Ultraviolet resonance Raman spectroscopy: studies of depolarization dispersion and strong vibronic coupling

The application of ultraviolet resonance Raman spectroscopy to the electronic excitations of the simple π-electron systems butadiene, cyclopentadiene, benzene, acetylene, dimethylacetylene (2-butyne) and diacetylene (1,3-butadiyne) is described. Raman scattering resonant with electronic excitations of these species provides new information that permits a check on the interpretation of the corresponding absorption transitions. The determination of the depolarization ratio for Raman scattering of totally symmetric modes and its variation with excitation wavelength is shown to be a useful way to demonstrate that an electronic band consists of two or more transitions with orthogonal polarization components. Raman spectra of acetylene, dimethyl-acetylene, and benzene show evidence of strong vibronic coupling. A quantitative analysis has been developed for the benzene case where pseudo-Jahn-Teller distortion is observed. The general utility of resonance Raman spectroscopy using ultraviolet radiation as a tool in molecular spectroscopy is illustrated with these studies. Highly excited vibrational levels not seen by other methods are often observed with high intensity, overlapping electronic transitions can be detected, and the nature of Franck-Condon displacements and vibronic coupling mechanisms can be determined.