poliakoff, R.A. Rosenberg, M.G. White, S. Southworth and D.A. Shirley, Proc. Fifth Annual SSRL User's Meeting (Oct., 1978) p. 44. [11] R.A. Rosenberg, M.G. White, E.D. Poliakoff, G. Thornton and D.A. Shirley, J. Phys. B: Atom. Molec. Phys. 11 (1978) L719. [12] J.E. Hansen, J. Opt. Soc. Am. 67 (1977) 754. [ 13 ] K.M. Monahan, V. Rehn, E. Matthias and E.D. Poliakoff, J. Chem. Phys. 67 (1977) 1784. [14] K.M. Monahan and V. Rehn, J. Chem. Phys. 68 (1978) 3814; SSRL Report No. 77110 (Oct., 1977). [15] K.M. Monahan and V. Rehn, J. Chem. Phys. 71 (1979) 2360. [16] R.V. Taylor, W.C. Walker, K.M. Monahan and V. Rehn, Proc. Sixth Annual SSRL Users' Meeting (Oct., 1979), SSRL Report No. 79]05, p. 59. [17] I.H. Munro, I. Peeht and L. Stryer, Proc. Natl. Acad. Sci. (USA) 76 (1979) 56. [18] I.H. Munro and A. Sabersky, in: Synchrotron Radiation Appfications, ed., H. Winick, in press, ch. 9. [19] C. Kunz, ed., Topics in Current Physics: Vol. 10, Synchrotron Radiation Techniques and Applications (Springer-Verlag, New York, 1979). [20] R.Z. Bachrach, F.C. Brown and S.B.M. HagstriSm, J. Vac. Sci. Techn. 12 (1975) 309; Vacuum Ultraviolet Radiation Physics, eds., E.E. Koch, R. Haensel and C. Kunz (Pergamon Press, Oxford, 1974) p. 795. [21] R.Z. Bachrach, M. Skibowski and F.C. Brown, Phys. Rev. Lett. 37 (1976) 40. [22] R.A. Rosenberg, M.G. White, G. Thornton and D.A. Shirley, J. Chem. Phys. 66 (1976) 2496. [23] R.A. Rosenberg, M.G. White, G. Gabor, E.D. Poliakoff, G. Thornton, S.H. Southworth and D.A. Shirley, Rev. Sci. Instr. in press. [24] M.L. Knotek, V.O. Jones and V. Rehn, Phys. Rev. Lett. 43 (1979) 300. [25] M.L. Knotek, V.O. Jones, V. Rehn and K.M. Monahan, Proc. Sixth SSRL Users' Meeting (Oct., 1979) SSRL Report No. 79]05. [26] R.T. Pack, J. Chem. Phys. 65 (1976) 4754; E.J. Heller, J. Chem. Phys. 68 (1978) 2066, 3891; J.A. Beswick and J. Jortner, Chem. Phys. 24 (1977) 1. [27] E. Gratton and R. Lopez-Delgado, Rev. Sci. Instr. 50 (1979) 789; K.M. Monahan, I.H. Munro and V. Rehn, unpublished. [28] K.M. Monahan and V. Rehn, Nucl. Instr. and Meth. 152 (1978) 255. [29] O. Benoist d'Azy, R. Lopez-Delgado and A. Tramer, Chem. Phys. 9 (1975) 327. [30] A.P. Sabersky and hH. Munro, in: Picosecond Phenomena, eds., C.V. Shank, E.P. Ippen and S.L. Shapiro, Chemical Physics Series 4 (Springer-Verlag, Berlin, Heidelberg, New York, 1978).

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[31] H.P. Haar and M. Hauser, Rev. Sci. Instr. 49 (1978) 632. [32] J.M. Ramsey, G.M. Hieftje and G.R. Haugen (Indiana University) private communication. [33] K.M. Monahan, I.H. Munro and V. Rehn, to be published. [34] K.M. Monahan, I.H. Munro, L.F. Chase, B.A. Watson, M. Donald and J. Sheppard, SSRL Report No. 79/04, p. 121; and to be published. [35] A.P. Sabersky (Stanford Linear Accelerator Center,

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Stanford, Calif 94305) private communication. [36] P.B. Wilson, R. Servranckx, A.P. Sabersky, J. Gareyte, G.E. Fischer and A.W. Chao, Proc. 1977 Practical Accelerator Conference, Chicago (16-18 March 1977); See also ref. [30]. [37] P.L. Csonka, SSRL Report No. 79/04 (July, 1979). [38] D.A.G. Deacon, L.R. Elias, J.M.J. Madey, H.A. Schwettman and T.I. Smith, Proc. Soc. Photo-Optical Instrumentation Engineers, Vol. 121, Optics in Adverse Environments, eds., E. Bernal and H.V. Winsor (1977) p. 89.

V. EXPERIMENTAL TECHNIQUES