Strong laser-field effects in hydrogen: High-order above-threshold ionization and photoelectron angular distributions

The problem of the interaction of the hydrogen atom with a: strong laser pulse has been tackled by solving the time-dependent Schrodinger equation via the state-specific expansion approach [Th. Mercouris et al., Phys. Rev. A 50, 4109 (1994)]. We present above-threshold ionization (ATI) spectra and study the frequency ((h) over bar omega(laser)=2-4 eV) and intensity (I-0 less than or equal to 2x10(14) W cm(-2)) dependence of the plateaulike structure appearing at high photoelectron energies. Our findings are qualitatively consistent with experimental data recently obtained in rare gases and render support to the proposed tunneling-rescattering mechanism for the interpretation of the observed features. Additional information on the dynamics of the laser-atom interaction is obtained by studying the frequency and intensity dependence of the photoelectron angular distributions (PADs). In agreement with recent experimental discoveries in rare gases, an appreciable sidelobe structure is found in PADs corresponding to high-order ATI peaks, for low laser frequencies ((h) over bar omega(laser) less than or equal to 3 eV). The connection of the sidelobe structure in the PADs to the ATI plateau is explored and evidence for a common underlying mechanism is produced. Our results suggest that the electron tunneling-rescattering mechanism is not the only one contributing to the appearance of the sidelobe structure in the PADs and the formation of the ATI plateau.