Infrared Studies of the Symmetry Changes of the 28SiH4 Molecule in Low-Temperature Matrixes. Fundamental, Combination, and Overtone Transitions

Infrared spectra of (SiH4)-Si-28 in argon and nitrogen matrixes at low temperature 6.5-20 K in the region of overtone and combination transitions were recorded for the first time. Additionally, the high-resolution spectra were obtained in the fundamental region. The frequencies and the relative intensities of all bands were determined. The set of experimental data suggests that the symmetry of molecules studied in the matrixes is different from the symmetry of the free molecules because of an interaction with the environment. The symmetry of (SiH4)-Si-28 changes from T-d to C-3v on transition from the gas phase to a nitrogen matrix and to D-2d on transition to an argon matrix. A modeling of SiH4 molecule force fields explains the experimental data as a change of a force constant of the selected SiH bond in the case of SiH4 in the nitrogen matrix or force constants of two opposite angles in the case of SiH4 in the argon matrix. In spite of small values of these changes, they result in noticeable spectroscopic effects: the band splitting and appearance of new bands in matrix spectra compared with spectra of free SiH4. The interpretation of transitions in the fundamental and combination regions was performed.