1540 J. Opt. Soc. Am. B/Vol. 7, No. 8/August 1990

Isomerization of cis-stilbene in rare-gas clusters:direct measurements of trans-stilbene

formation rates on a picosecond time scale

Hrvoje Petek and Keitaro Yoshihara

Institute for Molecular Science, Myodaiji, Okazaki 444 Japan

Yoshihisa Fujiwara

Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan

Jeremy G. Frey

Department of Chemistry, Southampton University, Southampton S09 5NH, UK

Received October 20, i989; accepted March 6, 1990

Photochemical isomerization of cis-stilbene in Ar and Kr clusters is studied by selectively measuring the trans-stilbene product formation rate with less than 1-psec time resolution. The observed trans-stilbene rise timesare biexponential: The fast component is 1.7 ± 0.4 psec in Ar and 3.0 ± 0.3 psec in Kr, and the slow compo-nent is 16 ± 1 psec independently of the rare-gas solvent. It is proposed that the biexponential rise time is dueto different decay rates for the vibrationally hot and relaxed phantom intermediate state.

INTRODUCTION

Few problems have attracted as much interest frompicosecond spectroscopists as the photoisomerization ofcis- and trans-stilbene. 2 Although theoretical calcula-tions predict similar electronic structures for both cis-and trans-stilbene,3 the Si-state photophysical propertiesof the two isomers are quite different. It is well estab-lished that the trans-stilbene S, state decays by a ther-mally activated unimolecular barrier crossing, which isfollowed by internal conversion to the ground state cis-or trans-stilbene. 2'4'5 Fisher and his co-workers haveshown that at room temperature in solution cis-stilbene inthe SI state can isomerize to form either trans-stilbenewith a 35% quantum yield or dihydrophenanthrene (DHP)with a 10% quantum yield6 :

trans-stilbene cis-stilbene DHP

Femtosecond transient absorption2' 7 and multiphoton ion-ization' experiments have shown that in solution the cis-stilbene Sl-state decay in -1.4 psec is weakly dependenton solvent temperature and friction. This decay processhas been attributed to rapid torsion around the centralC = C bond to a 900 twisted geometry, which is thoughtto be the common intermediate for both cis- and trans-stilbene Si-state decay and has historically been desig-nated as the phantom state.2'7' 9 The point of bifurcation

between the cis-stilbene to trans-stilbene and the cis-stilbene to DHP channels is not known. Spectroscopy ofcis-stilbene homologs that have been structurally pre-vented from isomerizing to trans-stilbene homologs butthat can form DHP homologs shows that the initial mo-tion on the cis-stilbene S surface is along the DHP reac-tion coordinate; the nonradiative decay of some of thesemolecules is similar to that of cis-stilbene.10 The DHPchannel involves much smaller amplitude phenyl-ringmotion than does the cis-trans isomerization, which mayexplain why a strong solvent effect on cis-stilbene non-radiative decay is not reported in Ref. 2. In order to es-tablish the mechanism for cis-stilbene nonradiative decayand to investigate the role of the phantom state in cis-trans isomerization, we present preliminary measure-ments of cis-stilbene dynamics in rare-gas clusters thatare formed by supersonic expansion.

To establish the role of cis-trans isomerization on cis-stilbene Sl-state decay, it is necessary to determine therelationship between cis-stilbene decay and trans-stilbeneformation rates. It has been appreciated for a long timethat the direct measurements of trans-stilbene formationrate could show evidence of any intermediates in trans-stilbene formation; however, the 20-psec time resolutionof early experiments was not sufficiently short to measurethe trans-stilbene rise time.""2 Doany et al.,7 who mea-sured a multiexponential transient absorption at 312.5 nmwith a 100-fsec time resolution, observed a 1.35-psecdecay and a 3 ± 2 psec rise; the former has been assignedto the cis-stilbene Sl-state decay rate and the latter to thetrans-stilbene formation rate.7 The delay between thedecay of cis-stilbene and the formation of trans-stilbene

0740-3224/90/081540-05$02.00 ©D 1990 Optical Society of America

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has been attributed to the formation of a phantom statewith a lifetime of 3 ± 2 psec. One difficulty in interpret-ing these experiments is that the probe wavelength is ab-sorbed by both the ground- and excited-state cis-stilbene,trans-stilbene, and DHP; therefore it is problematic to un-ambiguously assign the observed dynamics to a particularspecies. Advancing the understanding of cis-stilbeneexcited-state dynamics will require experiments that candistinguish between the two isomerization channels andidentify the reaction intermediates.

