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.

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

Using a pulsed tunable dye laser pumped by a nitrogen laser, the 5s5p 1P10 level in Sr I was selectively populated from the 5s2 1S0 ground state. A simultaneously pumped broad-band dye laser provided a background continuum for photographic registration of absorption spectra. Transitions from the 5s5p 1P10 level to 5sns 1S0 levels and 5snd 1D2 levels were observed with principal quantum number up to n = 36 and n = 52, respectively. Substantial population in the lower lying energy levels 5s4d 1D2 and 5s4d 3D1,2,3 was obtained through radiative and collisional processes, making it possible to get simultaneous registrations of long absorption series out of these energy levels. Transitions from the 5s4d 1D2 level to 5snp 1P10 levels and 5snf 1F03 levels were observed up to n = 29 for both series. From the 5s4d 3D1,2,3 energy levels, transitions were obtained to the 5snf 3F02,3,4 series up to n = 28. An ionization energy of 45 932.09 ± 0.15 cm-1 has been determined for series converging to the 5s 2S1/2 level of Sr II.

https://iopscience.iop.org/article/10.1088/0031-8949/18/4/002/pdf

Rydberg Series in Strontium Found in Absorption by Selectively Laser-Excited Atoms

Static dipole polarizabilities for the neutral and ionized $M$, ${M}^{+}$, and ${M}^{2+}$ group 2 elements from $M=\mathrm{Ca}$ to Ra are predicted from relativistic coupled-cluster calculations using finite field techniques together with carefully optimized Gaussian-type basis sets. Useful relations between the dipole polarizability and the ionization potential are established.

Four-component and scalar relativistic Douglas-Kroll calculations for static dipole polarizabilities of the alkaline-earth-metal elements and their ions from Ca n to Ra n ( n = 0 ,+1,+2)

Extensive new data on the bound even-parity J=2 spectrum of Ba provide an excellent opportunity for the application of multichannel quantum-defect theory (MODT) to this spectrum. The parametrisation of the J=2 energy levels is successfully carried out using a nine-channel MQDT model involving four limits. The influence of large spin-orbit coupling effects and configuration interaction is analysed. The mutual interaction of the 6snd1D2 and 6snd3D2 Rydberg series as well as the perturbations of these by levels pertaining to the 5dns (n=7,8), 5dnd (n=6,7) and 6p2 configurations are clearly interpreted.

Multichannel quantum-defect analysis of the bound even-parity J=2 spectrum of neutral barium

Observations of the Stark structure of barium for Rydberg states in the region of n=12 are presented. The Stark manifold includes a valence state which perturbs the 1D2 series. General features of the interaction of the valence and Rydberg states in the presence of the electric field are explained by simple arguments based on configuration mixing.

Stark structure of barium Rydberg states

Using a pulsed tunable dye laser either of the 6s6p 1P10, 5d6p 3D10 or 5d6p 3Pd10 levels in Ba I could be selectively excited from the 6s2 1S0 ground state. A simultaneously pumped broad-band dye laser provided a back-ground continuum for photographic registration of absorption spectra. By quantum defect analysis, series members and perturbing terms of 6sns 1S0 were extended from n=9 to n=31, of 6snd 1D2, from n=41 to n=52, and of 6snd 3D2, from n=11 to n=28. In addition, a change of previous assignments is suggested for 6p2, 8s, 9s 1S0 and 10d, 11d 3D2. Argon or neon was used as a buffer gas. For lines going to levels with high quantum numbers (n>or approximately=12), argon gave a pressure shift to the red of about 0.3 cm-1 per 100 Torr and neon a blue-shift of about 0.015 cm-1 per 100 Torr.

https://iopscience.iop.org/article/10.1088/0022-3700/10/3/011/pdf

J R Rubbmark

Papers

Absorption spectroscopy of laser-excited barium