Relativistic and resonant effects in the ionization of heavy atoms by ultra-intense hard X-rays

Abstract

An accurate description of the interaction of intense hard X-ray pulses with heavy atoms, which is crucial for many applications of free-electron lasers, represents a hitherto unresolved challenge for theory because of the enormous number of electronic configurations and relativistic effects, which need to be taken into account. Here we report results on multiple ionization of xenon atoms by ultra-intense (about 1019 W/cm2) femtosecond X-ray pulses at photon energies from 5.5 to 8.3 keV and present a theoretical model capable of reproducing the experimental data in the entire energy range. Our analysis shows that the interplay of resonant and relativistic effects results in strongly structured charge state distributions, which reflect resonant positions of relativistically shifted electronic levels of highly charged ions created during the X-ray pulse. The theoretical approach described here provides a basis for accurate modeling of radiation damage in hard X-ray imaging experiments on targets with high-Z constituents.

Nonlinear Coherence Effects in Transient-Absorption Ion Spectroscopy with Stochastic Extreme-Ultraviolet Free-Electron Laser Pulses

“…Only for sufficiently high field strengths, at which more than one photon can interact with the quantum system on its intrinsic time and energy scale, these phenomena are accessible. Modern ultrafast lasers are effective driver and control tools for nonlinear effects at visible frequencies and have become the "working horses" for nonlinear coherent spectroscopies [9], in time domain and frequency domain, including the quantum control of boundbound electronic transitions (see, e.g., [10] and references therein).Since the advent of short-wavelength free-electron lasers (FELs) [11,12] the field of nonlinear spectroscopy is being extended into the extreme-ultraviolet (XUV) and x-ray spectral ranges [13][14][15][16][17][18][19][20][21][22][23][24]. One advantage of employing x-rays is the ability to access bound-bound electronic transitions associated with the spatially localized innerelectronic shell and the potential to probe site-specific spectroscopic information of a sample.…”

Breakdown of frustrated absorption in x-ray sequential multiphoton ionization

“…We employ the XATOM toolkit [42,43] to describe ionization dynamics of atoms, which has been tested for a series of gas-phase XFEL experiments [12][13][14][15]32,35]. It has been extended to incorporate both resonance and relativistic effects [31], which makes it suitable to explore ionization dynamics of heavy atoms over a wide range of x-ray pulse parameters in terms of photon energy, fluence, and pulse duration.…”

“…First, we investigate multiphoton multiple ionization of Xe at 1200 eV, where ionization dynamics are mainly initiated by M-shell (n = 3) ionization. The number of coupled rate equations, which is equivalent to the number of electronic configurations that are formed by removing 0, 1, or more electrons from initially occupied subshells of Xe and placing them into (n, l ) subshells (3 n 30 and 0 l 7), is estimated to be ∼4.2 × 10 60 [15,34]. To handle such a huge number of rate equations, we use the Monte Carlo on-the-fly scheme [14].…”

“…Figure 6(a) shows the final mean charge of Xe in the hard-x-ray range for several fluences. At 5.5 keV, the multiphoton multiple ionization dynamics of Xe is initiated by L-shell ionization [14,15], but this direct one-photon absorption channel closes at Xe 22+ . At low fluences (2 × 10 12 to 1 × 10 13 ph/μm 2 ), the mean charge follows the trend expected from frustrated absorption, whereas at high fluences (2 × 10 13 and 5 × 10 13 ph/μm 2 ), the mean charge increases slightly for decreasing pulse duration.…”

X-ray induced ultrafast dynamics in atoms, molecules, and clusters: experimental studies at an X-ray free-electron laser facility SACLA and modelling

“…Furthermore, various levels of experimental methods and theoretical modelling are within reach for these relatively simple targets. Because of these reasons, atoms [6][7][8][9][10][11], molecules [12][13][14][15][16][17][18][19][20], and atomic clusters [21][22][23][24][25][26][27][28] irradiated by an XFEL pulse have been investigated extensively.…”