Core-hole Decay Research Articles

Nuclear dynamics during the N1sautoionization of physisorbedN2

We demonstrate that even for adsorbed molecules, the vibrational fine structure of resonant core hole decay channels after narrow-band excitation can show a dependence on the variation of the primary excitation energy as known from isolated molecules. Despite several adsorption-induced broadening mechanisms, the so-called detuning effect can be observed in the autoionization of a ${\mathrm{N}}_{2}$ monolayer physisorbed on Xe/Ru(001) for the $1{\ensuremath{\pi}}_{u}^{\ensuremath{-}1}$ final state of participant decay, in good agreement with theoretical predictions and observations for isolated ${\mathrm{N}}_{2}.$ However, contrary to the corresponding decay channel of isolated ${\mathrm{N}}_{2}$ and to the theoretical predictions for it, the decay into the $3{\ensuremath{\sigma}}_{g}^{\ensuremath{-}1}$ final state shows clearly different behavior much closer to that of normal Auger decay. This is partly due to final-state broadening.

Three-body and one-body channels of the Auger core-valence-valence decay: A simplified approach

We propose a computationally simple model of Auger and APECS line shapes from open-band solids. Part of the intensity comes from the decay of unscreened core-holes and is obtained by the two-body Green's function $G^{(2)}_{\omega}$, as in the case of filled bands. The rest of the intensity arises from screened core-holes and is derived using a variational description of the relaxed ground state; this involves the two-holes-one-electron propagator $G_{\omega}$, which also contains one-hole contributions. For many transition metals, the two-hole Green's function $G^{(2)}_{\omega}$ can be well described by the Ladder Approximation, but the three-body Green's function poses serious further problems. To calculate $G_{\omega}$, treating electrons and holes on equal footing, we propose a practical approach to sum the series to all orders. We achieve that by formally rewriting the problem in terms of a fictitious three-body interaction. Our method grants non-negative densities of states, explains the apparent negative-U behavior of the spectra of early transition metals and interpolates well between weak and strong coupling, as we demonstrate by test model calculations.

Probing the time-dependent decay of molecular core-excited states: The Auger resonant Raman effect forO2

We use our recently developed explicitly time-dependent theory for one-step resonant excitation-deexcitation processes of core-excited states in diatomic molecules to investigate the $\mathrm{O} 1s$ core-hole decay electron spectra of ${\mathrm{O}}_{2}.$ Looking at the changes of the spectra as the exciting radiation frequency is tuned to and away from the excitation resonance allows us to follow the time evolution of the nuclear degrees of freedom of the molecule in the short-living core-excited state. We explicitly demonstrate that here, in contrast to other diatomic molecules such as ${\mathrm{N}}_{2}$ or CO investigated earlier, the relative softness of the intramolecular bond in the core-excited state allows us to interpret changes in some gross features of the decay spectra with varying photon energy in terms of the classical time evolution in the excited state. In particular, the low-energy tail of the electron spectra, due to the fraction of the core-excited state molecules surviving longest, gets visibly suppressed if the exciting radiation is detuned away from the x-ray-absorption resonance.

Substrate Mediated Suppression of Postcollision Interaction Effects

The line shapes and kinetic energies of argon ${2p}_{3/2}$ and ${2p}_{1/2}$ photoelectrons from monolayers adsorbed on a Ru(001) substrate were measured with an electron time-of-flight detector in the near-threshold region. Compared with gas phase results, the two main effects due to postcollision interaction, i.e., line broadening and redshift of the kinetic energy of threshold electrons, are strongly suppressed on the surface. We show that these changes are brought about by an effective, screening induced reduction of the energy loss encountered by the threshold electron upon transition from a singly to a doubly charged central argon atom at core-hole decay.

