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

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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 $2.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ ${\mathrm{W}/\mathrm{cm}}^{2}$ 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.