Overtone spectrum of SiH stretching in H₂SiCl₂

The overtone spectra of H₂SiCl₂ molecule in the regions of 2000-9000cm^-1 and 12000-12900cm^-1 at room temperatures have been studied by use of high-resolution Fourier transform spectroscopy and sensitive-intracavity-laser absorption spectroscopy, respectively. The variations of vibrational quantum numbers ΔV _SiH=1, 2, 3, 4 and 6 for the overtones of the SiH stretching have been analysed and assigned with the local mode model and the normal mode model. The values of harmonic frequency ω_m, anharmonicity constant Χ_m, bond coupling constant λ, the Morse oscillator parameters D_e, α, and interaction force constant f _rr′ are derived from the experimental spectrum with nonlinear least-squares fitting. The most striking feature of the SiH₂Cl ₂ is that the larger the vibrational energy, the smaller the energy difference between a couple of lowest stretching states of a given manifold, and finally, the couple of lowest stretching states are degenerated within the experimental error for ΔV_SiH ≥4 vibrational manifolds. The degenerate energy level structure resembles that of a diatomic Morse oscillator; the transition energies show a remarkable fit to the Birge-Sponer relation. The high vibrational states can be described straightforward with a SiH diatomic Morse oscillator wavefunction, this is an indication of vibrational bond localization.