Two-Photon Excitation of trans-Stilbene: Spectroscopy and Dynamics of Electronically Excited States above S1.

The photoisomerization dynamics of trans-stilbene have been well studied in the lowest excited state, but much less is known about the behavior following excitation to higher-lying electronically excited states. This contribution reports a combined study of the spectroscopy and dynamics of two-photon accessible states above S1. Two-photon absorption (2PA) measurements using a broadband pump-probe technique reveal distinct bands near 5.1 and 6.4 eV. The 2PA bands have absolute cross sections of 40 ± 16 and 270 ± 110 GM, respectively, and a pump-probe polarization dependence that suggests both of the transitions access Ag-symmetry excited states. Separate transient absorption measurements probe the excited-state dynamics following two-photon excitation into each of the bands using intense pulses of 475 and 380 nm light, respectively. The initially excited states rapidly relax via internal conversion, leading to the formation of an S1 excited-state absorption band that is centered near 585 nm and evolves on a time scale of 1-2 ps due to intramolecular vibrational relaxation. The subsequent evolution of the S1 excited-state absorption is identical to the behavior following direct one-photon excitation of the lowest excited state at 4.0 eV. The complementary spectroscopy and dynamics measurements provide new benchmarks for computational studies of the electronic structure and dynamics of this model system on excited states above S1. Probing the dynamics of molecules in their higher-lying excited states is an important frontier in chemical reaction dynamics.