Sudden perturbation of hydrogen atoms by intense ultrashort laser pulses

Abstract

We study theoretically how hydrogen atoms respond to intense ultrashort laser pulses of duration τ shorter than the inverse of the initial-state energy ei−1. An analytical expression for the evolution operator S is derived up to the first order of the sudden perturbation approximation. This approximation treats the laser-atom interaction beyond the dipole approximation and yields S as a series in the small parameter eiτ. It is shown that the effect of realistic laser pulses on atoms begins at the first order of eiτ. Transitions between atomic (nlm) states of different m become possible due to the action of the pulse’s magnetic field. Transitions between states of same m and arbitrary l become possible if the static Coulomb potential is taken into account during the pulse.