Photoisomerization of trans-stilbene in moderately compressed gases: Pressure-dependent effective barriers

A systematic experimental study of the bath gas and pressure dependence of the photoisomerization of trans-stilbene in the low to intermediate pressur e regime is presented. The analysis of the results by a detailed numerical master equation simulation reveals specific bath gas influences in the effe ctive specific rate constants for isomerization that are already observable at pressures of about 1 bar. The low-pressure regime of the unimolecular r eaction in the S-1 state can be located in the pressure range well below 1 bar for most of the bath gases. The effective "high-pressure limit" of the photoisomerization rate constant that can be extracted from the simulation is found to be pressure dependent, approaching a bath gas specific plateau value in the 10 bar range for bath gases such as methane, ethane, propane, or xenon. The existence of a bath gas dependent plateau of the effective hi gh-pressure limit is consistent with the proposition of a pressure- or dens ity-dependent effective barrier of the reaction. The values obtained for th e pressure-dependent barrier height agree with the trend observed in earlie r experiments in highly compressed gases and liquids and confirm a substant ial contribution of "static" lowering of the reaction barrier. Additional " dynamic" lowering of the effectice barrier due to collision-induced intramo lecular vibrational energy redistribution cannot be ruled out, however, and the relative importance of these two contributions remains an open questio n. The recently proposed effect of vibrational Franck-Condon cooling upon o ptical excitation of trans-stilbene definitely is not consistent with the e xperimental results.