UNAMBIGUOUS ASSIGNMENT OF THE VAN-DER-WAALS MODES OF BENZENE-AR BY ANALYSIS OF THE ROTATIONALLY RESOLVED UV-SPECTRA AND COMPARISON WITH MULTIDIMENSIONAL CALCULATIONS

We present calculations of the van der Waals (vdW) states of electroni cally excited benzene-Ar in which the coupling to the vibronic angular momentum of the excited 6(1) state of benzene is explicitly included. It is predicted, in particular, that the vibrational angular momentum of the degenerate (E(1)) vdW bending fundamental leads to parallel Co riolis coupling between two substates and consequently to a perturbed rotational structure of the corresponding parallel band in the UV spec trum. 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 n ot 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 ch anges of rotational constants upon vdW excitation, and the intensities calculated with-some of the available model potentials, and the isoto pic 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 th e intermolecular vibrations in vdW complexes. The libration of the vib ronic transition dipole moment of the monomer gives an additional sour ce of intensity to the (non-totally symmetric) bending modes. This all ows the hitherto doubted observation of the transition to the vdW bend ing fundamental. (C) 1996 American Institute of Physics.