Intra- and intercycle interferences in XUV+ IR ionization

GRAMAJO, A. A.; R. DELLA PICCA; LÓPEZ, S. D.; ARBÓ, D. G.

Budapest

Conferencia; The International Conference on Many Particle Spectroscopy of Atoms, Molecules, Clusters and Surfaces; 2018

In laser-assisted XUV photoelectric effect (LAPE) the XUV and optical lasers overlap in space and time. When the XUV pulse is longer than the laser period, a photoelectron is emitted into the optical dressing field where one or more quanta of energy can be absorbed and emitted. As a result, the photoelectron spectrum shows sidebands (SBs) separated at energy values given by [1] En = nωL + ωX - Ip - Up,(1) where ωX(L) is the frequency of the XUV (IR) pulse, Ip is the ionization potential, Up the ponderomotive energy, and n is the number of absorbed/emitted IR photons.In this work, we study the angle-resolved energy distribution of photoelectron for the case that both fields are linearly polarized in the same direction making use of the semiclassical model (SCM) [2,3,4]. We thoroughly analyze and characterize two different emission regions in the angle-energy domain: The parallel-like and the perpendicular-like regions. In the former, two classical electron trajectories per optical cycle contribute to the (intracycle) interference pattern which modulates the sidebands stemming from the (intercycle) interference of the electron trajectories at different optical cycles. In the latter, there are four classical trajectories per optical cycle contributing to the intracycle factor.In figure 1, we show the respective contributions of intracycle (figure a) and intercycle (figure b) factors to the SCM emission probability (figure c). The intracycle factor depends on both photoelectron energy and angle and it has a richer structure in the perpendicular-like region (four contributing electron trajectories) than in the parallel-like region (two contributing trajectories). The intercycle factor is mirrored as periodic stripes (sidebands) separated by the laser frequency at energies En according to Eq. (1). When both intra- and intercycle factors are considered, we obtain the spectra plotted in figure (c), where we observe that the intracycle interference pattern works as a modulation of the intercycle interference pattern.We have studied the dependence of our SCM as a function of the time delay between the IR and the XUV pulses and also as a function of the laser intensity. We also have checked the accuracy of 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 [4].Figure 1. Angle-resolved photoelectron spectra of atomic hydrogen (a) SCM intracycle factor, (b) SCM intercycle interference factor considering two optical cycles, and (c) the product both previous factors. The IR and XUV laser amplitudes are FL0 = FX0 = 0.05 a.u. with frequencies ωX = 1.5 a.u. and ωL = 0.05 a.uThis work has been supported by CONICET (PIP0386), ANPCyT (PICT-2016-0296 and 2014-2363), and UBACyT 20020130100617BA.References[1] V Véniard et al., Phys. Rev. Lett. 74, 4161 (1995).[2] A A Gramajo et al., Phys. Rev. A 94, 053404 (2016).[3] A A Gramajo et al., Phys. Rev. A 96, 023414 (2017). [4] A A Gramajo et al., J. Phys. B: At. Mol. Opt. Phys. 51, 055603 (2018).