Shifts of the vibration–rotation absorption lines of diatomic molecules perturbed by diatomic polar molecules. A theoretical analysis

Abstract

In the frame of the second order time‐dependent theory of the molecular line shape, the line shifts for diatomic molecules perturbed by diatomic ones are carefully analyzed. Particular emphasis is put on two types of physical situations depending upon the nature of the perturber with respect to that of the active molecule. The available literature presents data which may be classified in two categories. The first one (here called ’’normal shifts’’) is illustrated by the vibration–rotation line shifts of HCl–HCl, HF–HF, and HCl–DCl and exhibits an aperiodic behavior of the line shifts vs the initial rotational quantum number. The second one (called ’’oscillating shifts’’) is illustrated by the vibration–rotation line shifts of DCl–HCl and DF–HF and is characterized by an oscillating behavior with a semiperiod of one rotational quantum number. These behaviors are particularly well reproduced by the present calculations in a quantitative way. A detailed analysis of the physical mechanisms responsible for these two types of behavior is presented and permits a precise determination of the nature of the collision induced transitions included in these mechanisms. Moreover, an extension of this study is proposed in order to predict changes of the phase of the oscillations. This is the case, for instance, for HCl perturbed by HF in the O–2 band.