State-of-the-art argon $KLL$ Auger spectra measured using photon energies of $h\nu =3216$ and 3400 eV are presented along with an Ar $\left[1s\right]$ photoelectron spectrum (square brackets indicate holes in the respective orbital). The two different photon energies used for measuring the Auger spectra allow distinguishing between the shake transitions during the Auger decay and the Auger transitions of the photoelectron satellites. A complete assignment of satellite transitions is provided, partially based on configuration-interaction calculations. In addition, Ar $\left[1s3(s,p)\right]n^{\prime }{l}^{\prime }\to \left[2p^2{(}^1{D}_2)\right]$ transitions are observed, which can be explained by knock-down transitions leading to a direct exchange of angular momentum between the excited electron and the Auger electron. The lifetime broadenings of the Ar $\left[2s\right]$ single-core-hole state and the $\left[2s^2\right]$ and $\left[2s2p\right]$ double-core-hole states are also determined, confirming previously observed trends for double-core-hole states.

High-intensity ultrashort X-ray free-electron laser (XFEL) pulses are revolutionizing the study of fundamental nonlinear x-ray matter interactions and coupled electronic and nuclear dynamics. To fully exploit the potential of this powerful tool for advanced x-ray spectroscopies, a noninvasive spectral characterization of incident stochastic XFEL pulses with high resolution is a key requirement. Here we present a methodology that combines high-acceptance angle-resolved photoelectron time-of-flight spectroscopy and ghost imaging to enhance the quality of spectral characterization of x-ray free-electron laser pulses. Implementation of this noninvasive high-resolution x-ray diagnostic can greatly benefit the ultrafast x-ray spectroscopy community by functioning as a transparent beamsplitter for applications such as transient absorption spectroscopy in averaging mode as well as covariance-based x-ray nonlinear spectroscopies in single-shot mode where the shot-to-shot fluctuations inherent to a self-amplified spontaneous emission (SASE) XFEL pulse are a powerful asset.

We demonstrate experimentally and theoretically the presence of the post-collision interaction (PCI) effect in sulfur ${\mathrm{KL}}_{2,3}{\mathrm{L}}_{2,3}$ Auger electron spectra measured in the gas-phase thiophene and in solid-state organic polymers: polythiophene (PT) and poly(3-hexylthiophene-2,5-diyl), commonly known as P3HT. PCI manifests itself through a distortion and a blueshift of the normal Auger S ${\mathrm{KL}}_{2,3}{\mathrm{L}}_{2,3}$ spectrum when S $1s$ ionization occurs close to the ionization threshold. Our investigation shows that the PCI-induced shift of the Auger spectra is stronger in the solid-state polymers than in the gas-phase organic molecule. Theoretical modeling within the framework of the eikonal approximation provides good agreement with the experimental observations. In a solid medium, two effects influence the interaction between the photoelectron and the Auger electron. In detail, stronger PCI in the polymers is attributed to the photoelectron scattering in the solid, which overcompensates the polarization screening of electron charges which causes a reduction of the interaction. Our paper demonstrates the general nature of the PCI effect occurring in different media.

A comprehensive electron spectroscopic study combined with partial electron yield measurements around the Br 1s ionization threshold of HBr at ≅13.482 keV is reported. In detail, the Br 1s⁻¹ X-ray absorption spectrum, the 1s⁻¹ photoelectron spectrum as well as the normal and resonant KLL Auger spectra are presented. Moreover, the L-shell Auger spectra measured with photon energies below and above the Br 1s⁻¹ ionization energy as well as on top of the Br 1s⁻¹σ* resonance are shown. The latter two Auger spectra represent the second step of the decay cascade subsequent to producing a Br 1s⁻¹ core hole. The measurements provide information on the electron and nuclear dynamics of deep core-excited states of HBr on the femtosecond timescale. From the different spectra the lifetime broadening of the Br 1s⁻¹ single core-hole state as well as of the Br(2s⁻²,2s⁻¹2p⁻¹,2p⁻²) double core-hole states are extracted and discussed. The slope of the strongly dissociative HBr 2p⁻²σ* potential energy curve is found to be about −13.60 eV Å⁻¹. The interpretation of the experimental data, and in particular the assignment of the spectral features in the KLL and L-shell Auger spectra, is supported by relativistic calculations for HBr molecule and atomic Br.

A K⁻²V photoelectron spectrum of the CO molecule, showing several core-ionized core-excited states, has been recorded and the different spectral features have been interpreted in terms of their direct or conjugate nature.

Ultrafast charge transfer along the polymer chains is triggered by a selective resonant core-excitation of the sulfur atom in P3HT films and powders. Our approach opens perspectives for studies on intra-molecular conductivity in organic molecules.