Knowledge Bank

At first glance the exchange of energy and angular momentum between a particle as light as an electron and a system as heavy as a molecular cation seems improbable and inefficient. Molecular Rydberg spectra can reveal the nature and strength of the $e-$/Ion coupling mechanisms. Our studies of the alkaline earth monohalides (CaF, CaCl, BaP), examples of uniquely simple $e-/MX+$ system consisting of two closed shell ions $(M2+$ and $X-$) where $MX+$ has enormous and easily calculable multipole moments, have revealed the outline of a simple picture of the $e-/MX+$ interaction mechanisms. There are two types of Rydberg series, \textbf{core-non penetrating} (near-integer effective principal quantum number $n\ast$,negligible {l}-mixing, rapid {l}-uncoupling, unusual sensitivity to isotopic substitution or vibrational excitation through dependence of the multipole moments on the $M2+$ to center-of-mass distance) and {core-penetrating} (severe {l}-mixing among all of the penetrating-{l}-values as manifest in $s\sim p\sim d\sim f$ supercomplexes, slow and incomplete {l}-uncoupling, and $n+-$ scaling of fine structure $parameters1$). The structure and dynamics of non penetrating series are well described by a long-range multipole $model2,3$. Each penetrating series may be viewed as built on an $n3/2$ scaled replica of a valence $state1$, where the valence state is well described by ligand field $theory4$. The picture is completed by outside the core dipole and quadrupole interactions between penetrating and non penetrating $series5$. Although by revealing the values of the ion-core multipole moments and polarizabilities, the non penetrating series should pay a central role in describing the $e-$/ion interaction, these series have been surprisingly resistant to direct, systematic exploration.