Separating the vibrationally resolved Auger decay channels for a CO core hole state

The K-VV Auger spectrum of carbon monoxide (CO) excited by C 1s photoionization has been investigated with a novel electron-electron coincidence setup. The energy resolution is sufficiently high to resolve the vibrational energy levels of the core-ionized intermediate state and of most dicationic final states in the two-dimensional electron energy map. We demonstrate how the influence of vibrational states on a molecular Auger spectrum can be accessed experimentally without the constraint of averaging over all intermediate state energies.     

Breakdown of the two-step model in K-shell photoemission and subsequent decay probed by the molecular-frame photoelectron angular distributions of CO2

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay.     

Interatomic Auger processes and the density of states

The correlation between the line shape of Auger peaks and the density of states near the surface has been the subject of recent controversy. In certain cases, it has been possible to obtain the density of states by numerical deconvolution of a KVV peak (Amelio, 1970) or directly using a KLV peak (Cardona et al., 1973). However, the extension of this technique to transition metals (Cu, Zn) has encountered serious difficulties, related to the perturbation created by the presence of localized charges either in the initial or in the final state, although it is not yet clear why this perturbation is strong only in certain cases. The purpose of the present communication is to show a series of results that can throw some light on the abovementioned problem. The main point is that Auger processes of interatomic type, as those occurring in the INS technique of Hagstrum, are free of these perturbations. Recently, the authors have studied the line shape of the Auger peaks of O, C, N and S adsorbed on Cu, Ni and Fe. These results show that only that part of the Auger structure originated by interatomic transitions between substrate and adsorbate atoms can be related to the local density of states (LDOS). The rest of the structure, due to normal intraatomic processes, is dominated by the spectral terms in the final configuration of the ion. This new interpretation allows a separation of perturbation effects and clarifies the contribution of the LDOS to the peak line shape. In this communication, we present the line shape analysis of the L_2,3 VV and KVV Auger peaks of Mg and O in MgO. Due to the strong ionic character of this compound, the L_2,3 VV peak of Mg⁺⁺ is mainly due to interatomic processes between Mg⁺⁺ and O⁼ ions, whereas the KVV peak of O is mainly due to interatomic processes. This analysis shows that good agreement exists between the L_2,3VV Mg⁺⁺ Auger peak and the self-convolution of MgO density of states, whereas the KVV Auger peak of O⁼ is dominated by the spectral terms of the final configuration. Only a small peak in the high energy side of the latter peak can be related to the density of states and could be interpreted as an interatomic transition between two neighboring oxygen ions, in agreement with the interpretation given by others.     

Decay channel dependence of the photoelectron angular distributions in core-level ionization of Ne dimers

For K-shell photoionization of neon dimers, we report Ne 1s photoelectron angular distributions for Ne2++Ne+ and Ne++Ne+ channels exhibiting quite different patterns. Noninversion-symmetric patterns of the former obtained by the fast interatomic Coulombic decay of Auger final states show direct evidence of core-hole localization. Dipolar patterns of the latter obtained by the slow radiative decay of the other Auger final states clearly show that the radiative process is slow enough to allow dicationic dimers to rotate many times before fragmentation.     

On the satellite in the Auger electron spectrum

The satellite line profile in the C–VV Auger electron spectroscopy spectrum depends upon how the initial core hole state and the final multiple hole state are created. Even if the numbers of the holes in the satellite states are the same, the Auger electron spectroscopy spectral profiles of the final multiple hole states are different. The localization of the three hole satellite state is discussed.     

KVV radiative Auger emission spectrum of metallic silicon

The KVV radiative Auger region in the K X-ray emission spectrum of silicon in metallic powder form has been ionometrically recorded and compared with the expected spectrum calculated by using the measured L_2,3VV Auger spectrum available in the literature. It is found that the experimental KVV radiative Auger spectrum has some additional features. An attempt has been made to identify these as being due to various VV levels as final states.     

