11 - Rotational excitation II

Summary

Rotational excitation of photofragments is a wide field with many subtleties. In the foregoing chapters we have considered exclusively the scalar properties of rotational excitation, i.e., the distributions of final rotational states of the products and the forces that control them. For this purpose, it was sufficient to study the case that the total angular momentum of the entire molecular system is zero, J = 0. This restriction drastically facilitated the theoretical formulation and allowed us to concentrate on the main effects without being intimidated by complicated angular momentum coupling. In Section 11.1 we will extend the theory of rotational excitation to general total angular momentum states J ≠ 0. Our aim is the investigation of final rotational product states following the photodissociation of single rotational states of the parent molecule (Section 11.3). Before doing so, however, we discuss in Section 11.2 the distribution of the various electronic fine-structure states (Λ-doublet states) if the fragment possesses a nonzero electronic angular momentum which couples with the angular momentum of the nuclear motion. Important examples are OH and NO.

The vector of the electromagnetic field defines a well specified direction in the laboratory frame relative to which all other vectors relevant in photodissociation can be measured. This includes the transition dipole moment, μ, the recoil velocity of the fragments, v, and the angular momentum vector of the products, j. Vector correlations in photodissociation contain a wealth of information about the symmetry of the excited electronic state as well as the dynamics of the fragmentation. Section 11.4 gives a short introduction.