Unambiguous assignment of the van der Waals modes of benzene–Ar by analysis of the rotationally resolved UV-spectra and comparison with multidimensional calculations

Abstract

We present calculations of the van der Waals (vdW) states of electronically excited benzene-Ar in which the coupling to the vibronic angular momentum of the excited 61 state of benzene is explicitly included. It is predicted, in particular, that the vibrational angular momentum of the degenerate (E1) vdW bending fundamental leads to parallel Coriolis coupling between two substates and consequently to a perturbed rotational structure of the corresponding parallel band in the UV spectrum. A detailed analysis of these bands in the UV spectra of C6H6-Ar and C6D6-Ar is given and found to agree with the theory. This yields not only a set of accurate rotational and Coriolis coupling constants, but also an unambiguous assignment of all observed vdW transitions in the UV spectra of these complexes. The vibrational frequencies, the changes of rotational constants upon vdW excitation, and the intensities calculated with some of the available model potentials, and the isotopic shifts in these quantities are in good agreement with experiment. It is noteworthy that the Franck-Condon principle, which holds for the vibronic intensities in normal molecules, does not simply apply to the intermolecular vibrations in vdW complexes. The libration of the vibronic transition dipole moment of the monomer gives an additional source of intensity to the (non-totally symmetric) bending modes. This allows the hitherto doubted observation of the transition to the vdW bending fundamental.