The present work takes a unique approach to investi-gation of cis-stilbene dynamics; namely, cis-stilbene issolvated in cryogenic Ar or Kr clusters that are formedby rare-gas:cis-stilbene supersonic expansions. Prelimi-nary spectroscopic work on cis-stilbene: rare-gas clustersshowed an unexpectedly long 17.2-nsec fluorescence decayof the Si state of cis-stilbene. 0

13 This was interpreted as

evidence for a shallow minimum on the Si state, which hasa noticeable effect on cis-stilbene dynamics only in thelow-temperature environment of the cluster. It was pro-posed that the role of rare-gas solvent atoms is to removeexcess vibrational energy on a time scale that competeswith isomerization, thus trapping a fraction of the mole-cules in the Si state minimum.'3 In the present study,the cis-trans isomerization channel is studied by selec-tively measuring the trans-stilbene formation rate witha less than 1-psec time resolution in a pump-probe ex-periment. Identical pump and probe pulses are used toinduce cis-stilbene isomerization and to excite the trans-stilbene product fluorescence. Measurement of the trans-stilbene laser-induced fluorescence as a function of thedelay after photolysis is equivalent to measuring selectivetrans-stilbene transient absorption in a molecular-beam-cluster environment. It is hoped that this experimentwill form the basis for future picosecond investigations- ofsolvation effects on reaction dynamics.

EXPERIMENT

The experimental apparatus consists of (i) a molecularbeam apparatus in which cis-stilbene: rare-gas clustersare formed, (ii) a picosecond laser system for pump-probemeasurements, and (iii) fluorescence detection and signalprocessing instrumentation. The molecular-beam ap-paratus has been described previously. 3 Briefly, the su-personic expansion is formed by mixing either Ar or Krwith cis-stilbene within a pulsed nozzle and then expand-ing the sample through a 0.5-mm orifice. The partialpressure of cis-stilbene is controlled by heating the tip ofthe nozzle, where cis-stilbene is contained, to 50-65°C,which gives a cis-stilbene partial pressure of <0.1 Torr.cis-Stilbene is purified to >99.9% by spinning columndistillation in order to remove the trans-stilbene im-purity; trans-stilbene fluorescence is not observed underlinear excitation conditions. Typical stagnation pressuresare 1000 Torr for Kr and 3000 Torr for Ar, which yieldsdilution ratios of cis-stilbene to rare gas of 1:10000 orbetter. The molecular beam is intersected by the laserbeam at -10 mm from the orifice. The cluster size dis-tribution cannot be measured with the present molecular-beam apparatus.

The picosecond light source for pump-probe measure-

ments is an amplified, synchronously pumped, hybridlymode-locked picosecond dye laser. Light from a CoherentAntares cw mode-locked Nd:YAG laser is used both topump a Coherent 702-1 dual jet hybridly mode-locked dyelaser and as a seed for a Quantel RGA60-10 regenerativeamplifier. The dye laser, operating with Rhodamine 6Gdye, produces -800-fsec (autocorrelation width) pulses ata 76-MHz repetition rate between 615 and 580 nm. The-1-nJ pulses from the dye laser are amplified to -1 mJ ata 10-Hz repetition rate in a three-stage amplifier thatuses Sulforhodamine 101/methanol (605 nm) or KitonRed/H 20 (594 nm) dyes, which is pumped by 20-mJ 100-psec duration pulses from the Quantel regenerative ampli-fier. The autocorrelation width of amplified pulses is-1 psec. The light for pump-probe measurements is gen-erated by frequency doubling the amplified dye laserpulses in a 0.5-mm, ,f-BaBO3 crystal. Since both cis- andtrans-stilbene absorb near 300 nm, the pump and probepulses are identical. After frequency doubling, the UVpulse is split into -50% parts; one beam is sent througha fixed delay, while the other beam transverses a variabledelay. The beams are rendered nearly parallel and arefocused into the molecular-beam chamber by a 700-mmfocal-length lens, such that the UV beams intersect in themolecular beam -100 mm short of the focus.