Core-hole decay mechanism in Ru studied by Auger–photoelectron coincidence spectroscopy

Abstract In order to explore the photoexcitation and decay mechanism of holes created in the 4p core level in Ru, the N 2 , 3 VV Auger spectrum of the Ru(0001) surface has been measured in coincidence with several features of the photoemission spectra and vice versa. The main features of the core level photoemission spectrum and the Auger spectra can be successfully modelled by atomic-like multiplets of the p 5 d 8 and d 6 configurations respectively, meaning that the core hole is suddenly screened. Furthermore, the photoemission spectrum cannot be simply described by a 4p 1/2 ,4p 3 / 2 spin-orbit doublet, but the angular momentum coupling between the 4p core hole and the 4d open shell must be considered. The Auger electrons emitted in coincidence with the satellite feature, shifted 10 eV below the 4p core level, have a high energy threshold that exceeds the threshold of conventional Auger measurements by an amount corresponding to the downward energy shift of the satellite feature. This indicates that the excitation associated with the satellite feature is long-lived and participates in the Auger decay. Remaining differences between the experimental and the model spectra for the Auger emission could be explained by correlation effects.

Inverse photoemission spectra of the Nb(111) surface

The k -resolved inverse photoemission spectra of the single crystal Nb(111) surface were measured with normally incident electrons in the energy range 21–52 eV. Unoccupied electronic states were studied along the Γ–P–H symmetry direction in the ΓHPN symmetry plane. The experimental results are in a reasonable agreement with the energy-band calculations made by the linear muffin-tin orbital method with the atomic-sphere approximation (LMTO-ASA). Two characteristic features appeared in the spectra at the fixed emitted photon energy. First, we obtained an enhancement of the inverse photoemission intensity at the plasma frequency energy of 19.7 eV, and second an enhanced emission at 31.0 eV corresponding to the fluorescence decay of the Nb 4p core holes.

Ionic fragmentation of inner-shell excited molecules

The combination of tuneable synchrotron radiation and multicoincidence time-of-flight mass spectrometry is an effective means of studying the core-hole decay and fragmentation of inner-shell excited molecules. In addition to aiding spectral interpretation, partial ion and ion-pair yields enable both the more specific identification of fragmentation channels and the identification of site and/or state selective fragmentation processes. The power of photoelectron-photoion-photoion coincidence (PEPIPICO) and related techniques is illustrated using selected examples from recent high resolution spectroscopic and ionic fragmentation studies of PF 3 and SPF 3.

And x-ray emission lines of rare-earth trifluorides and

X-ray emission spectra due to the rare-earth and electronic transitions in rare-earth compounds have been measured with x-ray excitation. The satellite structure due to the 4d - 4f exchange interaction in the final state is observed in Ce compounds, as seen in other heavier rare-earth compounds. The intensity ratio of the satellite to the main band in the line is larger than that in the line, which is conspicuous in heavy rare-earth compounds. The experimental results are well explained by the theoretical calculation, where the multiplet couplings and the dependence of the 4d core-hole decay rate on the multiplet terms are taken into account. It is found that the spectral shapes of and resemble each other as a result of the phase matching of the wave functions between the intermediate and final states.

Desorption of H ions from water chemisorbed on Si(100) by O 1s excitation - an Auger electron-photoion coincidence spectroscopy study

Auger electron-photoion coincidence (AEPICO) spectroscopy, which has been recently developed and proved to be a very powerful technique for investigating the dynamics of desorption induced by the core-level excitation, is applied to the investigation of Auger-stimulated ion desorption from the chemisorbed-water-Si(100) surface induced by O 1s excitation. It is shown that the fast relaxation of the excited state with a core hole and an excited electron takes place before the core hole decay, and that the desorption yield is enhanced by the shake-up (and/or shake-off) excitation. The relative cross-section for Auger-stimulated ion desorption is estimated, and is shown to increase as holes are created at deeper levels of the valence bands as the final state of the Auger decay. A comparison is also made with condensed H 2O.

Decay of core holes in cesium chloride studied by the luminescence spectroscopy

The relaxation of the electronic excitations created in the CsCl single crystal after the Auger decay of Cs + 4d core hole was studied by the measurements of crossluminescence (CL), STE emission, excitation spectra and decay kinetics in the energy range 70–140 eV and at temperatures from 10 to 350 K using synchrotron radiation from the SRS storage ring. CL decay time at hv > 80 eV decreased with temperature in the range 10–150 K, then increased with it up to 350 K, but did not reach the value observed at 10 K. At 140 K < T < 350 K and excitation energies hv > 80 eV, a molecular-type band structure with the separation between the peaks of ~ 0.24 eV superimposed over the CL emission spectrum was observed similar to the CN − spectrum in other alkali halides. Possible relaxation channels for the electronic excitations involved are discussed.

