An improved dipole moment model for the intensities of the vibrational absorption of XY4 near the local mode limit

A bond-bond coupling dipole model is discussed for the absorption of the local-mode vibrations of XY4 (Y = H, D). In this model the interaction between individual bonds is considered and cross terms occur in the dipole moment. The general forms of the coupling parameters αii, βij, χij for XY4 are reported and their values for the molecules (X = Si, Ge) are calculated. This model is tested with experimental intensity data for silane and germane, and the results agree. The reason for the (3000) band of silane or (4000) band of germane being much weaker than the (2100) band of silane or (3100) band of germane can be explained with this model.