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Journal of Physics B Atomic and Molecular Physics

Configuration interaction and singlet-triplet mixing probed by hyperfine structure of 6s14d1D2and3D1,2,3barium Rydberg states

Jörg Neukammer, Herbert H. Rinneberg

Published Nov 14, 1982• 13 citations
Publisher

Abstract

The hyperfine structure of the 6s14d configuration of barium has been measured by optical-optical double resonance. From the observed positions for the hyperfine components of the 1 D 2 and 3 D 1,2,3 Rydberg states, the singlet and triplet character as well as the amount of admixture of the 5d7d perturbing configuration was deduced. Although agreeing qualitatively, the authors' results differ quantitatively from predictions made by multichannel quantum defect theory (MQDT).

DOI
10.1088/0022-3700/15/21/005

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The hyperfine structure of the 6s14d configuration of barium has been measured by optical-optical double resonance. From the observed positions for the hyperfine components of the 1 D 2 and 3 D 1,2,3 Rydberg states, the singlet and triplet character as well as the amount of admixture of the 5d7d perturbing configuration was deduced. Although agreeing qualitatively, the authors' results differ quantitatively from predictions made by multichannel quantum defect theory (MQDT).
The hyperfine structure of the 6snd1D26snd^{1}D_{2}6snd1D2​ (10< n< 5010<~n<~5010< n< 50) Rydberg states of barium has been measured and analyzed in terms of singlet-triplet mixing. A dispersionlike resonant behavior of the mixing coefficient $\ensuremath{\Omega}$ was observed for barium and strontium in the vicinity of doubly excited configurations. In barium strong disagreement with multichannel-quantum-defect-theory wave functions was found. The present results are compared with the spin polarization and anisotropy of photoelectrons.
The hyperfine structure of 6snd1D26snd^{1}D_{2}6snd1D2​ Rydberg states of 135,137Ba^{135,137}\mathrm{Ba}135,137Ba is reported for principal quantum numbers ranging between n=10n=10n=10 and n=50n=50n=50. From our data, singlet-triplet mixing coefficients have been deduced. A three-channel quantum-defect model has been used to analyze the resonance in singlet-triplet mixing of 6snd1D2and3D26snd^{1}D_{2} \mathrm{and} ^{3}D_{2}6snd1D2​and3D2​ Rydberg states in the vicinity of the 5d7d1D25d7d^{1}D_{2}5d7d1D2​ perturber. For the determination of the scattering matrix, hyperfine-structure data were found to be essential. Good agreement has been achieved between calculated and experimental singlet-triplet mixing coefficients.
Odd-parity barium Rydberg states have been populated by stepwise excitation via M1-E1 and E2-E1 transitions, starting from the 6s2 1S0 ground state. Using two cw dye ring lasers and employing thermionic detection, Doppler-free spectra of 6snp 1,3P and 6snf 1,3F Rydberg states have been recorded with excellent signal/noise ratios. A systematic study of the hyperfine structure of 6snp 1P1 Rydberg states has been carried out. Between n=20 and n=23 a resonance in singlet-triplet mixing occurs caused by configuration interaction with the 5d8p 1P1 perturbing state, located close to the first ionization limit. Our results are in disagreement with an existing multichannel quantum defect analysis of this series. The singlet-triplet mixing of 6snp 1P1 and 6snd 1D2 Rydberg series is compared.
Singlet-triplet mixing of 4snd1D24snd ^{1}D_{2}4snd1D2​ and 3D2^{3}D_{2}3D2​ states of Ca was determined between principal quantum numbers 7< n< 427<~n<~427< n< 42 with the use of Doppler-free two-photon spectroscopy. The hyperfine structure of the 4snd1D24snd ^{1}D_{2}4snd1D2​ states was used to probe admixtures of triplet character into the singlet wave functions. It turned out that, in the region of series crossings, an enlarged singlet-triplet mixing was observed.
The MQDT of hyperfine structure is greatly simplified in extremely high Rydberg states. A four (or three)-channel quantum defect model is derived for the hyperfine structure of extremely high Rydberg states of msnd configuration in odd alkaline-earth isotopes, in which the transformation matrix U i(F)i' is given analytically except for one parameter which measures the mixing of singlet 1 D 2 and triple 3 D 2 induced by spin-orbit interaction in even isotopes, and in which the four eigen quantum defects mu i' (i'=1, 2, 3, 4) are related to the quantum defects of the fine structure in even isotopes by mu i' =mod( delta i' ). This model has been used to interpret the observed sublevel structure of 5snd (n>100) configurations in 87 Sr. An excellent agreement between the theoretical and experimental results was achieved.
A systematic study of the hyperfine structure of 5snd5snd5snd 1D2^{1}D_{2}1D2​ and 3D2^{3}D_{2}3D2​ states of 87Sr^{87}\mathrm{Sr}87Sr was carried out by Doppler-free two-photon excitation. In the region of strong singlet-triplet mixing around n=16n=16n=16, the hyperfine structure exhibits a marked nnn dependence which is related to the amount of singlet-triplet mixing.