Observation of Two-Photon Above-Threshold Ionization of Rare Gases by xuv Harmonic Photons

Miyamoto, Naoyuki; Kamei, Masaki; Yoshitomi, Dai; Kanai, Teruto; Sekikawa, Taro; Watanabe, S.; Nakajima, Takashi

doi:10.1103/physrevlett.93.083903

Cited by 87 publications

(40 citation statements)

References 25 publications

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“…The very first results were reported in breakthrough experiments at the very limit of intensities that can be obtained with these high-order harmonic sources. These included two-photon single and double ionization of He [4][5][6] and single ionization of the valence shells of Ar and Xe [7]. However, in focused beams, FLASH yields irradiance levels which can approach 10 16 W cm À2 [8], and so it provides a platform for a dramatic expansion of the range of sequential and simultaneous multiphoton ionization experiments that could be performed at EUV wavelengths.…”

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Two-Photon Inner-Shell Ionization in the Extreme Ultraviolet

et al. 2010

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We have observed the simultaneous inner-shell absorption of two extreme-ultraviolet photons by a Xe atom in an experiment performed at the short-wavelength free electron laser facility FLASH. Photoelectron spectroscopy permitted us to unambiguously identify a feature resulting from the ionization of a single electron of the 4d subshell of Xe by two photons each of energy ð93 AE 1Þ eV. The feature's intensity has a quadratic dependence on the pulse energy. The results are discussed and interpreted within the framework of recent results of ion spectroscopy experiments of Xe obtained at ultrahigh irradiance in the extreme-ultraviolet regime. DOI: 10.1103/PhysRevLett.105.013001 PACS numbers: 32.80.Rm, 32.80.Fb, 32.80.Hd, 42.50.Hz The advent of high-intensity extreme-ultraviolet (EUV) and x-ray free electron lasers (FELs) has heralded a new era in the study of nonlinear optical processes. These facilities put this well established area into a new domain where the optical field exhibits a high degree of coherence in addition to an unprecedented combination of high average and peak intensity at high photon energy. As a result, inner-shell electrons and, indeed, highly correlated multielectron excited states can become important mediators of the multiphoton-matter interaction. Since it began operation in 2005, the EUV Free Electron Laser in Hamburg (FLASH) [1] has hosted a wide range of investigations in atomic and molecular physics including experiments on dilute targets such as molecular ions and highly charged ion beams, few-photon few-electron ionization processes, two-color coherent processes, and ultrafast pump-probe experiments [2,3].The simplest nonlinear process one can drive in an intense EUV laser field is two-photon ionization. Consequently, it is a prototypical process and its pursuit has become an important benchmark experiment for the study of the response of the simplest quantum systems, ranging from atoms to clusters, to intense high-frequency electromagnetic fields. Some years ago, high-order harmonic sources developed to the point where they could attain the threshold intensities needed to drive two-photon processes in an atom. The very first results were reported in breakthrough experiments at the very limit of intensities that can be obtained with these high-order harmonic sources. These included two-photon single and double ionization of He [4][5][6] and single ionization of the valence shells of Ar and Xe [7]. However, in focused beams, FLASH yields irradiance levels which can approach 10 16 W cm À2 [8], and so it provides a platform for a dramatic expansion of the range of sequential and simultaneous multiphoton ionization experiments that could be performed at EUV wavelengths. A good example is an experiment where FLASH permitted the full kinematics of two-photon double ionization of the valence shell of Ne to be recorded by using the cold target recoil ion momentum spectroscopy technique [9].Exceptional behavior of the light-matter interaction at high irradiance has been demonstrated in two...

