FEMTOSECOND STUDIES OF THE PHOTOISOMERIZATION OF CIS-STILBENE IN SOLUTION

Femtosecond laser studies on cis-stilbene photoisomerization in alkane solvents show a weak friction dependence on the excited state lifetime. A possible barrier crossing process is indicated when these results are compared with various potential models. The anisotropy values of the transient absorptions allow the assignment of the various excited states to A type in C2 symmetry. Anisotropy measurements in the regions of product ground state absorptions demonstrate a high alignment between reactant cis and product trans and a low alignment between cis and product dihydrophenanthrene (DHP) transition dipoles. These results indicate a significant angular displacement of the ethylene bond during isomerization. In addition, time resolved absorption studies detecting product trans fluorescence estimate an initial internal temperature of 725 +/- 100 K which decays with ca. 14 ps time constant.