Odd Rydberg spectrum of Ar-40(I): high-resolution laser spectroscopy and MQDT analyses of the nd, J=4 levels and the ng ', J=4 resonances

Using transverse resonant two-photon, single-mode laser excitation of metas table Ar-40(4s P-3(2)) atoms in a collimated beam in combination with a cal ibrated travelling Michelson interferometer involving digital fringe interp olation, 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 uncer tainty of 8 x 10(-8). The Rydberg atoms are formed in a region of low elect ric and magnetic fields and detected by electron transfer to SF6 molecules in a skimmed supersonic beam. A single-channel quantum defect analysis of t he experimental data is performed, revealing weak perturbations of the rr = 12 and 20 levels by interaction with the Ar(P-2(1/2) 7g', J = 4) and AT(P- 2(1/2) 8g', J = 4) levels, respectively. The value for the ionization energ y of the Ar-40(+)(P-2(3/2)) limit has been determined as 21 647.076(2) cm(- 1) relative to the Ar-40(4p, J = 3) level and as 127 109.836(3)(+/-0.05) cm (-1) relative to the ground state of Ar-40 (see also note added in proof). Low-lying autoionizing ng', J = 4 resonances (n = 9-13) have been investiga ted and their quantum defects and reduced width determined for the first ti me. A multichannel quantum defect theory analysis yields an estimate (0.76 cm-l) for the energy separation between the perturbed 20d level and the per turbing 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 determin ed from the 8g'-induced perturbation in the discrete nd, J = 4 spectrum.