Odd Rydberg spectrum of40Ar(I): high-resolution laser spectroscopy and MQDT analyses of thend,J= 4 levels and theng´,J= 4 resonances

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Abstract
Using transverse resonant two-photon, single-mode laser excitation of metastable 40Ar(4s 3P2) atoms in a collimated beam in combination with a calibrated travelling Michelson interferometer involving digital fringe interpolation, we have measured the energies for the Ar(4p,J = 3 nd,J = 4) transitions with principal quantum numbers n = 12-100 with a relative uncertainty of 8 × 10-8. The Rydberg atoms are formed in a region of low electric and magnetic fields and detected by electron transfer to SF6 molecules in a skimmed supersonic beam. A single-channel quantum defect analysis of the experimental data is performed, revealing weak perturbations of the n = 12 and 20 levels by interaction with the Ar(2P1/2 7g´ J = 4) and Ar(2P1/2 8g´J = 4) levels, respectively. The value for the ionization energy of the 40Ar+(2P3/2) limit has been determined as 21 647.076(2) cm-1 relative to the 40Ar(4p,J = 3) level and as 127 109.836(3)(±0.05) cm-1 relative to the ground state of 40Ar (see also note added in proof). Low-lying autoionizing ng´,J = 4 resonances (n = 9-13) have been investigated and their quantum defects and reduced width determined for the first time. A multichannel quantum defect theory analysis yields an estimate (0.76 cm-1) for the energy separation between the perturbed 20d level and the perturbing 8g´ level, predicts the transition intensity to the 8g´ level to be much weaker than that to the 20d level and shows that the g´-g coupling in the autoionization region is much stronger than the g´-d coupling determined from the 8g´-induced perturbation in the discrete nd, J = 4 spectrum.

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