Colloquia

CaF is as “not-atom” as a diatomic molecule can be. The core-penetrating and core-nonpenetrating Rydberg states of CaF are observed by two-color Resonance Enhanced Ionization spectroscopy. The observed rovibronic energy levels are input to an energy- and internuclear distance-dependent Multichannel Quantum Defect Theory fit model. The fitted quantum defect matrix, μ(E,R), accounts for nearly all spectra and dynamics of CaF. A “zone of death” is observed, where selection-rule-shattering “indirect” interactions of all Rydberg states with each other, is caused by one repulsive electronic potential curve. A STIRAP-like, multiphoton, chirped pulse, millimeter wave scheme for “jumping over” this zone of death is being developed. Progress toward “pure electronic spectroscopy” and magnetic resonance-like manipulation of molecular Rydberg states requires taking a step that Arthur Schawlow would have liked, back from CaF, with its one atom too many, to the Ca atom. 5 kilo-Debye Rydberg-Rydberg transitions in Ca are directly detected by Free Induction Decay signals, rather than indirectly, via ions or UV fluorescence, in a pulsed supersonic jet.