Monte Carlo calculation of ion, electron, and photon spectra of xenon atoms in x-ray free-electron laser pulses

Abstract

When atoms and molecules are irradiated by an x-ray free-electron laser (XFEL), they are highly ionized via a sequence of one-photon ionization and relaxation processes. To describe the ionization dynamics during XFEL pulses, a rate equation model has been employed. Even though this model is straightforward for the case of light atoms, it generates a huge number of coupled rate equations for heavy atoms like xenon, which are not trivial to solve directly. Here, we employ the Monte Carlo method to address this problem and we investigate ionization dynamics of xenon atoms induced by XFEL pulses at a photon energy of 4500 eV. Charge state distributions, photo-/Auger electron spectra, and fluorescence spectra are presented for x-ray fluences of up to 10 13 photons/µm 2 . With the photon energy of 4500 eV, xenon atoms can be ionized up to +44 through multiphoton absorption characterized by sequential one-photon single-electron interactions.

Ultrafast Dynamics of a Nucleobase Analogue Illuminated by a Short Intense X-ray Free Electron Laser Pulse

“…These values are close to the ones for CH 3 I (τ ¼ 9 fs and R ¼ 0.37 fs −1 [31]), suggesting that the model employed is indeed of general applicability. It may be worth noting also that these values of R are comparable to the Auger decay rates of individual decay stages in Xe, as calculated by Son and Santra [40]. The details of parameter dependence of SP 2 ðA; BÞ and SP 3 ðA; B; CÞ are presented in Ref.…”

Probing ultrafast electronic and molecular dynamics with free-electron lasers

“…However, already the first experiments on heavy elements, in this case, xenon, demonstrated a drastic violation of this simple rule [50]. As illustrated in figure 5(a), the highest charge state detected upon the irradiation of xenon atoms by 1500 eV 2.7 mJ LCLS pulses is Xe 36+ , although the last ionic state which can be further ionized by a single 1500 eV photon is Xe 25+ [60]. Therefore, not only the highest charge state, but even the most abundant charge state observed (Xe 30+ ) is well beyond the intuitive expectations for sequential ionization model.…”

“…Early LCLS experiments on the photoionization of light elements (e.g., neon, nitrogen) [41,55,56] along with extensive theoretical work [57][58][59] firmly established sequential single-photon absorption as the dominant mechanism of x-ray ionization at intensities up to 10 18 W cm −2 . Within this picture, the highest charge state produced is typically defined by the last ionic state that can still be ionized with one photon.…”

“…[55,57,60], implies that another efficient ionization process must play a role here. An insight into the possible mechanism can be obtained from the fluorescence spectra measured simultaneously with the ion charge state distributions ( figure 5(b)).…”

Electron and fluorescence spectra of a water molecule irradiated by an x-ray free-electron laser pulse

“…So far, the manifestation of ionization dynamics in spectra has only been investigated for a single xenon atom [34]. In molecules, the radiation damage can be significantly different from what one would expect for single atoms [35,36].…”