The emission is collected by f/i optics, focused on a slitof a 250-mm spectrograph, and detected by an intensifiedoptical multichannel analyzer (EG&G model PAR 1420).For pedagogical purposes the emission spectrum is mea-sured as a function of delay between the pump and theprobe pulses (Fig. 1). The spectrum shows two compo-nents: (i) a structured trans-stilbene emission in the UV,which increases in intensity with the delay between thepump and the probe pulses and which is not observed

26 psec

, /D kS\

350 400 450

Wavelength (nm)

Fig. 1. Emission spectra of cis-stilbene:Ar clusters after excita-tion with pump and probe pulses at 302.5 nm, delayed by the timeinterval indicated. The optical multichannel analyzer integra-tion time is 1 min, and the stagnation pressure is 3000 Torr of Arfor each spectrum. The small amount of trans-stilbene emissionat 0-psec delay is not due to impurity in cis-stilbene, rather itoccurs because nascent trans-stilbene isomerization productfluorescence is excited within the pump pulse under nonlinearexcitation conditions. Dotted lines indicate the 10% transmis-sion limits of the 329-nm filter used in the kinetic measure-ments. The apparent increase in cis-stilbene intensity withincrease in the delay is due to the overlap of cis-stilbene emissionwith the tail of trans-stilbene emission.

Petek et al.

1542 J. Opt. Soc. Am. B/Vol. 7, No. 8/August 1990

under linear single-pulse excitation conditions; and(ii) structureless cis-stilbene emission with a maximum at417 nm, which is independent of the delay and is observedeven under single-photon excitation conditions.

The actual kinetic data on the trans-stilbene formationrate is recorded with a photomultiplier by detecting thetrans-stilbene emission intensity through a 329-nm inter-ference filter as a function of delay between the pump andthe probe pulses. The laser intensity is measured with aphotodiode for each pulse in order to normalize the trans-stilbene emission intensity. Both the photomultiplier andphotodiode signals are measured with a sample-and-holdcircuit and digitized, and trans-stilbene emission is nor-malized by the laser power. Linear normalization is usedbecause under present excitation conditions the cis-stil-bene absorption is saturated. The trans-stilbene inten-sity is measured at 266.7-fsec/point intervals; 250 pointson each side of zero delay are measured in each scan.The delay is repetitively scanned in both directions;5-10 scans are summed to minimize problems that aredue to long-term changes in cis-stilbene partial pressureand other effects. Great care is taken to keep the inten-sity in both beams equal and to ensure optimal overlapthroughout the scans.

RESULTS AND ANALYSISThe trans-stilbene rise time after photolysis of cis-stil-bene that is solvated in Ar and Kr clusters is shown in

.r4Ca,

C,

C0

ELU

at

a)

a

-0Ca,

52

C

.

n

c

E

W

LUaI

a)M

a,

2

-65 0

Fig. 2. The signal is symmetric with respect to timeorigin because the pump and the probe pulses are identi-cal. The logarithmic plots in Fig. 2 clearly show thattrans-stilbene production is biexponential. The risetimes are fit using the following expression:

Af(t) = Al + A2 - Al exp(-t/T1) - A2 exp(-t/T2), (1)

k 1 = 1/iT, k2 = 1/T2. (2)

The fitting parameters for measurements at 297.0 and302.5 nm are presented in Table 1. The calculated fastrise time (X1) for trans-stilbene formation in Ar clusters isprobably longer than the true rise time because the finiteexcitation pulse width is not accounted for in the fittingprocedure. From the autocorrelation measurements forthe amplified dye laser pulse, a 700-fsec pulse width canbe estimated when a hyperbolic secant square pulse shapeis assumed. The doubled pulse is expected to broaden by-250 fsec, owing to the group-velocity mismatch in thedoubling crystal, 4 and to shorten, owing to the intensity-squared dependence of the doubling process. Autocorre-lation of the UV pulse could not be measured to determinethe width precisely; however, an upper limit of 1 psecis assumed. The effects of the finite pulse width are that(i) the apparent formation rate is slower than the trueformation rate and (ii) less than 5% of trans-stilbenefluorescence, which is produced by isomerization of cis-stilbene on the time scale of the pulse duration, is observedunder nonlinear excitation conditions, even with only thepump pulse.

