Structure and vibrational dynamics of aniline and aniline-Ar from high resolution electronic spectroscopy in the gas phase

Rotationally resolved S-1<--S-0 electronic spectra of aniline and its single atom van der Waals complex with argon (An-Ar) have been observed. Analysis of these spectra leads to a determination of the vibrationally averaged structures of the bare molecule and the complex in the two electronic states. Aniline itself is pyramidally distorted at the NH2 group in the S-0 state. Attachment of the Ar atom on the side of the ring opposite the two N-H bonds converts the symmetric double well along the inversion coordinate into an asymmetric one, in the ground state. The excited state is quasiplanar along this coordinate. Analyses of the spectra of An-Ar at higher energies in the S-1 state provide a probe of the vibrational predissociation (VP) behavior of the complex. We observe in these spectra line broadenings and spectral perturbations from which the important role of inaa-intermolecular mode mixing (i.e., IVR) in promoting the VP process is elucidated. (C) 1996 American Institute of Physics.