Resonant Auger Spectra Research Articles

Ultrafast dynamics in C 1s core-excited CF4 revealed by two-dimensional resonant Auger spectroscopy

Following core excitation in an isolated molecule, ultrafast dissociation of one particular chemical bond can occur, where "ultrafast" is defined as taking place during the lifetime of the core hole, of the order of few femtoseconds. The signature of such phenomenon can be observed in resonant Auger spectra following core excitation. We present here an investigation of ultrafast dissociation following C 1s-to-σ* core excitation in CF4, with high-resolution resonant Auger spectroscopy. We are able to characterize final states of both the molecular ion and the CF3 (+) fragment. We use two-dimensional (2D) maps to record resonant Auger spectra across the resonance as a function of photon energy and to characterize ultrafast dynamics. This method provides immediate visual evidence of one of the important characteristics of the study of spectral features related to molecular versus fragment ionic final states, and namely their dispersion law. In the 2D maps we are also able to identify the dissociation limit for one of the molecular final states.

Site-selective resonant Auger spectroscopy of iso-dichloroethylene at the carbon K-edge

This study focuses on the two C1s-to-LUMO (lowest unoccupied molecular orbital) excitations of the iso-dichloroethylene (H2CCCl2) and the subsequent Auger decay. We investigate the effect of the two different carbon core excitations on the population of the cation produced after electronic relaxation. The resonant Auger spectra are interpreted by comparison to the valence shells photoionization spectrum and with the help of theoretical calculations. Several consequences of the core-hole localization on the electronic decay are observed. In particular, the resonant excitation of the C1s(CH2) to the LUMO leads to a large intensity increase in the region of the first satellite state, whereas no dramatic changes are observed for the C1s(CCl2) excitation.

Angle-resolved electron spectroscopy of the resonant Auger decay in xenon with meV energy resolution

The angle-resolved resonant Auger spectrum of Xe is investigated with a record high meV energy resolution in the kinetic energy region of 34.45–39.20 eV at hν=65.110 eV, corresponding to the resonant excitation of the Auger Xe*4d5/2−16p state. New lines have been observed and assigned in the spectra. The results of previous measurements concerning energies, intensities and angular distribution asymmetry parameters have been refined, complemented and, for some of the lines, corrected.

Effect of the Cl2p core orbital excitation on the nuclear dynamics of the three dichloroethylene isomers

In this article, we present the X-ray absorption spectra of the three isomers of dichloroethylene C2H2Cl2 in the vicinity of the chlorine L2 and L3 edges. These isomers were excited along the first Cl2p to lowest unoccupied molecular orbital transition and an analysis of the corresponding resonant Auger spectra shows that the C–Cl bond can break before the electronic decay. An investigation of a possible Auger Doppler-like effect is also presented.

Doppler effect in fragment autoionization following core-to-valence excitation inO2

Atomic autoionization following O $1s$ resonant excitation in ${\mathrm{O}}_{2}$ is studied using high-resolution electron spectroscopy. The resonant Auger spectrum showing evidence of ultrafast dissociation as well as the autoionization of oxygen atoms is measured and discussed. Evidence of the electronic Doppler effect is observed in both electronic decay mechanisms.

Resonant Auger decay of Ar2p3/2−14sand2p3/2−14pstates excited by electron impact

Auger spectra of resonantly excited $2{p}_{3/2}^{\ensuremath{-}1}4s$ and $2{p}_{3/2}^{\ensuremath{-}1}4p$ states in argon were measured by $(e,2e)$ technique. The 99.2-eV scattered electrons were detected in coincidence with ${L}_{3}--{M}_{23}{M}_{23}$ Auger electrons, and the experiment was performed at 343.6- and 344.9-eV electron impact to tune the energy loss to the energy of the dipole-allowed and the dipole-forbidden excitations, respectively. The resonant Auger spectra are obtained upon subtraction of the overlapping signal due to the outer-shell ionization, which was recorded at 340-eV electron-impact energy. The most intense groups of Auger transitions from $2{p}_{3/2}^{\ensuremath{-}1}4s (J=1,2)$ and $2{p}_{3/2}^{\ensuremath{-}1}4p (J=0,1,2,3)$ states are identified by comparison with the results of the two-step model, based on distorted-wave Born approximation with exchange and multiconfiguration descriptions of the relaxed states. The $4 s$ spectrum displays a substantially larger shake-up contribution than the one observed in photoexcitation experiments, which may be explained by the interference of the resonant decay path with the direct ionization excitation of the Ar $3p$ subshell. The majority of the observed $4p$ signal is assigned to the monopole and quadrupole excitations of the ground state.

Tautomerism in Cytosine and Uracil: A Theoretical and Experimental X-ray Absorption and Resonant Auger Study

The core level photoabsorption spectra of the nucleobases cytosine and uracil in the gas phase have been measured and the results interpreted with theoretical calculations using an ab initio Green’s function approach. A single tautomer of uracil is populated, in agreement with previous work, while three tautomers of cytosine are clearly identified, whose identity and relative populations at the temperature of the experiment were reported previously. The second-order ADC approach to polarization propagator was employed in calculations of X-ray photoabsorption energies and intensities. The theoretical spectra have been constructed as Boltzmann-factor-weighted sums of individual tautomer spectra. These theoretical spectra are in good agreement with the experimental photoabsorption results at the oxygen, nitrogen, and carbon edges. In addition we report resonant Auger spectra of the valence band of cytosine, which support previous assignments of the character of the valence band states.

