Atom under an Intense Laser Pulse:
Stabilization Effect and Strong-Field Approximation

E. A. Volkova^a, A. M. Popov^a*, M. A. Tikhonov^b, and O. V. Tikhonova^a

^a Skobeltsyn Institute of Nuclear Physics, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia

*e-mail: popov@mics.msu.su

^b Moscow State University, Vorob’evy gory, Moscow, 119992 Russia

Received March 21, 2007

Abstract—Direct numerical calculations of the single-photon ionization dynamics of the hydrogen atom were
compared with the data obtained within the strong-field approximation (SFA). An analysis showed that the SFA
model accurately determines the range of electromagnetic field intensities, upon reaching of which the ioniza-
tion mode deviates from that described within perturbation theory; in particular, the ionization rate decreases
with increasing intensity. It was demonstrated that the actual ionization mechanism under an intense pulse dif-
fers significantly from the SFA predictions. For example, an analysis of photoelectron angular distributions and
energy spectra showed that the strong-field ionization features within the SFA model are primarily controlled
by the ionization channel closing effect associated with the ponderomotive shift of the continuum boundary. At
the same time, the results of direct numerical calculations of the ionization dynamics suggest that the Kramers–
Henneberger atom is formed in a strong field, which is characterized by increased stability to strong-field ion-
ization.

PACS numbers: 33.80.-b, 33.90.+h

DOI: 10.1134/S1063776107090087

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