We study the ionization of atomic hydrogen in the direction of polarization due to a linearly polarized XUV pulse in the presence a strong field IR. We describe the photoelectron spectra as an interference problem in the time domain. Electron trajectories steming from different optical laser cycles give rise to intercycle interference energy peaks known as sidebands. These sidebands are modulated by a grosser structure coming from the intracycle interference of the two electron trajectories born during the same optical cycle. We make use of a simple semiclassical model which offers the possibility to establish a connection between emission times and the photoelectron kinetic energy. We compare the semiclassical predictions with the continuum-distorted wave strong field approximation and the ab initio solution of the time dependent Schrödinger equation. We analyze such interference pattern as a function of the time delay between the IR and XUV pulse and also as a function of the laser intensity.

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 intraand 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 Schrödinger equation.

A theoretical study of ionization of the hydrogen atom due to an XUV pulse in the presence of an IR laser is presented. Well-established theories are usually used to describe the laser assisted photoelectron effect. However, the well-known softphoton approximation firstly posed by Maquet et al in Journal of Modern Optics 54 1847 (2007) and Kazansky's theory in Phys. Rev. A 82, 033420 (2010) completely fails to predict the electron emission prependicularly to the polarization direction. Making use of a semiclassical model, we study the angle-resolved energy distribution of photoelectrons for the case that both fields are linearly polarized in the same direction. We thoroughly analize and characterize two different emission regions in the angle-energy domain: (i) the parallel-like region with contribution of two classical trajectories per optical cycle and (ii) the perpendicular-like region with contribution of four classical trajectories per optical cycle. We show that our semiclassical model is able to asses the interference patterns of the angle-resolved photoelectron spectrum in the two different mentioned regions. Electron trajectories stemming from different optical laser cycles give rise to angle-independent intercycle interference known as sidebands. These sidebands are modulated by an angle-dependent coarse-grained structure coming from the intracycle interference of the electron trajectories born during the same optical cycle. We show the accuracy of our semiclassical model as a function of the time delay between the IR and the XUV pulses and also as a function of the laser intensity by comparing the semiclassical predictions of the angle-resolved photoelectron spectrum with the continuum-distorted wave strong field approximation and the ab initio solution of the time dependent Schrödinger equation