Ionization-detected optical-optical double resonance spectroscopic studies of moderate energy Rydberg states of calcium monofluoride

This paper describes a systematic investigation of quasi-bound Rydberg states of calcium monofluoride (CaF) existing between the moleculeʹs u+ = 0 and 1 ionization thresholds. Experiments utilized ionization-detected optical-optical double resonance spectroscopy to assign states as belonging to one of the six core-penetrating ( l<=2) or to a core-nonpenetrating (l>= 3) Rydberg series. Most states observed had effective principal quantum number, n, between 12 and 18 and one quantum of vibrational excitation in the CaF+ ion-core, although lowern , u = 2 states were also identified. Core-nonpenetrating states were observed both directly and through avoided crossings with core-penetrating states. Five of the seven L components in the f-complexes derived from Ca+, 13f and n = 14f, have been identified. We present a detailed analysis of the CaF electronic structure for 12.5 <=n <=14.6, u = 1 using an effective Hamiltonian model to describe CaF+ ion-core-induced l -mixing between l<=3 (s,p, d, and f) Ca+ atomic orbitals. An observed avoided crossing between the 14.19 2S+, u = 1 and 14f (L = -3), u = 1 states implies that the previously identified 0.19 S+ core-penetrating series has 20-30% f 2u+-character. The effective Hamiltonian approach accounts for much of the data, however, a complete accounting requires the use of multichannel quantum defect theory (MQDT). An MQDT analysis of the data presented here is provided in a companion paper by Jungen and Roche in this issue. The effective Hamiltonian model enabled derivation of electrostatic properties of the CaF+ core as well as the 0.14D series quantum defect derivative, [dd/dR]Re+, which governs the exchange of energy between the Rydberg electron and the CaF+ ion-core. The CaF+ electric quadrupole moment, defined with the coordinate origin at the center-of-charge, is 11.3 ± 0.5 a.u.