Electron-energy-resolved measurements of the threshold shift of negative chlorine ions have been performed under the influence of a strong (4.5 x l013 W/m2) infrared laser field. The threshold shift is found to be in accordance with a full ponderomotive shift. Explanations are proposed for previously measured smaller-than-expected threshold shifts.

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Journal Phys. Rev. Lett.
Davidson, M. D, Wals, J, Muller, H. G, & van Linden van den Heuvell, H. B. (1993). Observation of full ponderomotive shift for the photodetachment threshold in a strong laser field. Phys.Rev.Lett., 71, 2192–2195.