Resonance feature in Al[Formula Presented] photodetachment below the Al[Formula Presented] threshold

Photodetachment of Al[Formula Presented] near the threshold of the first excited state of neutral aluminum has been studied both experimentally and theoretically. A 19-keV mass-resolved Al[Formula Presented] beam was intersected by a frequency-doubled neodymium-doped yttrium aluminum garnet pumped dye-laser beam, and the fast atoms created by detachment processes were detected. Measurements just below the threshold of the first excited state, Al[Formula Presented], find a large resonance peak in the detachment signal. An eigenchannel [Formula Presented]-matrix calculation generates a resonance structure that is qualitatively similar to the data and identifies the large resonance peak as resulting from a doubly excited negative-ion state having predominantly [Formula Presented] character. Density plots of the resonance state are presented to demonstrate its structure. Near the Al[Formula Presented] threshold, theory agrees also with recent relative measurements of D. Calabrese et al. [Phys. Rev. A 54, 2797 (1996)].