Odd Rydberg spectrum of ⁴⁰Ar(I): high-resolution laser spectroscopy and multichannel quantum defect analysis of the J = 2 and 3 levels

Using transverse resonant two-photon CW laser excitation of metastable ⁴⁰Ar(4s ³P₂) atoms in a collimated beam via the Ar(4p ³D₃) level, we have carried out a comprehensive high-resolution study of odd Ar(²P_3/2 ns, nd, J = 2, 3) Rydberg states (n = 13-90). The transition energies are determined with an uncertainty of 0.006 cm^-1 and perturbations in the spectra associated with the Ar(²P _1/2 8d′, 9d′, J = 2, 3) levels are clearly detected. Moreover, the low-lying Ar(nd′, J = 2, 3) autoionization resonances (n = 10-14) have been investigated and the respective widths (ranging from 5000 to 26 000 cm^-1) determined. A very sharp resonance (reduced width 27 cm^-1) has been detected and identified as a member of the ng′, J = 3 series. A multichannel quantum defect theory (MQDT) analysis of the measured bound Rydberg levels yields reliable eigenchannel quantum defect parameters which describe the odd J = 2 and 3 spectra of Ar in the bound as well as in the autoionization region. The MQDT-derived widths for the nd′ resonances are found to be in good agreement with the experimental results. The s-d coupling observed in the J = 2 spectrum is discussed in detail.