Picosecond dynamics and photoisomerization of stilbene in supersonic beams. II. Reaction rates and potential energy surface
Using picosecond excitation in a supersonic jet, we present a full account of our earlier report on the dynamics of state-selective photoisomerization of t-stilbene. Collisionless isomerization in this case indicates the twisting of the molecule about the ethylene bond away from the trans configuration Central to this reaction is the question of vibrational energy redistribution or IVR. From direct (single vibronic level) time-resolved measurements, relative fluorescence quantum yields from relaxed and unrelaxed states, and a thorough vibrational analysis from excitation and dispersed fluorescence spectra (previous paper), the following conclusions are reached: (i) The IVR yield is state selective being more extensive from combination modes than from fundamental modes of similar energy. The IVR yield becomes very significant above [approximately-equal-to]900–1000 cm^−1. The rate is much faster than the reaction at all energies studies. (ii) The barrier to isomerization is observed at 3.3±0.2 kcal/mol (1100–1200 cm^−1). The radiative lifetimes, measured from the 0° level fluorescence decays, are 2.7±0.1 ns (h12) and 2.5±0.1 ns (d12). (iii) The observed isomerization rates in the isolated molecule are approximately an order of magnitude less than the calculated RRKM rates and observed solution phase rates. (iv) The apparent non-RRKM behavior in the isolated behavior is explained by considering the nature of IVR and by adopting a diabatic representation of the reactive surface (i.e., an allowed surface) using a Landau–Zener–Stueckelberg model. (v) Finally, we compare t-stilbene with other related isolated molecules and to solution phase t-stilbene results in order to assess the role of mode mixing and the nature of the reactive surface.
Additional Information© 1984 American Institute of Physics. Received 18 April 1984; accepted 3 July 1984. We wish to acknowledge the support of this work by the National Science Foundation under Grant No. CHE-8211356. We are also grateful to Professor A. Warshel for illuminating discussions about nonadiabatic transitions and spectroscopy, Professor R.M. Hochstrasser for his interest in this work, and to Professor J. Jortner for providing us with a preprint of his paper. Finally, we wish to thank Professor P.G. Wolynes and Professor R.A. Marcus for enlightening discussions. [P.M.F. received an] IBM Predoctoral Fellowship. [A.H.Z. was a] Camille and Henry Dreyfus Foundation Teacher-Scholar. Arthur Amos Noyes Laboratory of Chemical Physics, Contribution No. 7014.
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