ABSTRACT
Chapter 3 explains how to determine equilibrium rotational constants using experimental ground state constants and rovibrational corrections calculated with the aid of an ab initio anharmonic force eld. This chapter demonstrates that in some cases it is possible to obtain these equilibrium constants using only spectroscopic information alone. As usual in molecular spectroscopy, the operators are written in boldface. In rovibrational spectroscopy, the experimental data are the frequencies of the transitions between the energy levels and one of the main tasks of the spectroscopist is to describe and explain the observed spectrum and to determine the molecular para meters. This chapter is not intended to be a review of molecular spectroscopy. Only the notions essential to understand how accurate (equilibrium) rotational constants are obtained will be given. The reader is directed to several reference books, starting from the historical [1-8], to the most recent [9-16]. Furthermore, a glossary as well as the denition of the quantum numbers, operators, and so on is given in Appendix IV.1.
