Intermolecular vibrations of o-xylene·Ar in the S₀ and S₁ states: Experiment and quantum three dimensional calculations

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.