Z+

C4

N+

C:Z

-

65 65

TIME(psec) TIME(psec)Fig. 2. trans-Stilbene emission intensity as a function of delay between pump and probe pulses in Ar and Kr clusters. The linear plot isshown on the left, and the corresponding logarithmic plot on the right. The best fit to the biexponential rise is shown by the solid curve.The rise times are symmetric with respect to zero time delay because the pump and probe pulses are nearly identical. Each half of thekinetic trace is fitted separately, and the results are averaged.

11000 Torr Kr

20 Ala All .

0 -

n I I

Petek et al.

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Vol. 7, No. 8/August 1990/J. Opt. Soc. Am. B 1543

Table 1. Parameters Obtained by Fittingtrans-Stilbene Rise Times in Ar

and Kr Clusters to Eq. 1

Al (o) A2 Ti (psec) T2

297.0 nmAr 59 5 41 ± 5 1.73 ± 0.28 15.8 ± 0.9Kr 60 4 40 ± 4 2.93 ± 0.11 15.2 ± 0.4

302.5 nmAr 57 4 43 ± 4 1.79 ± 0.39 16.1 ± 0.8Kr 59 6 41 ± 6 3.03 ± 0.37 16.7 ± 0.9

The dependence of trans-stilbene rise time on rare-gasstagnation pressure, nozzle temperature (cis-stilbene con-centration), laser power, and excitation wavelength is in-vestigated; however, no effect on the fitting parametersdetermined in Eq. (1) is found. Increasing the stagnationpressure, nozzle temperature, and laser power increasesthe total signal level. At the stagnation pressures formeasurements in Fig. 2, the trans-stilbene signal is at thehigh-pressure limit because the cluster size is sufficientlylarge that there is no dependence of the signal on the clus-ter size. It is assumed that trans-stilbene emission quan-tum yield is independent of the internal energy followingisomerization, i.e., the delay between the pump and theprobe pulses. The trans-stilbene emission from Ar:cis-stilbene clusters is approximately a factor of 2 strongerthan that from Kr clusters, which suggests that the iso-merization quantum yield in Ar is higher. The rise timesare not strongly dependent on the excitation wavelengthabove the trans-stilbene SI origin (322 nm for Ar clustersand 325 nm for Kr clusters),"0 although cis-stilbene absorp-tion strength is significantly higher at 297.0 nm thanat 302.5 nm.

DISCUSSION

The trans-stilbene formation rates measured in this studyshow previously unobserved biexponential production.The fast component clearly depends on the cluster envi-ronment (solvent), while the slower component is the samefor Ar and Kr. The fact that the trans-stilbene formationrate (k,) is slower in Kr clusters than in Ar clusters isconsistent with the smaller trans-stilbene formationquantum yield in Kr clusters and with the observation ofstronger cis-stilbene emission from Kr:cis-stilbene clus-ters.' 3 The slow production rate (k2 ) is the same for bothAr and Kr clusters, which suggests that it is due to a sol-vent-independent cis-stilbene dynamical process.

There are several dynamical processes that could con-tribute to the biexponential production of trans-stilbene;for instance, (i) twisting to a 900 configuration on thecis-stilbene Si surface, (ii) intramolecular or intraclustervibrational relaxation on ground- and excited-state sur-faces, (iii) internal conversion from the phantom state, or(iv) isomerization to DHP. The first two processes aremost likely to be influenced by the cluster environment;therefore these processes are likely to be responsible forthe fast rise time. Furthermore, the biexponential ratemay be a static effect that is related to cluster structure.

The friction that is experienced by cis-stilbene duringlarge-amplitude cis-stilbene torsion depends on the cluster

temperature, structure, solvent-solute and solute-soluteinteractions, and the solute mass. Although the clustertemperatures are not known, because of stronger bindingand larger mass the friction in Kr clusters is likely tobe greater than in Ar clusters. This is consistent withslower production of trans-stilbene, lower trans-stilbeneproduction quantum yield, and higher cis-stilbene emis-sion quantum yield in Kr clusters. The relative vi-brational relaxation rates in Ar and Kr clusters are moredifficult to predict. Generally, the lighter rare gases aremore efficient than the heavier ones in inducing vibra-tion-to-translation relaxation. However, possible reso-nances between the low-frequency phenyl-twisting motionin stilbene and Ar or Kr cluster phonons may result invibration-to-phonon energy transfer's being the dominantrelaxation pathway. If the rate-limiting process that isresponsible for ki is the vibrational relaxation of hot cis-stilbene, then the slower rise time in Kr than in Ar isinconsistent with a lower trans-stilbene production quan-tum yield. Vibrational relaxation may be responsible forthe second component in the trans-stilbene productionrate if the absorption cross section for the vibrationallyhot molecules is lower than that for cold molecules. How-ever, in this case the same rates would not be expected forAr and Kr clusters.

