Electronic transition dipole moments and excited-state alignment in linear molecules,with application to diatomic rare gas halides

The relationship between the direction of transition dipoles and the symmetry of electronic wavefunctions is investigated. In the case of $\text{Ω}\text{=}\text{1}\text{2}\text{?}\text{Ω′}\text{=}\text{1}\text{2}$ transitions, the dipole direction is not determined by symmetry alone. However, in all other cases the directions (parallel or perpendicular) are specified by simple selection rules based upon $\text{ΔΩ}$. In $\text{Ω}\text{=}\text{1}\text{2}\text{Ω′}\text{=}\text{1}\text{2}$ transitions, the transition dipole may accidentally have spherical symmetry, in which case a linearly polarized plane wave photon beam produces an unaligned excited-state distribution. These results are applied to the ultraviolet charge transfer transitions of the diatomic rare gas halides.