THEORETICAL-STUDY OF THE ELECTRONIC-SPECTRUM OF TRANS-STILBENE

The electronic spectrum of trans-stilbene in the energy range up to 6 eV has been studied using multiconfigurational second-order perturbati on theory (CASPT2). The study includes a geometry determination of the ground state. In all, 12 singlet and one triplet excited states were studied. The calculated spectrum makes it possible to assign the valen ce excited singlet states corresponding to the three bands observed in the low-energy region of the one-photon absorption spectrum. The most intense feature of the calculated spectrum corresponds to the 1(1)A(g ) --> 2(1)B(u) transition at 4.07 eV. The weakly allowed 1(1)B(u) stat e was found 0.3 eV below 2(1)B(u). Transition to the 3(1)A(g) state, c omputed at 4.95 eV, is responsible for the main band observed in the t wo-photon absorption spectrum. This state has a large weight (around 4 3%) of doubly excited configurations. The first singlet-singlet Rydber g transition (3s) appears at 5.33 eV and is weak. Implications of the present findings on the trans --> cis photoisomerization process in th e singlet manifold are discussed. A photoadibatic reaction involving t he two lowest states of B-u symmetry seems to be the most plausible me chanism to explain the origin of the small barrier observed on the S-1 surface.