A combined experimental and theoretical study of the intermolecular vibrations of the o-xylene·Ar van der Waals complex is reported for both the S0 and S1 electronic states. Two-color resonant two-photon ionization and fluorescence emission spectra of the vdW mode region of supersonic jet-cooled o-xylene·Ar exhibit five bands within 70 cm -1 of the electronic origin, which arise from low-frequency large-amplitude intermolecular vibrations. Accurate quantum 3D calculations of vdW vibrational levels were performed, based on the 3D discrete variable representation. Apart from the restriction to the J=0 state the calculated eigenstates are exact for the intermolecular potential energy surface (PES) employed. The PES is represented as a sum of Lennard-Jones (LJ) pair potentials, and the direct comparison between theory and experiment enabled calibration of the LJ parameters. Very good agreement was achieved for both the S0 and S1 states of o-xylene·Ar. The quantum 3D calculations provide a quantitative description of the vdW level structure up to ≈70 cm-1 above the vdW ground state. The low-energy eigenfunctions have nodal patterns analogous to the 2,3-dimethylnaphthalene·Ar complex. However, in the energy range 40-60 cm-1 the vdW mode eigenfunctions change over to 2D radial-oscillator-type wave functions, similar to those of benzene·Ar, but switch back to Cartesian type above 60 cm-1. The S1 state vdW levels of 2,3-dimethylnaphthalene [M. Mandziuk, Z. Bačić, T. Droz, and S. Leutwyler, J. Chem. Phys. 100, 52 (1994)] were recalculated with the present parameters, and the agreement between experimental and calculated frequencies is improved.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry