A systematic experimental study of the bath gas and pressure dependence of the photoisomerization of transstilbene in the low to intermediate pressure regime is presented. The analysis of the results by a detailed numerical master equation simulation reveals specific bath gas influences in the effective specific rate constants for isomerization that are already observable at pressures of about 1 bar. The low-pressure regime of the unimolecular reaction 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 high-pressure limit is consistent with the proposition of a pressure-or density-dependent effective barrier of the reaction. The values obtained for the pressure-dependent barrier height agree with the trend observed in earlier experiments in highly compressed gases and liquids and confirm a substantial contribution of "static" lowering of the reaction barrier. Additional "dynamic" lowering of the effectice barrier due to collision-induced intramolecular vibrational energy redistribution cannot be ruled out, however, and the relative importance of these two contributions remains an open question. The recently proposed effect of vibrational Franck-Condon cooling upon optical excitation of trans-stilbene definitely is not consistent with the experimental results.

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