D. G. Arbó

Above-threshold ionization of atoms by resonant XUV laser pulses

Above-threshold ionization of atoms by XUV short laser pulses with frequencies close to the resonant 1s?2p transition is investigated. We present a theory based on a variational expression using trial wavefunctions for the final and the initial states. For the former we use a Coulomb?Volkov wavefunction, and for the latter a close-coupling solution of the time-dependent Schr?dinger equation considering a few bound states. The close-coupling Coulomb?Volkov theory, fully accounting for the important 1s?2p transition, explains the photoelectron spectrum as well as the total ionization cross sections for the resonant case. We also compare the partial wave populations and angular distributions given by the theory with the numerical solutions of the time-dependent Schr?dinger equation.

Electron emission perpendicular to the polarization direction in laser-assisted XUV atomic ionization

We present a theoretical study of ionization of the hydrogen atom due to an XUV pulse in the presence of an IR laser with both fields linearly polarized in the same direction. In particular, we study the energy distribution of photoelectrons emitted perpendicularly to the polarization direction. By means of a very simple semiclassical model which considers electron trajectories born at different ionization times, the electron energy spectrum can be interpreted as the interplay of Intra and Intercycle interferences. The intracycle interference pattern stems from the coherent superposition of four electron trajectories giving rise to (i) interference of electron trajectories born during the same half cycle (Intrahalfcycle interference) and (ii) interference between electron trajectories born during the first half cycle with those born during the second half cycle (Interhalfcycle interference). The intercycle interference is responsible for the formation of the sidebands. We also show that the destructive interhalfcycle interference for the absorption and emission of an even number of IR laser photons is responsible for the characteristic sidebands in the perpendicular direction separated by twice the IR photon energy. We analyze the dependence of the energy spectrum on the laser intensity and the time delay between the XUV pulse and the IR laser. Finally, we show that our semiclassical simulations are in very good agreement with quantum calculations within the strong field approximation and the numerical solution of the time-dependent Schrodinger equation.

Intra- and intercycle interference of electron emissions in laser-assisted XUV atomic ionization

Fil: Gramajo, Ana Alicia. Comision Nacional de Energia Atomica. Gerencia del Area de Energia Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

A doubly distorted-wave method for atomic ionization by ultrashort laser pulses

We study a time-dependent distorted-wave formulation of atomic ionization by short laser pulses named the double-distorted Coulomb–Volkov (DDCV) approximation. The method takes into account the distortions introduced by the laser field in the initial and final channels, both by means of the Volkov phase. The DDCV model is applied to evaluate electron emission distributions for hydrogen atoms ionized by multi-cycle laser pulses. Results are compared with the predictions of an exact treatment based on the numerical solution of the time-dependent Schrodinger equation (TDSE) and with values derived from the standard Coulomb–Volkov (CV) approach, considering different intensities and frequencies of the laser field. Good agreement with the TDSE solution has been obtained for laser frequencies higher than 0.1 au, extending the range of applicability of the usual CV-type methods.

Intracycle interference in laser assisted XUV atomic hydrogen ionization

Laser assisted photoemission (LAPE) corresponds to two-color multiphoton ionization, where one of the fields has high frequency compared to the other. Depending on the pulse durations, two regimes can be distinguished: (i) the XUV pulse is much shorter than the IR period, i.e., streak camera [1], and (ii) the XUV pulse is longer than the laser period [2], with the ensuing formation sidebands (SBs) due to the absorption/emission of additional laser photons.

Non-constant ponderomotive energy in above threshold ionization by intense few-cycle laser pulses

We analyze the above threshold ionization of hydrogen atom by strong and short laser pulse with a simple envelope temporal dependence. We find additional peaks in the resulting ATI spectra originated from different values of the ponderomotive energy and the interference due to the electronic emission at different times.

Time delays in above-threshold ionization

We theoretically analyze time-resolved above-threshold ionization as probed by an interferometric measurement technique. We investigate the time delays in the ATI process as a function of the number of involved photons and the intensity of the ionizing UV field and identify IR-probe field induced contributions to the time shifts.

Doubly-differential intracycle interference in above threshold photoionization

We analyze the doubly-differential momentum distributions of electrons ejected at the interaction of strong IR laser pulses with atoms. With the help of a semiclassical model we clarify the interplay between intracycle and intercycle interferences. The model results are compared with numerical solutions of the time-dependent Schrodinger equation for atoms with long-range potentials. Similarities and differences will be discussed.

Interference of electrons ionized by short laser pulses

We analyze the two-dimensional momentum distribution of electrons ionized by short laser pulses by solving the time-dependent Schrodinger equation. Lindner et al., [Phys. Rev. Lett. 95, 040401 (2005)] identified oscillations in the electron emission spectrum for ionization by a few-cycle pulse as a time-double slit interference. We extend this analysis to interference fringes in the momentum distributions. For longer pulses we find a complex two-dimensional interference pattern that resembles ATI rings at higher energies and displays Ramsauer - Townsend - type diffraction oscillations in the angular distribution near threshold.