De-excitation in solid SiCl 4 following deep-core excitation at the K-edge: relation between ion desorption and auger decay

Photon-stimulated ion desorption from solid SiCl 4 following the deep-core excitations at the K-edges has been investigated together with the Auger decay spectra using synchrotron radiation. The major Auger-decay channel following the 1 s → σ ∗ resonance excitation was KLL spectator Auger decay, in which the excited electron remains in the σ ∗ orbital during the Auger transition. The photon-energy dependence of the ion desorption yields around the Cl K-edge revealed that the Cl + ions scarcely desorb in the photon energy region where Cl KLL normal Auger decay happens. This fact indicates that the spectator Auger decay is essential for the Cl + desorption because the existence of the spectator electron in the antibonding σ ∗ orbital reduces the SiCl bonding character resulting in the fragmentation. Among the Cl 1 s → σ ∗ resonances, Cl + desorption yield is high at the Cl 1 s → σ ∗ (8 a 1) resonance compared to that at the Cl 1 s → σ ∗ (9 t 2) resonance. The result is explained by the higher component of the antibonding Cl 3p ∗ in the 8a 1 orbital. Based on these results, it is concluded that the dissociation of the SiCl bond by the Cl 1 s → σ ∗ resonance excitation is faster than the core hole decay, which means that the Cl atom moves during the core life time, i.e. ultrafast non-Franck-Condon-like dissociation happens.

Core hole double-excitation and atomiclike Auger decay in N2.

Core hole decay spectra of the free N{sub 2} molecule show evidence for hitherto unobserved molecular resonances both below and above the {ital K}-shell photoionization threshold. Based on earlier calculations they are assigned to doubly excited neutral states which could not be seen below threshold in recent high resolution absorption spectra because of the more intense core-to-Rydberg excitations. By calculating the Auger spectrum of core-excited nitrogen atoms, we show that the features are atomiclike. {copyright} {ital 1996 The American Physical Society.}

One-step and two-step description of deexcitation processes in weakly interacting systems.

The origin of one- and two-step behavior of core-hole decay processes is identified in connection with high resolution autoionization spectra for Ar/Pt(111). The spectra simultaneously show features of the two types (resonant Raman vs Auger-like behavior). The character of the process is determined by whether excitations in the intermediate state created in the scattering process are detected in the decay process or not. The consequences for phenomena like resonant Raman Auger, resonant inelastic x-ray scattering, and resonant photoemission are discussed.

Classical trajectories studies of DIET from calcium fluoride

We investigate desorption of positive ions resulting from the repulsive environment created by core-hole Auger decay from relaxed CaF 2 surfaces. The molecular dynamics simulations in the lamina geometry (with two-dimensional ion-lattice summation) is used. For both (011) and (111) surfaces the simulation with changed charge without providing additional kinetic energy does not lead to the ejection of F + ion due to the lattice rearrangement and trapping of the ion. We also assume that the positive ion gains a substantial amount of kinetic energy at the onset of simulations, crudely mimicking ion-stimulated desorption. For the (011) surface the results are extremely sensitive to the size of the considered system, in sharp contrast to the ejection of positive ions from alkali halides. For a 384 ion system, ejection occurs if the kinetic energy, equal to 0.25 eV or more, is delivered to the F + ion at the start of the simulation. For a 768 ion system ejection occurs only for the initial kinetic energy of 4 eV. This result is probably caused by inadequate classical potential and lack of full convergence of the two-dimensional Ewald summation scheme for a highly disordered system. For the (111) surface with 1536 ions in the cell, ejection occurs for an initial kinetic energy of 0.4 eV.

Selectively excited core-to-core fluorescence of potassium halides.

