From the very beginning of our involvement in the investigation of light induced polarization of angular momenta of molecules we were fascinated by the variety of information about the properties of molecules which they bear. At the same time the description and interpretation of these phenomena appeared to us to be extremely complicated and unclear. In fact, at times it seemed as if our computers understood the problem better than we did.

This book is an attempt to clarify the processes during the course of which polarized (ordered) angular momenta distribution is created in an ensemble of molecules in the gas phase by the effect of light. We discuss the effect of static external magnetic and electric fields on the angular momenta distribution. In particular, we wish to emphasize the ‘geometric’ meaning and interpretation of the phenomena. This may, we believe, be a further step in attempts to simplify the theoretical description, thus making it more accessible to a wide range of users, both physicists and chemists.

The fundamental basis for optical polarization (alignment, orientation) of angular momenta is the law of conservation of angular momenta in photon–molecule interaction. In this book we examine a variety of macroscopic manifestations of spatial anisotropy of angular momenta, such as angular distribution and polarization of emitted light, including changes under non-linear absorption, and the influence of collisions and external fields. Quantum angular momentum theory, in particular that which is based on irreducible tensorial set representation, presents a well-developed approach that is widely used in subatomic, atomic and molecular physics.