Multiple ionization of neon by soft x-rays at ultrahigh intensity

Abstract

Abstract. At the free-electron laser FLASH, multiple ionization of neon atoms was quantitatively investigated at 93.0 eV and 90.5 eV photon energy. For ion charge states up to 6+, we compare the respective absolute photoionization yields with results from a minimal model and an elaborate description. Both approaches are based on rate equations and take into acccout a Gaussian spatial intensity distribution of the laser beam. From the comparison we conclude, that photoionization up to a charge of 5+ can be described by the minimal model. For higher charges, the experimental ionization yields systematically exceed the elaborate rate based prediction.PACS numbers: 42.50 Hz, 32.80.Rm, 32.80.Fb, 41.60.Cr With the construction of X-ray lasers during the last years applying the free-electron laser (FEL) concept of self-amplified spontaneous emission (SASE) [1,2,3,4], a number of experiments have become feasible which elucidate the principles of photon-matter interaction in a new parameter regime ([5, 6] and refs. therein). The combination of high frequency and high intensity allowed in particular new insight into the photoelectric effect and the photoionization process [7,8,9,10,11,12,13,14,15]. Even simple photoionization involves in a strict sense multi-electron dynamics as soon as the photoelectron is not ejected dominantly from the highest occupied orbital rendering a comprehensive theoretical description difficult. For more complex atomic targets such as xenon, multiphoton absorption in the energy range of the giant resonance poses in addition the question, which role collective electron excitations play in the high frequency -high intensity domain [10,16]. Due to these difficulties, the role of electron ‡

A non-invasive online photoionization spectrometer for FLASH2

“…Schram et al, 1966;Stockli & Fry, 1997). For the model used in the ion spectrometer and similar multipliers, this behavior could be confirmed already in earlier measurements at a synchrotron source (Juranić et al, 2009), at FLASH (Guichard et al, 2013) and at SACLA (Kato et al, 2012). Accordingly, intensity correction factors have been introduced for the ion spectra analysis,…”

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

“…At constant exposure, the mean charge is lower at shorter pulses, indicating that straight simultaneous multiphoton ionization may not be the underlying mechanism. This is not surprising because the multiphoton ionization at 93 eV photon energy and radiant exposure levels in the order of a few J=cm 2 have recently been well described in the case of Ne atoms by a sequential multiphoton scheme [20].…”

“…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].…”

The impact of pulse duration on multiphoton ionization in the soft X-ray regime

“…In contrast to simultaneous multiphoton processes (equation 5), the number of sequential n-photon processes per pulse does not explicitly depend on the pulse duration t as long as t is short enough, so that the atomic and ionic targets do not drop out of the interaction volume. This can also be directly seen from an analytical solution of the rate equation system of equation 8 with the simplification that the cross sections  ij are equal (= ) for all ij [70], which leads to a power law as a function of fluence (F) instead of irradiance (E):…”