Bound, odd-parity J = 1 spectra of the alkaline earths: Ca, Sr, and Ba

Abstract

We have used multiphoton excitation via selected intermediate states to observe and identify triplet Rydberg series of the type msnp3P0 up to high n in Ca, Sr, and Ba. Previous efforts have failed to identify these series beyond n = 7. We present details of our experimental method, give tables of the newly determined energy levels, and describe their analysis by multi-channel quantum defect theory (MQDT). For the most part, the msnp3P0 series in Ca and Sr can be described by a nearly constant quantum defect δ with δCa = 19.7 and δSr = 2.90. The 6s n p3P0 series in Ba is more heavily affected by the configuration interaction, but to a reasonable approximation has δBa ≈ 3.81. For all three elements, we find that the msnp¹P10 series are much more strongly perturbed than the ³P10 series. Most of this paper treats Ba. In addition to the 3P0 states, 3F2 and 1G4 states are measured and tabulated. Further results in Ba include new and/or revised values for some ¹P10 states a revised ionization limit, evidence for an energy-dependent configuration interaction, and an MQDT prediction of the photoabsorption cross section at the ionization limit. This prediction agrees with the most recent experimental cross section. The MQDT analysis of Ba is described in great detail to make it useful as a guide to the reader who wishes to become a user of MQDT.