Ionization by short uv laser pulses: Secondary above-threshold-ionization peaks of the electron spectrum investigated through a modified Coulomb-Volkov approach

The interaction between an atom and a short laser pulse is studied in the case where both photon energies are smaller than the ionization potential and perturbation conditions prevail. Under these conditions, a full numerical solution of the time-dependent Schr\"odinger equation for a hydrogen atom initially in its ground state shows that secondary peaks show up in the above-threshold ionization (ATI) spectrum. We introduce here an easy-to-implement approximation that sheds light on these features. This approach, which is based on Coulomb-Volkov-type states, is an extension of a previous theory that applies only when photon energies are greater than the ionization potential. We show that introducing a coupling to intermediate bound states into the approach permits to nicely reproduce the full numerical electron spectrum for smaller photon energies. Further, it allows us to trace back unambiguously the secondary peaks to a manifold process, i.e., excitation of transient bound states followed by multiphoton absorption, and to show that the main ATI peaks may be enhanced by intermediate state contributions. When bound states with 1n4 are included, the approach provides excellent predictions for photon energies as low as half the ionization potential.