Intra- and intercycle interference of angle-resolved electron emission in laser assisted XUV atomic ionization.

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 problem of laser assisted photoelectron effect. However, the well-known soft-photon 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 simple 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 Schr\"{o}dinger equation.