Characterization of spin–orbit autoionizing Rydberg states excited via one-photon absorption from the F 1Δ2 Rydberg state of HBr

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Abstract
Rotationally and parity resolved excitation spectra of autoionizing Rydberg states of HBr in the energy region between the 2Π3/2 and 2Π1/2 ionic thresholds have been obtained in a double resonant excitation scheme via single rotational levels of the vibrationless F 1Δ2 Rydberg state. A cursory examination of these spectra reveals the presence of s, p, d, and f Rydberg series. Apart from the f series, which show almost Hund’s case (e) coupling, these series clearly exhibit an angular momentum coupling scheme intermediate between Hund’s case (c) and (e). As a result it is difficult to assign them as converging upon specific ionic rotational thresholds. A detailed analysis of the excitation spectra has consequently been performed employing multichannel quantum defect theory calculations, allowing for a determination of the quantum defects of the Hund’s case (a) basis states and the relevant transition moments, and, concurrently, the assignment of nearly all the observed autoionizing resonances.