10638
J. Phys. Chem. 1993,97, 10638-10644
Picosecond Resonance Raman Studies of Vibrational Cooling of Electronically Excited trans-Stilbene in Alcohols and Alkanes Jun Qian, Sandra L. Schultz, Gregory R. Bradburn, and John M. Jean' Department of Chemistry, Washington University, One Brookings Drive, St. Louis, Missouri 63130 Received: June 2, 1993; In Final Form: July 23, 1993"
The vibrational cooling of electronically excited trans-stilbene has been studied in ethanol, hexanol, hexane, and decane by pump-probe resonance Raman ( R R ) spectroscopy employing pulses of 1.5-2.0 ps with 15 cm-I spectral width. Excitation 3200 cm-' above the electronic origin creates a transient rise in the stilbene vibrational temperature of approximately 150 K. The subsequent intermolecular transfer of this excess energy to the surrounding solvent leads to time-dependent changes in the peak position and bandwidth of the ethyleneic band a t 1565 cm-1. In agreement with previous studies, we find that the lineshape of this mode is nearly Lorentzian a t all delay times and that the width decays exponentially with increasing delay time. The peak position, however, shifts to higher frequency only after a 3-5-ps induction period. The vibrational cooling kinetics, as measured by the dynamics of the bandwidth, d o not depend on solvent, though the dephasing time of the ethylenic mode is faster in alcohols than in alkanes. We interpret these results as arising from coupling of the ethylenic motion to two or more low frequency modes that can exchange energy with the solvent via collisions. W e discuss the role of solvent-solute interactions and solvent thermal conduction in the cooling process.
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Introduction The rates and pathways by which hot solute molecules transfer vibrational energy to the surrounding medium (vibrational cooling, VC) iscurrently of considerable interest not only due to the insight it may provide with respect to specific solute-solvent interactions but also due to its relevance to the field of ultrafast condensed phase reaction dynamics1 Standard theoretical descriptions of solution phase reactions employ a reaction coordinate picture in which modes orthogonal to the reactive motion are considered to remain in thermal equilibrium with one another and with the solvent during the course of the reaction.24 While this may be a good approximation on the time scale of activated processes, wherevibrational T1 times are short compared to barrier crossing times, it is not expected to hold for reactions with little or no barrier. In such cases, vibrational relaxation can occur on time scales comparable to, or even slower than, motion along the reaction coordinate. Several theoretical attempts have been made to incorporate finite relaxation rates for nonreactive modes.'lq6 These ideas have also generated considerable experimental interest. Recently, vibrational cooling of the trans photoproduct in the cis-trans isomerization of stilbene has been studied by transient absorption' and resonance Raman (RR) spectroscopy8 with the goal of providing further insight into the nature of the reactive motion in this prototypical barrierless process. Numerical simulations of the fluorescence spectra of cis-stilbene showed that the S1 state of the cis-isomer is vibrationally hot on the time scaleof photoisomerization (- 1 P S ) . ~Understanding the factors that control vibrational relaxation rates is thus essential for providing a detailed picture of the molecular motions that lead from reactants to products in ultrafast chemical reactions. Numerous spectroscopic techniques have been employed to prepare and probe molecules with excess vibrational energies ranging from several hundred to tens of thousands of wavenumbers. The subsequent return of the solute molecule to thermal equilibrium results from both intramolecular and intermolecular processes. The excess vibrational energy, initially localized in one or more Franck-Condon active modes, is rapidly randomized among the entire vibrational space due to anharmonic coupling between modes. For moderate to large organic molecules in
* To whom correspondence should be addressed. S I
Abstract published in Aduance ACS Abstracts, September 15, 1993.
0022-3654/93/2097- 10638%04.00/0
solution this intramolecular vibrational redistribution (IVR) process generally occurs on the subpicosecond time scale; however, the recent work of Sension et al.7 has shown that IVR in cisstilbene is not complete on the time scale of the cis-trans isomerization process. The redistribution of energy results in a vibrationally hot molecule that subsequently cools via collisional energy transfer to the solvent on the 5-50 ps time scale. Studies of the vibrational cooling process have been carried out largely via transient absorption techniques in both the visiblelJbl6 and infrared regionsl7-I9 of the spectrum. Chesnoy and co-workersZ0 have used femtosecond fluorescence decay measurements to probe both IVR and cooling of large organic dye molecules in a variety of solvents. In these experiments, IVR and VC lead to spectral evolution giving rise to wavelengthdependent kinetics. Interpretation of the kinetic data requires modeling the temperature dependence of the absorption or emission bands. More recently several groups have demonstrated the potential of time-resolved Raman spectroscopy for providing detailed information about the VC process in solution8*21-zsand in molecular crystalsUz6A major advantage of R R spectroscopy is that it provides mode-specific information about the vibrational dynamics. Transient absorption and time-resolved emission profiles receive contributions from potentially a large number of Franck-Condon active modes, which renders the analysis of the dynamics of individual vibrations impossible. The frequency resolution afforded by time-resolved RR spectroscopy also has the potential for providing detailed information with regard to intramolecular couplings and static solvent effects on potential energy surfaces. This has recently been demonstrated by Gustafson and co-workers.21J2 The gain in frequency resolution is also a disadvantage. Since increased resolution in one domain is necessarily gained at the expense of resolution in the other domain, the best time-resolution feasible is > 1 ps. On time scales shorter than this, RR lineshapes will be primarily instrumentally broadened, thus obscuring much of the information on molecular dynamical processes. This limits the use of R R in studying IVR in real time, though VC processes can be probed with sufficent resolution in both domains. Despite a number of excellent studies on a variety of systems, our knowledge of the microscope details of intermolecular energy transfer is far from complet,e. Several issues merit further 0 1993 American Chemical Society
Vibrational Cooling of trans-Stilbene
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10639
I
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n
BS
V
$ BBO
7
1
Nd:YLF
Figure 1. Schematic diagram of the time-resolved resonance Raman spectrometer used in these studies: X/2, half-wave plate; SA, saturable absorber;BS, 30/70beamsplitter;BBO, 1 "crystal of fl barium borate; SC, spining sample cell; Mono, 0.32-m monochromator;FI,holographic interferencefilter (590); F2, OG 550 filter; CCD, charge coupled device detector.
