Rotation of molecules around specific axes: Axes reorientation under rotational excitation

Abstract

Abstract Rotational energy levels of nearly spherical molecules in an isolated vibrational state are studied. It is shown that, in the limit of high rotational quantum number J , the rotational states may be interpreted as those of a stable rotation around axes properly oriented in the molecular frame. The orientation of the axes depends on J . Simple analytical solutions are given for the problem considered in the asymptotic and harmonic approximations. The results obtained possess a clear quantitative interpretation of the phenomena considered and, at the same time, agree quantitatively with the results of numerical diagonalization. The analogy between the effects of rearrangement of the rotational levels under the variation of J and the critical phenomena in macroscopic systems is discussed. The intensities of rovibrational transitions between totally symmetric vibrational states are calculated. A new selection rule is introduced which is due to a overlap of the rotational functions corresponding to the rotation around differently oriented axes.