Theory of rotational line strengths in coherent anti-Stokes Raman spectroscopy

The general expressions for rotationally resolved CARS transitions involving molecular electronic states of arbitrary electronic angular momentum and spin are derived. All possible types of resonances (triple, double, Raman) are considered and the explicit results are presented for Hund's coupling cases (a) and (b) and intermediate (a–b) coupling. The results given are applicable to arbitrary incident laser polarizations. Calculations of practical interest are performed for three diatomics, namely OH, C2 and CH, for the triple resonance. Complete sets of closed formulae are tabulated for Hund's cases (a) and (b) for 3Π ← 3Π and 2Σ ← 2Π type transitions. With minor modifications, these expressions are also applicable to resonant degenerate four wave mixing.