Rotational Relaxation of Molecules

Abstract

Rotational relaxation, the process of evolution towards equilibrium between rotational and translational degrees of freedom (R-T processes), takes place, for the majority of molecular gases, with a characteristic time of the same order as the time of translational relaxation. This is evident from (1.1), if the rotational relaxation time T RT is rewritten as: $$^\tau RT = \frac{{E_{rot}^0}}{{\left( {d{E_{rot}}/dt} \right){E_{rot}} = 0}}$$ (2.1) Here, E rot is the rotational energy of molecules per unit volume and E 0rot is the equilibrium value of E rot. To estimate τ RT, let us assume, as in Chap. 1, that relaxing molecules BC (rotators with mass 2m B) are admixed as a small impurity of density N BC in an atomic gas A (atoms with mass m A) assumed in equilibrium at temperature TA. As E 0rot = NBC kT A, the order of magnitude of the ratio τRT/τTT will be determined by the ratio ∆∈ rot(c) /∆∈(c)where ∆∈rot(c)and ∆∈(c) are the rotational and translational energy, respectively, transferred to a molecule BC, at rest and non-rotating, in a single collision with an atom A of velocity c in absolute value.KeywordsDiffusion ApproximationRotational EnergyRotational TemperatureRotational LevelInterstellar CloudThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.