Spin-electronic-rotational frame transformation for photoionization and Rydberg spectra of diatomic molecules

The spin-rotational-electronic fine structure of molecular Rydberg and continuum states is considered. The frame transformation from the laboratory frame [Hund's case (e)] to the molecular frame [Hund's case (a)] is derived. Symmetry requirements are fully incorporated into the transformation, thus facilitating its practical use. Expressions for the dipole transition amplitudes in Hund's case (a), again fully symmetrized, are given, as well as their transformation to Hund's case (e) appropriate for excitation to discrete Rydberg states and the ionization continuum. Various examples of the transformation within the framework of collision theory and of multichannel quantum defect theory are described, and it is shown how the general frame transformation reduces to the transformations for molecules without electron spin. The present formulation can also be applied to the photoionization of aligned and oriented molecules, as well as to ``complete'' experiments.