On the theory of spectral line broadening : vibrational dephasing and resonance energy transfer in molecular liquids

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Abstract
Starting from a microscopic description a theory is presented which investigates the vibrational line-broadening mechanisms in dense molecular fluids. The paper consists of two parts. (i) Within the Zwanzig-Mori representation of the vibrational correlation functions, a formalism is developed which allows a unified and systematic description of the various line-broadening processes (pure dephasing, vibrational relaxation, resonant and non-resonant vibrational transfer) within the concepts of liquid-state theory. It is found that the vibrational linewidth cannot be represented as a sum of individual linewidths related to these particular mechanisms. (ii) Including only quasi-elastic processes (i.e. transition frequency modulations and vibrational resonance transfer), semiquantitative models are presented which describe the explicit time behaviour (in particular the exponential decay time of the vibrational correlation function. By means of a simplified interaction model we obtain numerical estimates for the dephasing time τ of liquid nitrogen that agree very well with experimental observations. It turns out that the most significant contribution to τ arises from the weak anharmonicities of the molecular oscillator.