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We studied the elementary processes of excitation and ionization of atomic hydrogen in an intense 800-nm pulse with intensity in the 1.0 to 2.5×1014 W/cm2 range. By analyzing excitation as a continuation of above-threshold ionization (ATI) into the below-threshold negative energy region, we show that modulation of excitation probability and the well-known shift of low-energy ATI peaks vs laser intensity share the same origin. Modulation of excitation probability is a general strong field phenomenon and is shown to be a consequence of channel closing in multiphoton ionization processes. Furthermore, the excited states populated in general have large orbital angular momentum and they are stable against ionization by the intense 800-nm laser—they are the underlying reason for population trapping of atoms and molecules in intense laser fields

Fine structures in the intensity dependence of excitation and ionization probabilities of hydrogen atoms in intense 800-nm laser pulses