attention. For example, to what extent does the cooling rate reflect the magnitude of the solute-solvent coupling? What role does thermal diffusion in the solvent play in this process? If this is slow, then the dissipation of thermal energy within the solvent may be rate determining. With regard to the mechanism of cooling, what vibrational mode(s) of the solute act as mediators in theexchange of energy between solute and solvent? Theanswer to these questions will only be obtained through a thorough investigation of both solvent effects and solute structural effects on vibrational cooling kinetics. This paper reports our initial studies on the VC process in electronically excited stilbenes using a spectrometer with time resolution of 1.5-2.0 ps and frequency resolution of 15-16 cm-1. The photodynamics of the SI state of trans-stilbene has been well-studied due to the importance of this system as a prototype for activated barrier crossing processes.27 As a result of these investigations a considerable body of information exists with regard to its vibrational structure in the SI state. A normal coordinate analysis has been carried out28 and resonance Raman spectra of various isotopically substituted species have been assigned.29 This detailed understanding of the vibrational structure of electronically excited trans-stilbene makes it an excellent candidate for a probe of solution phase vibrational cooling using a mode-specific technique such as time-resolved R R spectroscopy. Indeed, two groups have recently reported picosecond R R studies of S1 trans-stilbene in a l k a n e ~ ~ ~and ,*3 acetonitrile.21 We report on VC studies of trans-stilbene in both alkanes and alcohols employing a R R spectrometer with better time resolution than those employed in the two previous studies. Our results provide further insight into the vibrational dynamics of this system.
Experimental Section trans-Stilbene was obtained from Aldrich Chemical and used without further purification. The solvents used in our studies were HPLC grade and obtained from Fisher. Excitation of each solvent at 290 nm yielded no detectable fluorescence. The concentration of trans-stilbene in all the experiments was 25 mM. The solvent temperature was 295 f 1 K. The transient resonance Raman studies were carried out using the apparatus shown in Figure 1. The laser system consists of a dual jet, Rhodamine 6G dye laser (Quantronix 4500 DC) equipped with a two-plate birefringent filter and pumped by the second harmonic of a mode-locked Nd:YAG laser (Quantronix 4126). DQOCI is used as the saturable absorber. The 76-MHz
output (0.8 nJ/pulse) is amplified by the Q-switched output of a diode-pumped Nd:YLF laser (Spectra-Physics TFR) operating at 1 kHz. The TFR second harmonic pulse energy at this repetition rate is approximately 200-220 pJ with a temporal duration of 5.5 ns. Timing is accomplished by synchronizing the firing of the Q-switch of the TFR laser to the YAG mode-locker driver after suitable delay (Stanford Research Systems DG 3500 Digital Delay Generator). The 527-nm output of the TFR is split with a 30/70 beamsplitter and used to pump a two-stage, multipass amplifier similar to the design of Becker et al.30 The first stage consists of a double confocal cavity employing singlestack, dielectrically coated mirrors with a radius of curvature of 18 cm. The seed pulse, focused to -60 gm, makes three passes through this stage (active medium Rhodamine 610 in ethylene glycol; 200 pm jet). The nearly perfect TEMm mode of the TFR laser allows for excellent mode matching with the dye laser beam and we achieve a gain of approximately 1000. The second stage (rhodamine 610 in methanol) consists of a 3 mm diameter quartz flow cell (Clark Instrumentation) and employs two passes using mirrors with larger radii of curvature to allow looser focusing (- 1 mm). The two-pass gain of this stage is 4-5 and the overall amplification efficiency is -2%. Theamplifiedoutput (-4pJperpulse,585.8nm) is frequency doubled using a 1-mm crystal of BBO (Cleveland Crystals) to produce the 293-nm pump beam. The visible beam is separated via a dichroic mirror and recombined with the pump beam after traversing a computer-driven optical delay stage (Aerotech). Both the pump and probe pulses are focused with a 5-cm lens onto the sample contained in a spinning quartz cell. Spatial overlap of the pump and probe beams and the zero time position of the stage were determined by monitoring the transient absorption of the probe beam. The beam diameter and spinning rate of the cell were such that the illuminated volume was exchanged between laser shots. Care was taken to minimize exposure of the sample to the UV pump beam. Absorption measurements taken before and after exposure to the UV excitation beam revealed