Core-to-core fluorescence transitions from resonantly excited K 2${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$3d states in KF and KI have been measured and specific interaction energies of the final 3${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$3d configuration were determined. Delocalization of the excited electron in the 2${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$3d states was observed to compete with the core hole decay. Furthermore, evidence for an influence of the chemical surrounding on potassium properties such as the 3s core hole lifetime, the 3s3d exchange splitting, and the configuration interaction of the 3s core hole state was found.

The dynamics of core hole excitation and decay in adsorbed and condensed molecules on solid surfaces

In isolated molecules, the decay of neutral and ionic core-excited species leads to distinctly different decay electron spectra which are usually termed autoionization and Auger spectra, respectively. If a molecule is coupled to a substrate in an adsorbate state, or to other surrounding species in a condensed phase, this coupling can modify or, in the extreme, eliminate this difference; also, satellite structure can be influenced. This results from charge and/or energy transfer between the target molecule and its surroundings before the decay event, and can therefore be used to obtain information about such processes on time scales on the order of the core hole life time or shorter. Additional information can be derived from the registration of fragmentation of the molecule as a consequence of the primary excitation and/or the final states of decay. In this case, a third time scale, that of motion of atoms or groups of atoms, comes into play which in extreme cases can compete with those of hole decay and electronic coupling to the surroundings. Such coupling to nuclear motion is also at work in the interference of the electronic evolution with vibrational motion in the molecule which can be investigated with sufficient resolution in the primary excitation. Again, the change induced here into the behavior of the free molecule by the presence of a coupled medium (substrate or condensate) can be used to extract information on the relative time scales. At present much of this information is of a qualitative nature; adequate theories would be very helpful for more detailed understanding.

High resolution studies of the electronic decay of core holes in molecules: there is no threshold at the “XPS threshold”

Using high resolution excitation with synchrotron radiation we have studied the electronic decay of core hole excited states in CO 2 as the excitation energy is varied across the threshold for the core electron photoemission. All throughout this energy region the core hole decay spectra develop continuously and there is no threshold behavior observed in these spectra. These experimental observations are in agreement with recent theoretical work treating the excitation of core electrons continuously across the ionization threshold.

A time-of-flight spectrometer for energy-resolved, Auger-electron, multiple-ion coincidence spectrometry

A time-of-flight mass spectrometer, optimized for energy-resolved, Auger-electron, multiple-ion coincidence spectrometry suitable for the study of core-hole decay in molecules, is presented. The mass spectrometer collects the entire 4π angular distribution of energetic fragment ions up to 20 eV, while maintaining a low potential gradient in the ionization region to allow Auger-electron energy analysis.

High-resolution and high-sensitivity instrumentation for detection of visible-ultraviolet luminescence from products of core-hole decay

High-resolution and high-sensitivity instrumentation has been designed and assembled for the study of vibrationally- and rotationally-resolved luminescence from products of core-hole decay in molecules. The luminescence is produced by the interaction of a high-density molecular beam with either high-energy electrons or with monochromatic synchrotron radiation and is detected by a large-aperture 1-m spectrograph equipped with an intensified photodiode array.

Deexcitation processes in adsorbates.

It is shown theoretically that there is competition between core-hole decay and delocalization of a resonantly excited electron from the adsorbate to the substrate metal with simultaneous substrate-to-ligand charge-transfer core-hole screening [relaxation from the resonantly core excited neutral state to the x-ray-photoemission-spectroscopy (XPS) core ionized state] or between the core-hole decay and the charge-transfer core-hole screening from the substrate metal to the adsorbate (relaxation from the XPS core ionized state to the resonantly core excited neutral state). In the former case a spectral feature due to the Auger decay from the ionic core-hole state appears in the autoionization spectrum, while in the latter case a spectral feature arising from the spectator and participant decay from the neutral screened core-hole state appears in the Auger spectrum measured high above the core ionization limit. The relaxation time can be determined from the relative Auger and autoionization spectral intensity. Applications of the Auger-photoelectron coincidence spectroscopy and x-ray-emission spectroscopy to adsorbates are also discussed.