Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. I. Spectral theory

A unified non-Markovian theory for the vibrorotational Raman and infrared s pectra of polar diatomic molecules diluted in nonpolar fluids is presented. From this theory, the physical basis of the spectra can be interpreted in terms of a few molecular properties of the isolated diatomic and of the tim e autocorrelation functions determining the collective effects of the solve nt molecules on the vibrorotational dynamic of the diatomic. The spectrum i s obtained as a diagonal part, constituted by an additive superposition of lines accounting for the integrated intensity, and an (exact) nondiagonal p art accounting for the redistribution of intensity due to interbranch and i ntrabranch mixing effects. This theory generalizes previous theoretical fra mes based on a secular contribution modified by an (approximate) interferen ce term. Also it allows the comparative analysis of the Raman and infrared spectra, and gives a clear and consistent interpretation of the theoretical lines building up the spectra. (C) 1999 American Institute of Physics. [S0 021-9606(99)51047-7].