Rydberg series of BaF: perturbation-facilitated studies of core-non-penetrating states

Core-non-penetrating molecular Rydberg states sample the electronic structure of the ion core in a uniquely simple way. However, radial overlap and orbital angular momentum propensity rules (Delta l = +/-1) can severely restrict spectroscopic access to l > 3 non-penetrating states, even via. two-colour two-step excitation schemes. Owing to the profoundly different l-uncoupling behaviour of moderate-n* (n* approximate to 6-15) core-penetrating versus non-penetrating states, numerous and systematically repeated perturbations of the Hund's case (b) core-penetrating states (accidentally located near integer n*) by the case (d) non-penetrating states (always located near integer n*) can provide a valuable window into these information-rich, yet difficult to observe, core-non-penetrating states. We report analysis of core-penetrating (l less than or equal to 3)-core-non-penetrating (l > 3) perturbations in the fluorescence detected optical-optical double resonance spectrum of BaF. One of the perturbers of the 0.86(2) Phi series is assigned as the lowest energy component, L = +5, of an h complex (l = 5). Other core-nonpenetrating perturbers are assigned as components of g and h complexes. A critical aspect of the assignments of these quite weak and fragmentary interactions between one core-penetrating 'bright' state and many core-non-penetrating 'dark' states is the systematic (n* scaling of both matrix element and N values of the level crossing) repetition of each family of perturbations in each successive n* complex.