Detection and assignment of the ’’phantom’’ photochemical singlet of t r a n s-stilbene by two-photon excitation

We have made a complete polarization study of the two-identical-photon excitation (TIPE) spectrum of stilbene in solution in the energy region 34 400 to 45 000 cm−1, which shows a strongly allowed two-photon transition with a maximum at 41 000 cm−1. The upper state of this transition is of vibronic symmetry Ag. We believe that the electronic part of the upper state is also of Ag symmetry and that it is identifiable with the ’’phantom’’ singlet state postulated by Saltier and by Orlando and Siebrand as the major pathway for cis–trans photoisomerization of this molecule.