24 - Rotationally resolved spectroscopy of Mg⁺–rare gas complexes

Summary

Concepts illustrated: ion–molecule complexes; photodissociation spectroscopy; Hund's coupling cases; rotational structure in open-shell molecules; least-squares fitting of spectra.

This Case Study follows on from the previous one. However, rotationally resolved photodissociation spectra are the focus here, specifically for Mg⁺–Ne and Mg⁺–Ar. Although these ions are diatomic species, their rotationally resolved spectra are not trivial to analyse. The reason for this is the presence of an unpaired electron, which gives rise to a net spin angular momentum which can interact with the overall rotation of the complex (spin–rotation coupling). In addition, in some electronic states there may also be a net orbital angular momentum, and this can interact both directly with the molecular rotation (giving rise to the phenomenon known as Λ doubling) and with the electron spin. The latter is much the strongest of these angular momentum interactions and its effect can be readily seen in the rotationally resolved spectra, as will be discussed below.

Duncan and co-workers have recorded partly rotationally resolved electronic spectra for the A²Π˗ X²Σ⁺ transitions of Mg⁺–Ne and Mg⁺–Ar, and these form the basis of the Case Study described here [1, 2]. A photodissociation technique was employed as detailed in Chapter 23. Before describing the spectra and their analysis, the expected rotational energy level structure for the X²Σ⁺ and A²Π electronic states is considered. Much of this description is similar to that met for NO in Chapter 22.