Multimode Resonant Auger Scattering from the Ethene Molecule

Resonant Auger spectra of ethene molecule have been measured with vibrational resolution at several excitation energies in the region of the C1s(-1)1b(2g)(π*) resonance. The main features observed in the experiment have been assigned and are accurately interpreted on the basis of ab initio multimode calculations. Theory explains the extended vibrational distribution of the resonant Auger spectra and its evolution as a function of the excitation energy by multimode excitation during the scattering process. As a result, the resonant Auger spectra display two qualitatively different spectral features following the Raman and non-Raman dispersion laws, respectively. Calculations show that two observed thresholds of formation of non-Raman spectral bands are related to the "double-edge" structure of the X-ray absorption spectrum.

Interference effects during the Auger decay of theC∗O1s−1π∗resonance studied by angular distribution of theCO+(A)photoelectrons and polarization analysis of theCO+(A−X)fluorescence

Angular distributions of the ${\text{CO}}^{+}(A\text{ }^{2}\ensuremath{\Pi})$ photoelectrons and of the ${\text{CO}}^{+}(A\text{ }^{2}\ensuremath{\Pi}\ensuremath{\rightarrow}X\text{ }{^{2}\ensuremath{\Sigma}}^{+})$ fluorescence are computed in the vicinity of the $1{s}^{\ensuremath{-}1}{\ensuremath{\pi}}^{\ensuremath{\ast}}$ resonant excitation of the ${\text{C}}^{\ensuremath{\ast}}\text{O}$ molecule. In the calculations, lifetime vibrational interference and the direct transition amplitude for population of the $A\text{ }^{2}\ensuremath{\Pi}({v}^{\ensuremath{'}})$ states were taken into account ab initio. The weak direct photoionization channel induces broad exciting-photon energy range dispersions of the angular distribution parameters ${\ensuremath{\beta}}_{A}^{e}(\ensuremath{\omega})$ and $\ensuremath{\beta}{2}_{A}^{X}(\ensuremath{\omega})$, and the nuclear vibrational motion causes variations of the computed parameters across the positions of the ${\text{C}}^{\ensuremath{\ast}}\text{O}({v}_{r})$ vibronic states. Present calculations are in good agreement with available vibrationally and angularly resolved resonant Auger spectra. Theoretical $\ensuremath{\beta}{2}_{A}^{X}(\ensuremath{\omega})$ parameters are in agreement with the experimental results from the polarization analysis of the ${\text{CO}}^{+}(A\text{\ensuremath{-}}X)$ fluorescence induced by linearly polarized synchrotron radiation.

Chemical bond elongation following core-excitation of ammonia: resonant Auger spectra calculation

Theoretical resonant Auger decay spectra of ammonia with core-hole excited state dynamics simulation were investigated and some specific features of the experiment, except for a vibrational structure, were reproduced. A power spectral analysis with short-time maximum entropy method has been applied, and an obtained vibrational spacing assigned to the NH stretching mode was 340 meV, which was consistent with the experiment of 390 ± 10 meV. Proton dynamics of ammonia on the first core-excited state was discussed.

Resonant Auger decay of 2p hole in argon induced by electron impact

We present an experimental study of L-MM resonant Auger spectra of argon after electron impact excitation. The electron spectra were measured at ten different electron impact energies between 442.6eV and 461.7eV. During (e,2e) measurement the energy of the second electron was kept fixed at 209.6eV, corresponding to the energy of one of the strongest resonant Auger transitions from the [2p3/23d] state. Except for the monopole excitations, the recorded spectral structures are explained on the basis of photon impact data.

Angle-resolved photoion yield and resonant Auger spectroscopy for the doubly excited Rydberg states above the C1sthreshold of CO

Doubly excited core-hole states of carbon monoxide in the photon energy region of $300--305\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, i.e., directly above the C $1s$ ionization threshold, have been studied using both angle-resolved ion-yield and high-resolution resonant Auger spectroscopies. The leading configurations of the most prominent doubly excited Rydberg states are assigned by careful analysis of the ion-yield spectra and the final-state spectra to C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=1)$ $3s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1,2)$, C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=0)$ $3s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1,2)$, and C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=1)$ $4s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1)$, which can only be populated via a conjugate shake-up process. Analysis of the resonant Auger spectra provides an assignment of several two-hole--one-electron $(2h\text{\ensuremath{-}}1e)$ final states.

Angle-resolved study of the Ar 2p−11/23d resonant Auger decay

The Ar resonant Auger spectra have been recorded in Raman conditions at a photon energy of 249 eV corresponding to the Ar 2p1/2→ 3d excitation. The high experimental resolution allowed us to resolve the fine structure in the resonant Auger spectra in much greater detail than obtained previously. Experimental individual angular anisotropy β parameter values of the Ar 2p−11/23d → 3p4nl transitions are compared with the results of theoretical calculations carried out in a multi-configuration Dirac–Fock approach with different configuration expansions.

