Translational and vibrational energy distributions in metastable laser pumped polyatomic molecules: A quasithermodynamic description

Abstract

A model is proposed to describe the metastable vibrational population distribution for a laser pumped polyatomic gas which undergoes vibration–vibration energy exchange on a time scale fast compared to vibration–translation/rotation relaxation. Such systems are shown to be characterized by a multiple temperature distribution with temperature relationships governed by the vibrational energy transfer pathway. In the harmonic oscillator limit each vibrational mode is at a single mode temperature, different from the translational/rotational temperature, leading to simple expressions for the steady state thermodynamic quantities. Additional constraint equations allow the complete determination of the temperature distribution and thermodynamic quantities in terms of two initial parameters, input energy and ambient temperature. Examples of multiple temperature distributions are presented for laser pumped CH3F undergoing various relaxation pathways and at a variety of initial conditions. The selection of the hottest vibrational mode is found to depend only on the path, and this mode may not necessarily be the one with the smallest energy spacing nor the one that is actually pumped. The extent of the vibrational enhancement of this mode is dependent on the initial conditions through the conservation equations. A stable multiple temperature condition is found to exist only if a restricted number of energy transfer paths are allowed.