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Two-Photon Inner-Shell Ionization in the Extreme Ultraviolet

et al. 2010

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“…When focused into a spot of a few micrometers in diameter, the radiation can reach peak irradiance levels of more than 10 13 W cm −2 where nonlinear effects such as atomic multiphoton ionization occur. [3][4][5][6] A key point for the understanding and theoretical description of nonlinear processes is, in general, their dependence on irradiance. Therefore, among other quantities such as pulse energy and duration, spot-size determination of focused high-intensity VUV and XUV radiation is mandatory.…”

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Method based on atomic photoionization for spot-size measurement on focused soft x-ray free-electron laser beams

Sorokin

Gottwald

Hoehl

et al. 2006

Applied Physics Letters

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A method has been developed and applied to measure the beam waist and spot size of a focused soft x-ray beam at the free-electron laser FLASH of the Deutsches Elektronen-Synchrotron in Hamburg. The method is based on a saturation effect upon atomic photoionization and represents an indestructible tool for the characterization of powerful beams of ionizing electromagnetic radiation. At the microfocus beamline BL2 at FLASH, a full width at half maximum focus diameter of ͑15± 2͒ m was determined. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2397561͔ Recent progress in developing pulsed high-power vacuum ultraviolet ͑VUV͒ and soft x-ray uv ͑XUV͒ sources, such as higher-harmonic generation ͑HHG͒ sources 1 and free-electron lasers ͑FELs͒, 2 has opened the door to extended research on nonlinear interaction of electromagnetic radiation with matter from the optical region to shorter wavelengths. When focused into a spot of a few micrometers in diameter, the radiation can reach peak irradiance levels of more than 10 13 W cm −2 where nonlinear effects such as atomic multiphoton ionization occur. 3-6 A key point for the understanding and theoretical description of nonlinear processes is, in general, their dependence on irradiance. Therefore, among other quantities such as pulse energy and duration, spot-size determination of focused high-intensity VUV and XUV radiation is mandatory.Recently, two conventional methods have been applied to measure the spot size of focused HHG beams, namely, the knife-edge 7 and the fluorescence screen technique. 1,[8][9][10] Both methods provide information on two-dimensional photon intensity distribution with a spatial resolution of 1 to 2 m. The possibility of applying these techniques to FELs is, however, limited. The FEL pulse energy levels of VUV and XUV radiation may be some orders of magnitude higher than those of HHG sources and can cause radiation damage on fluorescence screens and, in general, on any solid irradiated surface. Moreover, due to its statistical nature, a beam of FEL radiation based on self-amplified spontaneous emission 2 may strongly fluctuate from shot to shot, perpendicular to the propagation axis, and requires spot-size measurements which do not depend on the beam position. In this context, we describe a method which has been used to determine the beam waist and spot size of a focused beam at the XUV-FEL facility FLASH of the Deutsches Elektronen-Synchrotron in Hamburg. 11 It is based on a saturation effect upon photoionization of a rare gas and manifests itself by a sublinear increase in the ion yield as a function of the photon number per pulse. It is due to a considerable reduction in the number of target atoms within the interaction zone by ionization with a single photon pulse and becomes stronger with decreasing beam cross section. The method is indestructible and not affected by fluctuations of the beam position. Moreover, it can easily be realized in any ionization chamber by introducing a ͑rare͒ gas and detecting the photoionization signal as a fun...

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“…In order to overcome this limitation, an alternative two-XUV-photon ionization scheme has to be used. Possible schemes are the non-resonant direct double ionization (DDI), sequential double ionization (SDI) or ionic ionization (II) and above threshold ionization (ATI) (see figure 11) [34,35,48]. In these schemes, ionic products such as electrons or ions are to be detected utilizing energy resolved PE or TOF ion-mass spectroscopy, respectively.…”

Section: Alternate Two-xuv-photon Processes For Higher-order Harmonicmentioning

confidence: 99%

“…Since both schemes coexist in the ionization process, the temporal evolution of the system can be evaluated at different intensities using rate equations [50,51]. Nevertheless, according to previous studies [34,35,48,50,51] in He, Ar and Kr, the intensity regions where DDI or SDI could be used for aspulse-train characterization by means of second-order AC can be roughly estimated. Based on previous studies in He [51], for I XUV < 10 13 W cm −2 , far below the SDI saturation intensities, the TPDDI and ThPSDI rates can be expressed as…”

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