This monograph describes procedures for making quantum mechanical calculations of rotational energy levels and rotational line intensities in diatomic molecules. The procedures are illustrated by sample calculations. A familiarity with the material of this monograph should enable a practicing electronic spectroscopist to carry out, though in a rather mechanical way, his own theoretical calculations for molecules under experimental investigation.
A true understanding of the procedures described below can only be acquired by studying the theoretical reference material cited. These references are not exhaustive; they represent sources which the author finds convenient and instructive. Unfortunately, the material in the references is sometimes presented in a notation different from that used here. In addition, it is sometimes slightly too general or slightly too specific to apply directly to diatomic molecules.
The material of this monograph is aimed at electronic spectroscopists who have had the equivalent of one semester of graduate-level quantum mechanics.
Finally, I should like to point out, that the procedures described here for carrying out calculations are not new. Neither do they represent all possible correct procedures. They do represent, however, a unified approach to the problem, which, in the opinion of the author, is easier to understand than the original literature cited, and less likely to lead to error than some of the alternative calculation procedures.
Note added in proof. The reader is referred to the book Rotational Structure in the Spectra of Diatomic Molecules  by I. Kovács, which just appeared and which represents another author's discussion of much of the material in this monograph.