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

Abstract

The time-dependent multiphoton ionization of xenon atoms is studied with femtosecond pulses in the excitation range of the 4d giant resonance at the photon energy of 93 eV. Benefiting from a new operation mode of the free electron laser FLASH, the measurements are performed with varying pulse durations. A strong dependence of the ion charge distribution on the pulse duration allows the different multiphoton mechanisms behind the multiple photoionization of xenon to be disentangled up to a charge state of Xe 10þ . The results up to Xe 8þ are well explained by sequences of single photon, multiphoton, and Auger processes, but higher charge state generation suggests the need for collective electron multiphoton excitations. DOI: 10.1103/PhysRevLett.112.213002 PACS numbers: 32.80.Rm, 32.80.Fb, 41.60.Cr, 42.50.Hz The highly intense and ultrashort radiation pulses of the new free electron laser (FEL) facilities in Europe [1,2], the United States [3], and Japan [4,5] can significantly extend the study of interaction of x rays with matter to the nonlinear regime of multiphoton processes. 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].A prime example is the multiphoton multiple ionization of Xe in the vicinity of the so-called 4d giant resonance in the extreme ultraviolet (EUV) between 90 eV and 110 eV photon energy where charge states up to Xe 21þ were observed at irradiance levels in the order of 10 16 W=cm 2[22]. A sequential scheme requires 19 steps and almost 60 EUV photons to generate Xe 21þ from atomic Xe, and more than a single photon is required for each step from Xe 7þ on. The role of the 4d giant resonance and the impact of collective electron excitation on this particular behavior are still-open scientific questions of fundamental importance [22][23][24][25][26]. Since the different multiphoton schemes depend in different ways on the FEL pulse duration, as has recently been summarized [27], photoionization experiments at varying pulse duration may help to disentangle the underlying mechanisms. Experiments at the Linac Coherent Light Source (LCLS) have demonstrated that sequences of inner-shell excitation in the harder x-ray regime and successive refilling via A...

Theoretical characterization of the collective resonance states underlying the xenon giant dipole resonance

“…For example, high-harmonic generation spectra of xenon driven by an intense midinfrared laser display a striking enhancement of the plateau [15], which reflects the partial cross section of the 5p valence shell strongly modified by the GDR [16]. Also, * yi-jen.chen@cfel.de † robin.santra@cfel.de the GDR lies at the heart of the behavior of xenon exposed to free-electron lasers with ultrahigh XUV irradiance [17][18][19]. Hence, it is important to fully understand the nature of the xenon GDR.…”

Collective resonances of atomic xenon from the linear to the nonlinear regime

“…Second, our study is, to the best of our knowledge, the first successful attempt to find collective multipole excitations in a realistic many-electron system. As far as experiment is concerned, the collective resonances of Xe under consideration are still confined to the dipole mode(s) [13][14][15][17][18][19]. On the theory side, [27] examines the quadrupole transition matrix elements related to the np −1 ò f resonances of Xe in a many-body framework.…”

Sensitivity of nonlinear photoionization to resonance substructure in collective excitation

“…The case of xenon ionization under the unprecedented conditions at FELs has been the subject of several investigations11121314151617, which have stimulated speculations about the influence of collective effects on the process of multiple ionization1214; furthermore, a high harmonic generation experiment on xenon18 evidenced the impact of the 4 d giant resonance on a nonlinear optical process1920. Yet, all these observations can be well understood, as far as collectiveness is concerned, in terms of the 1-photon absorption cross-sections of the various charge states of xenon131721, that is, in terms of the spectral characteristics of its linear response.…”