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

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 omega. We believe that the field parameters. 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.