Rotational state distributions of the photofragments provide a wealth of information on the dissociation dynamics. The total available energy often exceeds 1 eV which suffices to make many rotational states energetically accessible. Since the torque imparted to the rotor is typically large, it is not unusual that 50 or even more rotational states become populated during dissociation. Modern detection methods, on the other hand, make it feasible to resolve even the most detailed aspects of rotational excitation: scalar properties, such as the final state distribution as well as vector properties, such as the orientation of the angular momentum vector of the fragment with respect to any axis of reference. In the same way as the vibrational distribution reflects the change of the bond length of the fragment molecule, the rotational distribution elucidates the change of the bond angle along the reaction path.

In this chapter we discuss only the scalar aspect of rotational excitation, i.e., the forces which promote rotational excitation and how they show up in the final state distributions. The simple model of a triatomic molecule with total angular momentum J = 0, outlined in Section 3.2, is adequate for this purpose without concealing the main dynamical effects with too many indices and angular momentum coupling elements. The vector properties and some more involved topics will be discussed in Chapter 11.