Observation of large populations in excited states after short-pulse multiphoton ionization.

Atoms subjected to an intense subpicosecond laser pulse (10 13 W/cm 2 , 100 fs) survive in excited states, as is shown by probing with a second laser. Electron spectroscopy with this 5-ns probe pulse enables identification of the resonant intermediate states as well as an unusuallyreliable energy calibration of the short-pulse electron spectrum. Only after incorporating population of intermediate levels is the mechanism for short-pulse multiphoton ionization consistent with the new data