LOWEST ENERGY EXCITED SINGLET-STATE OF ISOLATED CIS-HEXATRIENE

In a previous letter [J. Chem. Phys. 92, 4622 (1990)] we reported the first observation of the 2 1A(g) state of cis-hexatriene in a supersonic jet expansion by using resonance enhanced multiphoton ionization spectroscopy. Here, the vibrational analysis of the 1 1A(g) --> 2 1A(g) excitation spectrum of cis-hexatriene is presented. The excitation spectrum shows that cis-hexatriene in the 2 1A(g) state deviates slightly from planarity; a conclusion which is corroborated by ab initio calculations indicating that the nonplanarity primarily involves the terminal hydrogen atoms. Except for observable intensity in the low frequency modes associated with the small out of plane distortion, the vibronic development of the 1 1A(g) --> 2 1A(g) transition in cis-hexatriene is similar to that observed for other polyenes: the 0-0 transition is the most intense feature and the next most intense band is the C=C stretching fundamental. Thus the general features of the electronic structure of the cis-hexatriene 2 1A(g) state are analogous to those of other polyenes.