Showing papers by "John D. Bozek published in 2020"

Clocking Auger Electrons

Abstract: Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. As the relaxation occurs primarily via Auger emission, excited state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive at XFELs due to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here, we develop a new approach termed self-referenced attosecond streaking, based upon simultaneous measurements of streaked photo- and Auger electrons. Our technique enables sub-femtosecond resolution in spite of jitter. We exploit this method to make the first XFEL time-domain measurement of the Auger decay lifetime in atomic neon, and, by using a fully quantum-mechanical description, retrieve a lifetime of $2.2^{ + 0.2}_{ - 0.3}$ fs for the KLL decay channel. Importantly, our technique can be generalised to permit the extension of attosecond time-resolved experiments to all current and future FEL facilities.

Aerosol Synthesis and Gas-Phase Photoelectron Spectroscopy of Ag-Bi-I Nanosystems

Abstract: We report on the aerosol generation of ligand-free silver iodobismuthate (Ag-Bi-I) nanoparticles (NPs) and on in situ investigation of their electronic structure using synchrotron radiation soft X-...

Unravelling molecular interactions in uracil clusters by XPS measurements assisted by ab initio and tight-binding simulations.

Abstract: The C, N and O 1s XPS spectra of uracil clusters in the gas phase have been measured. A new bottom-up approach, which relies on computational simulations starting from the crystallographic structure of uracil, has been adopted to interpret the measured spectra. This approach sheds light on the different molecular interactions (H-bond, π-stacking, dispersion interactions) at work in the cluster and provides a good understanding of the observed XPS chemical shifts with respect to the isolated molecule in terms of intramolecular and intermolecular screening occurring after the core–hole ionization. The proposed bottom-up approach, reasonably expensive in terms of computational resources, has been validated by finite-temperature molecular dynamics simulations of clusters composed of up to fifty molecules.

Photoabsorption, photoionization, and Auger processes at the carbon K edge in C H 3 I

Abstract: The dynamics of photoabsorption, photoionization, and the associated Auger decay have been investigated at the carbon $K$ edge in methyl iodide ($\mathrm{C}{\mathrm{H}}_{3}\mathrm{I}$) using linearly polarized synchrotron radiation. Ion yield measurements were used to investigate transitions in the pre-edge region due to excitations into either unoccupied valence or Rydberg states. The assignment of these transitions was achieved through comparison with theoretical x-ray absorption spectra calculated using time-dependent density functional theory, within the Tamm-Dancoff approximation. Several of the Rydberg states belonging to series converging onto the $\mathrm{C}1s$ ionization limit exhibit significant vibrational structure that is also interpreted using theoretical calculations. The $\mathrm{C}1s$ in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{I}$ photoelectron spectrum was measured, and the observed vibrational structure was assigned with the aid of theoretical predictions. Polarization dependent, resonantly excited, valence shell photoelectron spectra were recorded at photon energies coinciding with the $\mathrm{C}\phantom{\rule{0.16em}{0ex}}1s\ensuremath{\rightarrow}{\ensuremath{\sigma}}^{*}, \mathrm{C}\phantom{\rule{0.16em}{0ex}}1s\ensuremath{\rightarrow}6s{a}_{1}$ and $\mathrm{C}\phantom{\rule{0.16em}{0ex}}1s\ensuremath{\rightarrow}6pe$ transitions in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{I}$, thereby allowing photoelectron angular distributions to be determined. The nonresonantly excited C(KVV) Auger electron spectrum was measured and some of the features observed at high kinetic energies were attributed to transitions into valence orbitals possessing significant iodine character. The contributions of participator and spectator Auger decay to the resonantly excited photoelectron spectra have been assessed. The influence of participator decay appears minor whereas spectator decay results in the enhanced population of satellite states.

Oxygen K-shell spectroscopy of isolated progressively solvated peptide.

Abstract: Gas-phase near-edge X-ray-absorption fine structure (NEXAFS) action spectroscopy around the oxygen K-edge and mass spectrometry were employed to probe isolated substance P (SP) molecular ions, both bare and progressively solvated with 4 and 11 water molecules. Detailed mass spectra of bare and hydrated precursors are presented for the resonant photon energy of 532 eV that corresponds to O1s → π(amide)* core excitation, triggering resonant Auger decay and fragmentation from the ionized radical molecular system. The fragmentation pattern of doubly protonated SP hydrated with 4 water molecules clearly shows a series of abundant doubly charged backbone fragments, as well as triply charged precursor with small neutral losses, all preserving full water cluster. This is drastically different from the collisional induced dissociation of the hydrated peptide where the water loss is a dominant relaxation process. Moreover, the action NEXAFS obtained from several resolved small backbone fragments revealed increased fragmentation of hydrated SP relative to the bare one, due to a resonant O1s excitation of the attached water molecules. Such unexpected result inspires further experimental developments to investigate possible nonlocal energy transfer from the solvent to the biomolecules within the first solvation shell. The experiment is supported by molecular dynamics and DFT calculations to estimate the intensity of the resonant X-ray absorption of bare and hydrated SP around peptide and water O1s excitation region.

Subthreshold erosion of an organic polymer induced by multiple shots of an X-ray free-electron laser

Abstract: Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the $\mathrm{C}---\mathrm{C}$ bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results.

Conjugate photoelectron recapture peaks in the high resolution Auger electron spectra following near-threshold Ar 2p photoionization

Abstract: We have observed formation of Rydberg series superimposed on the Ar (L3M2,3M2,3) Auger electron spectral profile with photoexcitation just above the 2p-13/2 photoionization threshold. This is due to the recapture of photoelectrons induced by post-collision interaction (PCI) between the Auger electron and photoelectron: Ar+ 2p-13/2+e- →Ar+ 3p-2ml+e- . The intensity distribution of the angular momentum state resolved Rydberg peaks revealed prominent conjugate peaks due to angular momentum exchange between photoelectrons and Auger electrons in the process. In addition, we measured the 2p-13/2nl→3p-2ml+e- resonant Auger spectra with high resolution below the 2p-13/2 threshold. Our results showed that conjugate shake-up/down 2p-13/2nd→3p-2mp+e- processes are stronger than the direct 2p-13/2nd→3p-2ms, (m-1)d+e- processes in the spectator Auger processes, implying a strong angular momentum exchange effect between photoelectron and Auger electron.

Surface chemistry of colloidal surfactant-free gold nanoparticles generated by laser ablation

Abstract: Surface chemistry of gold nanoparticles produced by laser ablation in liquids has been investigated using synchrotron radiation and the X-ray Photoelectron Spectroscopy technique. Thanks to the recorded core-level and valence spectra, combined with a detailed analysis based on Bayesian statistical methods, the oxide presence and its proportion on the nanoparticle surface have been determined.