Substrate-induced effects in the creation and decay of potassium 2p core excitations inultrathin films of KCl on copper

Substrate-induced effects on the transport properties in thin KCl films on the Cu(100) surface have beenstudied using K+ 2p photoelectron, photoabsorption and resonant Auger spectra. The measurements wereperformed at different KCl coverages ranging from a partial monolayer to a thick bulk-likefilm.The morphology and layer thickness were estimated from an analysis of the electron energy loss structureof the K+ 2p and Cl− 2p photoelectron peaks, and from the variations in the photoelectron peak fine structure.The resonant Auger spectator decay spectra measured at the photoabsorptionresonances show that the significant differences between the spectra of thesolid and of thin layers are related to the charge delocalization from theK+ 3d excited state into the metal substrate. The core-hole-clock approach yields an estimateof 1.5 fs for the corresponding charge transfer time at a single monolayer KCl coverage.

Strong nonmonopole shake transitions in the Br3d−1npπ(n=6–9)resonant Auger spectra of HBr

The high-resolution resonant Auger spectra of Br $3{d}^{\ensuremath{-}1}np\ensuremath{\pi}$ $(n=6\char21{}9)$ in HBr are presented. These spectra are extremely complex, with very intense shake and vibrational features. Especially surprising is the observation of at least five intense Auger lines for each intermediate state due to transitions into different final-state configurations; this number of lines can only be explained with strong nonmonopole transitions which prevent obtaining information on the symmetry character of the excited-state orbital in the intermediate state. They are explained in a semiclassical model with the interaction between the Auger electron and the excited Rydberg electron.

Interference effects in resonant Auger decay of atomic caesium

High-resolution electron spectroscopy with synchrotron radiation excitation was used to study the resonant Auger decay of atomic caesium. The resonant Auger spectra with different photon energies across the 4d5/2 → 6p photoabsorption resonances were measured and the results were compared to the multiconfiguration Dirac–Fock calculations. The high kinetic energy resolution made it possible to observe interference effects between the decay of different overlapping 4d5/2 → 6p resonant states. The partial electron yield covering transitions to 5p−26s6p final ionic states was calculated and observed experimentally. The interference effects were found to average out, resulting in the absorption spectrum.

Electronic Doppler effect in resonant Auger decay of CO molecules upon excitation near a shake-upΠresonance

We present an experimental observation of the electronic Doppler effect in resonant Auger spectra upon core excitation slightly above the carbon K edge of the CO molecule. Thus the electronic Doppler effect has been identified in above-threshold excitation, and in a transition of Pi symmetry. Ab initio calculations of the potential energy curves of the relevant states of CO and the wave packet technique have been employed to provide a theoretical background to the experimental studies. The weak feature around 299.4 eV in the photoabsorption spectrum, whose decay has been investigated by the present experiment, is assigned to double (core-valence) excitations to C 1s shake-up states vertical bar 1s(C)(-1)1 pi(-1)pi(*2)> with a strong dissociative character, and the Doppler splitting of the atomic peak has been reproduced by the simulation.

Experimental evidence for molecular ultrafast dissociation in O2 clusters

Resonant Auger spectra of O2 clusters excited at the O1s edge are reported. After excitation to the repulsive 1s-13σ* state, the resulting resonant Auger spectrum displays features that remain constant in kinetic energy as the photon energy is detuned. The shift between known atomic fragment features and these features is consistent with that observed for atoms and clusters in singly charged states in direct photoemission. These findings are strong evidence for the existence of molecular ultrafast dissociation processes within the clusters or on their surface.

Electronic structure of core-excited and core-ionized methyl oxirane

We report a detailed analysis of the electronic structure of methyl oxirane, including core-level photoelectron spectroscopy, X-ray absorption at both C and O K-shells, resonant Auger and normal Auger spectroscopy. The X-ray absorption spectra (XAS) around the C K-edge can be easily interpreted on the ground of the chemical shift between the carbon atoms with different chemical environments. The X-ray absorption data around the O K-edge are quite structureless, hinting for a likely fragmentation process (possibly implying a ring-opening reaction). In resonant Auger spectra obtained after excitation below both the C 1s and O 1s ionization thresholds we notice a predominance of spectator decay implying a strong mixing between empty molecular orbitals and Rydberg states.

Substrate-induced effects in the creation and decay of core excitations in ultrathin films of potassium chloride on copper

Substrate-induced effects in the creation and decay of 2p core excitations of K + and Cl − in ultrathin films of KCl on Cu(1 0 0) surface have been studied using photoelectron, photoabsorption and resonant Auger spectra, excited by synchrotron radiation. The measurements were performed on different overlayers ranging from sub-monolayer coverages up to continuous thick films. It is demonstrated that the formation and decay of core excitations in thin films of KCl at Cu(1 0 0) surface is strongly influenced by the hybridization of excited states of K + or Cl − origin with the metal substrate, which leads to the delocalization of the excited electron from the absorption site.