Two-colour experiments in the gas phase

Abstract

Abstract. First experiments on atomic photoionization and molecular dissociation have been performed by taking advantage of the unprecedented characteristics of the Free Electron Laser in Hamburg (FLASH) combined with a separate near-infrared (NIR) femtosecond laser. In a series of two-color experiments, the photoionization of rare gases in the presence of a strong NIR dressing field as well as the polarization dependence of this process were investigated systematically. A detailed analysis of the partial cross sections for the two-color two-photon ionization process was carried out for low dressing fields. Higher dressing fields give rise to multi-photon processes, which are observed and analyzed without undesirable interferences, a beneficial consequence of the monochromaticity of the FLASH radiation. The experimental results are compared with theoretical descriptions for two-color above threshold ionization obtained by employing second-order perturbation theory and the "soft-photon" approximation. In addition, complementary information was obtained on the sequential two-photon double ionization of Ne, which was made possible by the short and intense FLASH pulses. As a starting point for future time-resolved studies on molecular dissociation, a proof-of-principle experiment on the hydrogen diatomic system was carried out. In a typical pump-probe arrangement, excited neutral fragments, which were formed during photo-induced dissociation by the FLASH radiation, were identified via single-and multi-photon ionization induced by the time delayed optical laser.

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

“…Most of the theories of LAPE processes are based on the strong field approximation (SFA) [27][28][29]. For example, the soft photon approximation (SPA) [14], derived from the SFA in the velocity gauge for infinitely long XUV and IR pulses, depicts satisfactorily the experimental results [4,6,7,10,11,30]. Besides, the analytic angle-resolved PE spectra derived by Kazansky et al [31,32] and Bivona et al [33] are based on simplifications of the temporal integration within the SFA.…”

Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism

“…In the experiment, He atoms are ionized by XUV FEL radiation in the presence of an intense-near infra red (NIR) laser field. In the resulting photoelectron spectrum a series of lines, so-called sidebands, are observed on both sides of the normal photoline 27,28 . Since the sidebands are very sensitive to the spatial and temporal overlap and to the physical characteristics of both the XUV and NIR pulses, this type of experiment is widely used for studying multi-photon ionization dynamics 29 as well as for characterizing linearly polarized ionizing FEL pulses 30 .…”

Above-threshold ionization in neon produced by combining optical and bichromatic XUV femtosecond laser pulses

“…Many studies, both experimental and theoretical, of the sideband patterns in XUV + IR ionization have been performed (for example [18][19][20][21][22][23] sidebands [23][24][25][26][27][28][29][30][31][32][33][34]. Recently such experiments with circularly polarized XUV beams from FELs became feasible [35,36].…”