The solvent- and temperature-independent process thatis responsible for k2 may be due to the intrinsic lifetime ofa bottleneck intermediate state such as the phantom state,or an intermediate associated with the DHP isomerizationchannel. Internal conversion and isomerization throughthe triplet channel is a minor (<2%) process.6 Since in-termediates associated with the DHP channel and possiblealternative pathways for formation of trans-stilbene arenot established, it is most likely that 2 is the phantom-state lifetime.

Since the biexponential production of trans-stilbene isfirst established in cryogenic rare-gas clusters, it is impor-tant to consider the possibility that it is a property of thecluster environment. Since clusters have a large surface-to-volume ratio, it is possible that one rate is due to cis-stilbene molecules held on the surface and the other isdue to the molecules within the bulk of the cluster. Also,it is known that rare-gas clusters undergo order-disorderphase transitions and that the two phases can coexist witha cluster.' 5 The measured rates may reflect this phasecoexistence. However, the insensitivity of rates on theexpansion conditions, which would determine the clustersize distribution and the temperature, argues againstthese cluster-related effects.

Although the preliminary measurements in this work donot conclusively establish the mechanism for the observedtrans-stilbene production rates, the following mechanismis proposed. The fast production rate of trans-stilbene(kj) is for the formation of the vibrationally hot phantomstate, which is the rate-limiting step, followed by rapid in-ternal conversion to the So state. The dependence of thek, on the cluster environment suggests that the solventfriction is the rate-limiting step in the production of thehot phantom state. The time scale for formation of thevibrationally hot phantom state is consistent with previ-ous measurements of a 1.35-psec decay of the S state ofcis-stilbene,2'8 9 but the observed solvent dependence is

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1544 J. Opt. Soc. Am. B/Vol. 7, No. 8/August 1990

greater. This may be due to the different natures of thesolvents used here and in Ref. 2.

The slow production rate of trans-stilbene (k2) is tenta-tively assigned to the decay of the vibrationally relaxedphantom state. It is assumed that the internal conver-sion rate from the phantom state depends on the vibra-tional temperature. Doany et al.7 assigned a rise time of3 + 2 psec transient absorption at 312.5 nm to the pro-duction of trans-stilbene from the phantom intermediatestate. The shorter phantom-state lifetime in the earlierreport may be due to the much higher temperature (295 K)at which the measurement was performed. Since at 295 Kthe cis-stilbene internal temperature is higher than inrare-gas clusters, the rate observed by Doany et al. ismuch closer to the vibrationally hot phantom-state decay(kI) that is proposed in the present study, and therefore bi-exponential production has previously not been observed.

CONCLUSIONSSubpicosecond pump-probe measurements of the trans-stilbene formation rate in cis-stilbene: Ar and cis-stilbene:Kr clusters indicate a biexponential rise time, with a fastcomponent of 1.7 ± 0.4 psec for Ar and 3.0 ± 0.3 psec forKr and a slow component of 16 ± 1 psec that is independentof the cluster environment. The fast component is ten-tatively assigned to the solvent-friction-dependent for-mation of the vibrationally hot phantom state, and theslow component to the internal conversion from the vi-brationally relaxed phantom state. It is proposed thatthe phantom-state lifetime is strongly dependent on thephantom-state vibrational temperature. This mechanismwill be tested in the future with experiments designed tomeasure the cis-stilbene S-state lifetime, trans-stilbenevibrational relaxation, DHP formation rate, and so on, ina variety of cluster environments.

ACKNOWLEDGMENTSJ. G. Frey thanks the Royal Society for a travel grant thatsupported his visit to the Institute for Molecular Sciencefor the duration of the experiments performed in thiswork. H. Petek is indebted to Iwao Ohmine for many dis-cussions about the phantom state and other mysteries ofstilbene isomerization.

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