ANGULAR MOMENTA DYNAMICS IN MAGNETIC AND ELECTRIC-FIELD - CLASSICAL AND QUANTUM APPROACH

A standard description of the angular momentum in atomic or molecular physics is based on quantum mechanics. However, especially at large an gular-momentum limit in molecules, sometimes a classical approach grea tly simplifies problems that are extremely complicated from a quantum- mechanical viewpoint. The aim of this paper is to examine the relation ship between the classical and quantum descriptions of the angular-mom entum distribution in an ensemble of particles. At first glance, quant um and classical approaches appear to be absolutely different and have very little in common. In this paper this relationship is analyzed. I t is shown that similarities can be found between the precession of an gular momentum in the external field and the influence of this field u pon the wave-function phase of a microparticle. The importance of wave function phases is stressed even for stationary conditions, and some examples of coherent superposition of quantum states are presented. Th e general method of how to pass from the quantum approach to a classic al one is formulated. Results are visualized in the form of 3D distrib ution functions. This approach allows a graphical interpretation and t hus results in a better understanding, sometimes at first glance, of c ounterintuitive quantum results. As an example the phenomenon of align ment-orientation transition is considered. Rather than presenting only formal derivations, the emphasis is on the instructional aspects, and on illustrating the general approach.