S-2-S-0 spectroscopy of the van der Waals complexes of azulene with rare gases

The S-2-S-0 fluorescence emission and excitation spectra of jet-cooled azulene and its complexes with the rare gases, Ne, Ar, Kr, and Xe, have been measured. Features due to AZ . RG(n) (n = 1-4 when RG = Ar, Kr, Xe and n = 1 when RG = Ne) are observed in the S-2-S-0 fluorescence excitation spectra when azulene is coexpanded with the rare gases. The microscopic solvent shifts, delta (v) over bar, of the origin bands for each complex scale linearly with the polarizability of the adatom(s), indicating that binding is dominated by dispersive interactions. This conclusion has been confirmed in calculations of the separate contributions of dispersion and induction to delta (v) over bar. Mildly anharmonic progressions in very low frequency excited-state intermolecular (van der Waals) modes are attached to the origin bands of each AZ . Kr-n and AZ . Xe-n complex. The dominant progression is assigned to single quantum changes in that excited-state bending mode which involves motion of the adatom(s) in the plane perpendicular to the azulene ring containing the long (x) axis of the molecule. This assignment has been confirmed by calculating the frequencies of the bending and stretching vibrations in the ground state by using one-dimensional Morse and Taylor's series potential functions. The most stable geometries of the n = 1-4 complexes in their ground states have been calculated by using a summation of pairwise atom-atom Lennard-Jones 6-12 potentials. The potential minima of the 1:1 complexes are located over the seven-membered ring; no second minimum is found over the five-membered ring. The most stable 1:2 species appears to be the symmetric (1 + 1) complex in which one adatom is bound on each side of the azulene surface. Some evidence of the asymmetric (2 + 0) isomer is also found.