High-resolution measurements and multichannel quantum defect analysis of the Kr(4p⁵(²P_1/2)nd', J = 2, 3) autoionizing resonances

Using two-step laser excitation of metastable Kr(4p⁵5s³P_2,0) atoms via selected Kr(4p⁵5p J = 1,2) levels, we have recorded the low-lying autoionizing resonances Kr(²P_1/2 nd', J = 2, 3) (n = 6, 7) at high resolution in two different experiments (atomic beam spectroscopy and optogalvanic spectroscopy in a discharge). Accurate values for the resonance positions (quantum defects) and the (reduced) widths have been determined and profile indices are reported. For comparison, multichannel quantum defect theory (MQDT) analyses, using energy-dependent MQDT parameters, have been carried out for the odd Kr(J = 1, 2, 3) levels, yielding good agreement in the quantum defects and partial agreement in the predicted reduced widths for the nd' resonances with the experimental values. Moreover, simplified phase-shifted quantum defect theory analyses of the measured bound odd Kr(J = 2 and 3) Rydberg levels were carried out with the aim of providing improved insight into the dominant decay mechanism of the resonances. We also present a summary and comparative discussion of the reduced widths for the nd'[K]_J resonances of all the rare-gas atoms Ne, Ar, Kr and Xe.