Vibrational relaxation in liquids

Abstract

The infra-red spectrum of a molecular liquid is calculated directly as the power lost from a quantized, incident radiation field. Unlike previous theories the vibrational and reorientational relaxation effects of the liquid are included together with the radiation interaction. The spectrum emerges as the solution of generalized, steady-state Bloch equations in analogy with spinresonance spectroscopy. A formal expression for the vibrational dephasing time is obtained. This is devoid of particular assumptions about time scales or models and depends explicitly on reorientational matrix elements. The conventional assumptions of linear response and extreme motional narrowing need not be made in the derivation. Neither has it been assumed that the contributions of reorientational and vibrational relaxation are factorizable. In a following paper the formal expression for the dephasing time is evaluated.