15min:

T. EBATA, D. SHIMADA, R. KUSAKA AND Y. INOKUCHI, Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan; M. EHARA, Institute for Molecular Science, 38 Myodaiji, Okazaki 444-8585, Japan.

The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA-H₂O) in the S₁ state have been measured by picosecond pump-probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S₁ - S₀ origin is 8 - 9 ps. On the other hand, the lifetime of OMpCA-H₂O complex at the origin is 930 ps, which is 100 times longer than that. Furthermore, in the complex the S₁ lifetime shows rapid decrease at an energy of ~200 cm^-1 above the origin and becomes as short as 9 ps at ~500 cm^-1. Theoretical calculations with symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that in OMpCA, the trans - cis isomerization occurs smoothly without a barrier on the S₁surface, while in OMpCA-H₂O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA-H₂O is in good agreement with that estimated from the lifetime measurements.