46 research outputs found
The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(II) complexes
Oxygen binding to cobalt and iron phthalocyanines as determined from in situ X-ray absorption spectroscopy
Cobalt phthalocyanine (CoPc) and iron phthalocyanine (FePc) are possible oxygen reduction catalysts in fuel cells, but the exact functioning and deactivation of these catalysts is unknown. The electronic structure of the CoPc and FePc has been studied in situ under hydrogen and oxygen atmospheres by a combination of ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The results show that when oxygen is introduced, the iron changes oxidation state while the cobalt does not. The data show that oxygen binds in an end-on configuration in CoPc, while for FePc side-on binding is most likely
Ultrafast adsorbate excitation probed with sub-ps resolution XAS
We use a pump-probe scheme to measure the time evolution of the C K-edge
X-ray absorption spectrum (XAS) from CO/Ru(0001) after excitation by an
ultrashort high-intensity optical laser pulse. Due to the short duration of the
X-ray probe pulse and precise control of the pulse delay, the
excitation-induced dynamics during the first ps after the pump can be resolved
with unprecedented time resolution. By comparing with theoretical (DFT)
spectrum calculations we find high excitation of the internal stretch and
frustrated rotation modes occurring within 200 fs of laser excitation, as well
as thermalization of the system in the ps regime. The ~100 fs initial
excitation of these CO vibrational modes is not readily rationalized by
traditional theories of nonadiabatic coupling of adsorbates to metal surfaces,
e. g. electronic frictions based on first order electron-phonon coupling or
transient population of adsorbate resonances. We suggest that coupling of the
adsorbate to non-thermalized electron-hole pairs is responsible for the
ultrafast initial excitation of the modes.Comment: 16 pages, 16 figures. To be published in Physical Review Letters:
https://journals.aps.org/prl/accepted/c1070Y74M8b18063d9cd0221b000631d50ef7a24
Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy
We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The âŒ100ââfs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes
Atom-Specific Probing of Electron Dynamics in an Atomic Adsorbate by Time-Resolved X-Ray Spectroscopy
The electronic excitation occurring on adsorbates at ultrafast timescales from optical lasers that initiate surface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) of a simple well-known adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel [Ni(100)] surface, following intense laser optical pumping at 400 nm. We observe ultrafast (âŒ100ââfs) changes in both XAS and XES showing clear signatures of the formation of a hot electron-hole pair distribution on the adsorbate. This is followed by slower changes on a few picoseconds timescale, shown to be consistent with thermalization of the complete C/Ni system. Density functional theory spectrum simulations support this interpretation
Soft X-ray spectroscopy as a probe for gas-phase protein structure:Electron impact ionization from within
Preservation of protein conformation upon transfer into the gasâphase is key for structure determination of free single molecules, e.g. using Xâray freeâelectron lasers. In the gas phase, the helicity of melittin decreases strongly as the protein's protonation state increases. We demonstrate the sensitivity of soft Xâray spectroscopy to the gas phase conformation of melittin cations ([melittin+qH]q+, q=2â4) in a cryogenic linear radiofrequency ion trap. With increasing helicity we observe a decrease of the dominating carbon 1sâï°* transition in the amide C=O bonds for nonâdissociative single ionization and an increase for nonâdissociative double ionization. As the underlying mechanism we identify inelastic electron scattering. Using an independent atom model we show that the more compact nature of the helical protein conformation substantially increases the probability for offâsite intramolecular ionization by inelastic Auger electron scattering
Atom-Specific Probing of Electron Dynamics in an Atomic Adsorbate by Time-Resolved X-ray Spectroscopy
The electronic excitation occurring on adsorbates at ultrafast time scales
from optical lasers that initiate surface chemical reactions is still an open
question. Here, we report the ultrafast temporal evolution of X-ray absorption
spectroscopy (XAS) and X-ray emission spectroscopy (XES) of a simple well known
adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel
(Ni(100)) surface, following intense laser optical pumping at 400 nm. We
observe ultrafast (~100 fs) changes in both XAS and XES showing clear
signatures of the formation of a hot electron-hole pair distribution on the
adsorbate. This is followed by slower changes on a few ps time scale, shown to
be consistent with thermalization of the complete C/Ni system. Density
functional theory spectrum simulations support this interpretation.Comment: 33 pages, 12 figures. Submitted to Physical Review Letter
Symmetry-Resolved CO Desorption and Oxidation Dynamics on O/Ru(0001) Probed at the C K-edge by Ultrafast X-Ray Spectroscopy
We report on carbon monoxide desorption and oxidation induced by 400 nm femtosecond laser excitation on the O/Ru(0001) surface probed by time-resolved x-ray absorption spectroscopy (TR-XAS) at the carbon K-edge. The experiments were performed under constant background pressures of CO (6 Ă 10â8 Torr) and O2 (3 Ă 10â8 Torr). Under these conditions, we detect two transient CO species with narrow 2Ï* peaks, suggesting little 2Ï* interaction with the surface. Based on polarization measurements, we find that these two species have opposing orientations: (1) CO favoring a more perpendicular orientation and (2) CO favoring a more parallel orientation with respect to the surface. We also directly detect gas-phase CO2 using a mass spectrometer and observe weak signatures of bent adsorbed CO2 at slightly higher x-ray energies than the 2Ï* region. These results are compared to previously reported TR-XAS results at the O K-edge, where the CO background pressure was three times lower (2 Ă 10â8 Torr) while maintaining the same O2 pressure. At the lower CO pressure, in the CO 2Ï* region, we observed adsorbed CO and a distribution of OCâO bond lengths close to the CO oxidation transition state, with little indication of gas-like CO. The shift toward âgas-likeâ CO species may be explained by the higher CO exposure, which blocks O adsorption, decreasing O coverage and increasing CO coverage. These effects decrease the CO desorption barrier through dipoleâdipole interaction while simultaneously increasing the CO oxidation barrier
Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser
Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-rayâmatter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L3-edge absorption of metallic nickel thin films, measured with fluences up to 60âJ/cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale
Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL
Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very
promising technique that can be employed at X-ray Free Electron Lasers (FELs)
to investigate out-of-equilibrium dynamics for material and energy research.
Here we present a dedicated setup for soft X-rays available at the Spectroscopy
& Coherent Scattering (SCS) instrument at the European X-ray Free Electron
Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used
in transmission to create three copies of the incoming beam, which are used to
measure the transmitted intensity through the excited and unexcited sample, as
well as to monitor the incoming intensity. Since these three intensity signals
are detected shot-by-shot and simultaneously, this setup allows normalized
shot-by-shot analysis of the transmission. For photon detection, the DSSC
imaging detector, which is capable of recording up to 800 images at 4.5 MHz
frame rate during the FEL burst, is employed and allows approaching the photon
shot-noise limit. We review the setup and its capabilities, as well as the
online and offline analysis tools provided to users.Comment: 11 figure