Compton spectra of atoms at high x-ray intensity

Compton scattering is the nonresonant inelastic scattering of an x-ray photon by an electron and has been used to probe the electron momentum distribution in gas-phase and condensed-matter samples. In the low x-ray intensity regime, Compton scattering from atoms dominantly comes from bound electrons in neutral atoms, neglecting contributions from bound electrons in ions and free (ionized) electrons. In contrast, in the high x-ray intensity regime, the sample experiences severe ionization via x-ray multiphoton multiple ionization dynamics. Thus, it becomes necessary to take into account all the contributions to the Compton scattering signal when atoms are exposed to high-intensity x-ray pulses provided by x-ray free-electron lasers (XFELs). In this paper, we investigate the Compton spectra of atoms at high x-ray intensity, using an extension of the integrated x-ray atomic physics toolkit, \textsc{xatom}. As the x-ray fluence increases, there is a significant contribution from ionized electrons to the Compton spectra, which gives rise to strong deviations from the Compton spectra of neutral atoms. The present study provides not only understanding of the fundamental XFEL--matter interaction but also crucial information for single-particle imaging experiments, where Compton scattering is no longer negligible.Comment: 24 pages, 10 figures. This is an author-created, un-copyedited version of an article accepted for publication in the special issue of "Emerging Leaders" in J. Phys. B: At. Mol. Opt. Phys. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

Stability of the time-dependent configuration-interaction-singles method in the attosecond and strong-field regimes: A study of basis sets and absorption methods

We investigate the behavior of several spatial grid methods and complex absorbers for strong-field andattosecond scenarios when using the time-dependent configuration-interaction singles method to solve the multi-electron time-dependent Schr ̈odinger equation for atoms. We compare the pseudospectral grid, finite-element,and finite-element-discrete-variable-representation (DVR) methods with each other and discuss their advantagesand disadvantages. Additionally, we study the performances of complex absorbing potential (CAP) and smoothexterior complex scaling (SES) to absorb the outgoing electron. We find that SES performs generally better thanCAP for calculating high-harmonic generation spectra and XUV photoelectron spectra. In both of these cases,the DVR and even more the FEM grid representations show more reliable results—especially when using SES.Both absorbers show drawbacks when calculating photoelectron spectra in the strong-field regime

In situ characterization of few-femtosecond laser pulses by learning from first-principles calculations

The field of ultrafast spectroscopy is based on lasers being able to produce pulses that are as short as a few femtoseconds. Due to their broad bandwidth, these ultrashort light transients are strongly affected by propagation through materials. Therefore, a careful characterization of their temporal profile is required before any application. We propose a scheme for their characterization in situ, ensuring that the pulse parameters are measured in the region where the interaction with the sample takes place. Our method is based on first-principles calculations for strong-field ionization of rare-gas atoms and autocorrelation. We introduce a machine-learning algorithm, called vector space Newton interpolation cage (VSNIC), that uses the results from the first-principles calculations as input and reconstructs from a strong-field autocorrelation pattern for an unknown pulse the pulse length and spectral width by narrow margins

Stability of the time-dependent configuration-interaction-singles method in the attosecond and strong-field regimes: A study of basis sets and absorption methods

We investigate the behavior of several spatial grid methods and complex absorbers for strong-field andattosecond scenarios when using the time-dependent configuration-interaction singles method to solve the multi-electron time-dependent Schr ̈odinger equation for atoms. We compare the pseudospectral grid, finite-element,and finite-element-discrete-variable-representation (DVR) methods with each other and discuss their advantagesand disadvantages. Additionally, we study the performances of complex absorbing potential (CAP) and smoothexterior complex scaling (SES) to absorb the outgoing electron. We find that SES performs generally better thanCAP for calculating high-harmonic generation spectra and XUV photoelectron spectra. In both of these cases,the DVR and even more the FEM grid representations show more reliable results—especially when using SES.Both absorbers show drawbacks when calculating photoelectron spectra in the strong-field regime