Multiphoton Double Ionization of Ar in Intense Extreme Ultraviolet Laser Fields Studied by Shot-by-Shot Photoelectron Spectroscopy

Abstract

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ∼ 21 eV, ∼ 5 TW/cm²), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e⁻ and Ar²(+) + 2hν→Ar(+) + e⁻. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).

Time-Dependent Multiphoton Ionization of Xenon in the Soft-X-Ray Regime

“…A number of fascinating FEL experiments of multiphoton ionization have already been carried out on solids, clusters, molecules, and on free atoms [ [6,7,8,9], and references therein]. Whereas in the regime of optical femtosecond lasers, simultaneous multiphoton, above-threshold, and strongfield ionization are the mechanisms of relevance [10,11], the nonlinear interaction of soft and hard x rays with atoms at photon energies above the first ionization threshold has been shown to be dominated by sequential processes in which an excited atom or ion created in a preceding step represents the target for a subsequent step [12][13][14][15][16][17][18][19][20]. For higher ionization charges and ionization thresholds which exceed the photon energy, the mechanisms may be rather complex because simultaneous multiphoton ionization processes come into play for the higher steps of an ionization sequence [12,21].…”

Electron angular distributions of noble gases in sequential two-photon double ionization

“…Light-matter interaction with multiple photons of photon-energies ranging from the VUV into the hard X-ray regime, leverages the exploration of the creation of highly charged ions. In this regard, many experimental as well as theoretical investigations have been performed concerning fundamental multi-photon ionization processes [15][16][17][18][19][20]. In most of these studies, the experimental method was restrained to the detection of ions which helped to develop models of pathways up to the highest charged states as for example Xe 36+ [15].…”

Ultrafast Nonlinear Double Excitations of He in Intense EUV FEL Fields

“…The high collection efficiency allowed us to keep the ionization event rate low enough to avoid a space charge effect and to measure photoelectron spectra on the shot-to-shot basis [1,2]. The photoelectron spectroscopy is powerful in elucidating the non-linear processes in EUV-FEL fields, because (i) electronic states involved in the ionization process can be identified directly from the photoelectron energy and (ii) the spectral properties of fluctuating FEL pulses can be evaluated on the shot-by-shot basis.…”

“…However, unlike a non-linear process in the optical regime where only the valence electrons are involved, inner-valence or inner-core electrons can also participate in multiphoton process in EUV due to the large photon energy, which may open a new ionization channel. In our previous studies [1], we have demonstrated that single-shot photoelectron spectroscopy provides detailed information on the multi-photon processes of simple atoms and molecules utilizing the inherent fluctuation of FEL pulses. Here, we apply the single shot photoelectron spectroscopy to double excitation of He in ultrashort pulse duration of EUV-FEL pulses [2].…”