Viscosity dependence and solvent effects in the photoisomerization of cis-stilbene: Insight from a molecular dynamics study with an ab initio potential-energy function J. Chem. Phys. 111, 8987 (1999); 10.1063/1.480242 Vibrational energy redistribution and relaxation in the photoisomerization of cisstilbene J. Chem. Phys. 97, 5239 (1992); 10.1063/1.463822 Femtosecond laser study of energy disposal in the solution phase isomerization of stilbene J. Chem. Phys. 93, 9185 (1990); 10.1063/1.459707 Unimolecular reactions in isolated and collisional systems: Deuterium isotope effect in the photoisomerization of stilbeneFemtosecond laser studies have been performed on the photoisomerization reactions of cisstilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the -150 fs time scale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans-stilbene product using transient fluorescence and transient absorption, confirming that the reaction generates hot product states and that the Franck-Condon modes are largely spectators in the reaction; the anisotropy (alignment) of trans product molecules illustrating the effect of friction coupling overall motion to the reaction coordinate; and a theoretical treatment of three-pulse anisotropy experiments.

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After optical excitation of cis-stilbene, ground-state dihydrophenanthrene (DHP) is formed in 1.7 ps (hexadeane) via kinetically required intermediate(s) present during the first 300 f 200 fs. The DHP is formed hot, and its cooling takes place over the time scale of 35 ps. The DHP spectrum in the region around 450 nm is present throughout the time scale studied (200 fs to 50 ms). The angle between the cis and resulting DHP transition dipoles is 38' at the earliest times measured, which significantly deviates from the alignment expected from a one-dimensional reaction coordinate involving rotations of the phenyl rings. It is proposed that the ethylene bond axis rotates significantly during the isomerization to DHP, perhaps as a result of rotation of the whole molecule in the laboratory frame. Experiments with DHP indicate ultrafast (C500 fs) ring opening leading toward DHP and cis-stilbene ground-state products. IntroductienThe photoisomerization reactions of cis-and tram-stilbene have been studied extensively as model systems for understanding the dynamics of intramolecular rearrangements in solution. Many of the processes crucial to the isomerism occur very rapidly and therefore require ultrafast laser methods for their study.Electronic excitation of trans-stilbene yields an excited-state population that is able to flow over a small barrier corresponding to rotation around the ethylene double bond and return to the ground electronic state surface forming cis-and trans-stilbene in approximately equal amounts. This process is well characterized in gases and liquids, the barrier crossing being friction dependent in a manner predicted approximately by Kramers' theory.'By contrast, electronically excited cis-stilbene molecules (cis*) escape from the cis configuration region in about 1 ps even in relatively high friction solvents at room suggesting either unrestricted motion of the structure on the excited-state surface or motion over a very small barrier. In addition, the cis I , , 0 1991 American Chemical Society cis-Stilbene to Dihydrophenanthrene Reaction

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The vibrational population dynamics of HgI fragments in ethanol solution, resulting from the 320 nm photolysis of HgI 2 , are examined both experimentally and by a simulation. The experiments reveal an HgI population distribution which rapidly relaxes toward equilibrium. At the earliest times, the HgI exhibits vibrational coherent wave-packet motion that dephases with a time constant of ca. 1 ps. These data are used to gain insight into the character of the solvated potential energy curves. The population relaxation was adequately reproduced by master equations which were formulated to incorporate the HgI anharmonicity and a solvent frequency dependent friction. This treatment characterizes the spontaneous vibrational relaxation timescale for the nЉϭ1→0 transition to be ca. 3 ps, and is used to identify the relaxation rate constants for all other HgI level pairs. The simulations estimate that the initial excess energy of HgI is centered at nЉХ10 which corresponds to a total excess energy of ca. 1050 cm Ϫ1 .