Study of the stretching vibrational band intensities of XH4 molecules employing four-dimensional ab initio (X=C and Sn) and effective (X=C and Si) dipole moment surfaces

Stretching vibrational band intensities of XH4 molecules were investigated employing four-dimensional ab initio (X=C and Sn) and effective (X=C and Si) dipole moment surfaces (DMS) in combination with the local mode potential energy surfaces. The ab initio DMS of CH4 and SnH4 calculated at the coupled cluster CCSD(T) level of theory reproduced most of the observed intensities within a factor of 1.5. The effective DMS of CH4 and SiH4 were obtained by adjusting some selected high-order terms in the ab initio DMS to fit the observed intensities. They were applied to the corresponding deuterated isotopomers yielding better results than the ab initio DMS. The intensities of the combination bands are mainly due to the interbond cross terms in the DMS for SiH4, GeH4, and SnH4, while for CH4, both diagonal and cross terms are important. The relatively strong combination band that has comparable intensity with the pure overtone was predicted at the fourth local mode manifold for SnH4. (C) 2002 American Institute of Physics.