RAMAN-VIBRONIC DOUBLE-RESONANCE SPECTROSCOPY OF BENZENE DIMER ISOTOPOMERS

The results of Raman-vibronic double resonance experiments on benzene dimer are reported. The results were obtained by mass-selective, ionization-detected stimulated Raman spectroscopies. The data pertain to the nu-1, nu-2, and nu-6 fundamentals of numerous dimer isotopomers. The results are discussed in terms of the geometry of the dimer. They show that the species is characterized by two inequivalent benzene sites with one of the sites of low and the other of higher symmetry. These two sites give rise to different Raman resonance frequencies, different vibrational dynamics, and markedly different S1<-- -->S0 vibronic spectra. It is argued that all of the experimental results are consistent with a T-shaped equilibrium geometry in which the benzene moiety at the top of the T is freely rotating about its C6 axis.