Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments
In this and the following paper, we present a full account of our earlier report [Syage et al., Chem. Phys. Lett. 88, 268 (1982)] on the spectra and picosecond dynamics of stilbene isomerization in supersonic jets. The jet-cooled excitation and dispersed fluorescence spectra of t-stilbene-h12 and -d12 are reported and assigned for the Bu <-- Ag electronic excitation. The 000 wavelengths for h12 and d12 (in excitation) are 3101.4 and 3092.5 Å, respectively. Previously unidentified low frequency modes (as low as 20 cm^−1 in S0 for -h12) have been observed and tentatively assigned as out-of-plane modes of au symmetry in C2h. This indicates that t-stilbene has a propeller-like geometry involving phenyl rotation (i.e., C2 symmetry). A Franck–Condon analysis of the low frequency modes and particularly the ag, nu25 in-plane symmetric bend mode indicates that a large geometry change takes place upon electronic excitation possibly due to a delocalization of double bond character from the Ce–Ce bond to Ce–[cursive phi] bond. The geometry change of the in-plane Ce–Ce–[cursive phi] between S1 and S0 was determined from the Franck–Condon and a normal mode analysis to be 1.3°±0.3°. The rms amplitude of this bend motion for the symmetric nu25 bend mode (for one quanta in S0) is |^2|^1/2=1.0±0.2°. Most ag modes involving benzene-type vibrations (other than C–H stretch modes) have been assigned. Dispersed fluorescence spectra exhibited a broad background indicative of IVR which increased rapidly with S1 vibrational energy. The spectra were completely diffuse above 1200 cm^−1 which is consistent with the barrier for isomerization being at about 1100–1200 cm^−1. The excitation spectra show a rapid decline in intensity at higher energies due to the process of isomerization which competes with radiative decay. However, sharp (albeit weak) structure could still be discerned at energies well in excess of 2000 cm^−1. In the accompanying paper, we present results on the dynamics of isomerization and its dependence on mode mixing and the nature of the reactive surface (adiabatic vs diabatic).
Additional Information© 1984 American Institute of Physics. Received 18 April 1984; accepted 3 July 1984. We gratefully acknowledge the National Science Foundation for support of this work under Grant No. CHE-8211356. We are particularly indebted to Professor A. Warshel for providing us with the normal mode analysis of t-stilbene and the figure of the ν25 normal coordinate. We also wish to thank Professor D.A. Evans and Mr. Robert Dow for assistance in recording the gas chromatographs of t-stilbene. [P.M.F. was an] IBM Predoctoral Fellow. [A.H.Z. was a] Camille and Henry Dreyfus Foundation Teacher-Scholar. Arthur Amos Noyes Laboratory of Chemical Physics, Contribution No. 7013.
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