Photoselection and the structure of highly excited states: Rotationally resolved spin–orbit autoionization spectrum of HCl

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Abstract
Ultraviolet two‐photon photoselection, followed by visible one‐photon absorption is applied to HCl to record the first double‐resonant spin–orbit autoionization spectrum of a hydrogen halide, and the first rotationally resolved such spectrum of HCl. The J=2 level of the F 1Δ2(v=0) Rydberg state serves as the intermediate two‐photon resonance. The ionization‐detected absorption spectrum from this initial state, scanned across the 634 cm−1 interval between the lower 2Π3/2 and upper 2Π1/2 thresholds, shows a complex system consisting of hundreds of sharp lines converging to the accessible rotational limits of the upper spin–orbit threshold. The complexity of the spectrum is attributed to the relaxed selection rules associated with dipole transitions from a state in Hund’s case (a) to a manifold approaching Hund’s case (e), in concert with the irregularities expected for angular momentum coupling intermediate between the limits of case (c) and case (e). A simple case (e) fit over the central portion of the spectrum yields tentative assignment of a number of series converging to the J+=1/2, 3/2, and 5/2 levels of the ion. No evidence is found for series converging to higher rotational levels of the ion, even though such series are not excluded by angular momentum selection rules; transitions to Rydberg electronic angular momenta required for access to states of higher J+ are found associated with series observed converging to lower rotational thresholds.