Application of the gauge-invariant formalism to the description of ionization by ultrastrong few-cycle pulses

摘要

We analyze energy spectra of photoelectrons ejected by strong laser pulses, using the manifestly gauge-invariantnapproach. Gauge-invariant expressions for the ionization amplitude can be obtained by grouping together allnterms of the same order in atomic potential in the standard strong-field approximation (SFA) expansion. Wencalculate energy spectra of photoelectrons for the hydrogen atom, and compare them with results based onnnumerical solutions of the time-dependent Schr¨odinger equation. Agreement, at least qualitative, is observed fornan electric-field amplitude higher than 1 a.u., and for larger energies of ionized electrons, basically beyond thenregion, where the standard SFA is usually applied.