Vibrational self-consistent field approach to anharmonic spectroscopy of molecules in solids: Application to iodine in argon matrix

An extension of the vibrational self-consistent field (VSCF) method is deve loped for quantitative calculations of molecular vibrational spectroscopy i n a crystalline solid environment. The approach is applicable to fields suc h as matrix-isolation spectroscopy and spectroscopy of molecular crystals. Advantages of the method are that extended solid vibrations and their coupl ing to intramolecular modes are incorporated, and that the treatment includ es anharmonic effects, both due to the intrinsic property of individual mod es and due to coupling between modes. Suitable boundary conditions are adop ted in treating the solid environment. In applications, e.g., molecules in rare-gas crystals, hundreds of coupled molecular and matrix modes can be ha ndled computationally. The method is applied to the vibrational matrix-shif t of iodine in an argon matrix, and the calculated overtone frequencies are compared to experimental values obtained from both time-domain coherent Ra man and frequency-domain Resonance Raman measurements. The physical origin of the shifts is interpreted in detail, and the properties of the iodine-ar gon interactions essential to obtain the correct sign and magnitude of the shift are elucidated. An I-2-Ar potential, based on anisotropic atom-atom i nteractions and fitted to ab initio calculations, gives the best agreement with experiment. The results show that the VSCF solid-state approach is a p owerful tool for matrix spectroscopy. (C) 2001 American Institute of Physic s.