Retrieving intracycle interference in laser assisted XUV ionization of Ar

ARBÓ, D. G.; LÓPEZ, S. D.; KUBIN, M.; HUMMERT, J.; VRAKKING, M. J. J.; KORNILOV, O.

Budapest

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

Photoelectron spectra have been described in the literature as an interference problem in the time domain. Whereas trajectories stemming from different optical laser cycles give rise to intercycle interference energy peaks known as sidebands, these sidebands are modulated by a coarse grained structure coming from the intracycle interference of the electron trajectories born during the same optical cycle [1,2,3,4]. In this work, we show that even in experiments where interferences are missing or hidden by the varying intensities in the focal volume, the intracycle interference evidenced in the modulation of the sidebands emerges when the two angle-resolved and energy-resolved distributions taken at somewhat different laser intensity are subtracted.Figure 1. Difference of photoelectron distributions for two somewhat different laser intensities: around I ~ 6 × 1013 W/cm2 for the Ar atom ionized by an XUV pulse due to the 19th harmonic of a laser pulse of  = 800 nm. (a) Averaged TDSE calculations, (b) Experiment. The red color corresponds to positive values and the blue to negative. The experiments presented here are carried out using recently constructed XUV time delay compensating monochromator beamline [4] and a velocity map imaging spectrometer. This system is capable of producing a wavelength-selected, but short XUV pulse and thus is best suitable for studying laser-assisted XUV ionization processes. In the experiments, Atoms are ionized by 29.5 eV photons in the presence of IR field, which leads to generation of several sidebands.In Fig. 1 we observe the difference of the angle and energy-resolved distribution photoelectron distribution recorded for two slightly different laser intensities. The red color corresponds to positive values and the blue to negative ones. Near zero values of the difference are in white color. We see the resemblance between the calculated map by numerically solving the time-dependent Schrödinger equation and the measured map. In both, the distributions align around the polarization axis of both IR and XUV fields (0 and 180 deg.). We also checked that the strong field approximation is very reliable to cope with laser assisted XUV ionization. This work has been supported by the Argentine-Germany collaboration Conicet-DAAD 2015, CONICET (Argentina PIP0386), ANPCyT (Argentina PICT-2016-0296 y 2016-3029).References[1] A. K. Kazansky and N. M. Kabachnik, Journal of Physics B 43, 035601 (2010).[2] A. A. Gramajo et al., Phys. Rev. A 94, 053404 (2016).[3] A. A. Gramajo, et al., Jour. Phys. B 51, 055603 (2018).[4] M. Eckstein et al, J. Phys. Chem. Lett. 6, 419 (2015) ; M. Eckstein et al, arXiv:1604.02650 [physics.ins-det].