Multipeak envelope of the above-threshold-ionization energy spectrum in a strong circularly polarized laser field

Abstract

We develop a well-known strong-field approximation in the length gauge for a two-dimensional model atom in a strong circularly polarized laser field. For excited initial states (with the principal quantum number $n=2$) outgoing photoelectrons usually show energy spectra with an envelope having two or three peaks: one always below and one always above the ponderomotive energy ${U}_{P}$. In contrast, for the ground state (with $n=1$) there is always a single peak near ${U}_{P}$. Our numerical calculations show that the multipeak effect appears for sufficiently high laser intensities $I$ and is more pronounced for higher laser frequencies $\ensuremath{\omega}$. We believe that the field parameters $\ensuremath{\omega}$ and $I$, for which the multipeak envelope in the energy spectrum of above-threshold ionization exists, will enable its experimental observation in the near future.