CO adsorption on alkali-metal covered Ni(100)

The effect of preadsorbed alkali metal atoms Na, K and Cs on CO adsorption on Ni(100) has been studied using Auger spectroscopy and thermal desorption. It was found that the presence of alkali metals causes an appearance of several more tightly bound states in the CO thermal desorption spectra. The observed difference in carbon and oxygen Auger peak line shape on a bare and alkali modified Ni(100) is indicative that the presence of alkali adatoms induces CO decomposition on the Ni(100) surface. The fraction of dissociated CO increases with the amount of alkali adatoms present. At the same overlayer coverage the dissociation probability increases in the sequence Na, K, Cs. A comparison of the strength of the promoting effect on CO dissociation with the changes in the surface electron density in the presence of alkali adatoms has shown that at low overlayer coverages the electronic factor plays a major role in explaining the action of the surface modificators.     

Direct emission and Auger decay from multiplet final states in Zr observed by ultraviolet photoemission CFS techniques

Photoemission techniques have been used to investigate the structures accompanying excitation of shallow p-core states in crystalline Zr. The continuum nature of synchrotron radiation has been exploited between hv = 20 and 45 eV to determine the Constant Final-state-energy Spectra (CFS) over a range of kinetic energies of the emitted electrons. The results reveal strong direct emission from the multiplet final states and indicate that the final states are broadened by configuration interaction. Auger processes appear to be a probable mode of decay of the p-hole. Secondaries resulting from the Auger processes give rise to structures in the CFS's which compare well to structures in the optical absorption coefficient.     

Interaction of dimethyl methylphosphonate with oxidized iron and the effect of coadsorbed water

The behavior of dimethyl methylphosphonate (DMMP), dosed at 100 K with and without coadsorbed water on oxidized iron has been examined by temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). Molecular and dissociated states of DMMP are readily distinguished by the P(LMM) Auger lineshape. At low coverages DMMP undergoes complete decomposition during heating, leaving carbon, phosphorus and oxygen residues on the surface. The major low temperature decomposition products are CH₃OH, H₂O, CO, H₂ and a surface phosphate species. The DMMP decomposition is limited and large exposures lead to molecular DMMP desorption characteristics of multilayers (200–210 K). Pre-exposure to H₂O increases the extent of DMMP decomposition.     

Nuclear dissociation after the O 1s $\rightarrow (^4\Sigma_\text{u}^-)$3sσ excitation in O$_2$ molecules

We investigate the dissociation dynamics of core-excited $\mathrm{O}_2$ molecules using a high-resolution energy-resolved electron-ion coincidence experimental setup. The excited cationic states with two valence holes and one Rydberg electron are created after spectator Auger decay induced by $\mathrm{O}$ 1s $\rightarrow (^4\Sigma_{\rm u}^-)3{\rm s}\sigma$ core excitation in $\mathrm{O}_2$. From the energy correlation between the kinetic energy of the Auger electron and the ion kinetic energy release, we distinguish several dissociation channels. Rather complex dissociation channels of the spectator Auger final states are disclosed, which can be explained by the increased number of the crossing point due to the existence of Rydberg electron. The quantum system will evolve into different dissociation limits at each crossing point between the potential energy curves.     

Low energy Auger and loss electron spectra from magnesium and its oxide

Data have been obtained from Auger and energy loss processes in clean metallic Mg, Mg during stages of oxidation, and UHV cleaved MgO(100) surfaces. Particular attention has been paid to twenty features below 200 eV in the Auger spectra from these surfaces. A comparison of spectra from the metal, oxidised metal surface, and single crystal MgO has enabled estimates to be made of surface charging effects, and the MgO steady state surface potential is found to be near + 10 V above ground. All the Auger features are given assignments, two of which are interfacial processes involving ionic initial states and metallic final states. Several features in the low energy Auger spectrum are attributed to diffraction of true secondary electrons.     

Auger spectroscopy of polyethylene and poly (ethylene oxide)

The X-ray excited Auger spectra of polyethylene and poly(ethylene oxide) have been corrected for Auger electron energy losses due to interactions with the solid and compared to the corresponding spectra of gas phase molecular analogs. The corrected polyethylene spectrum is an extrapolation of trends observed in the spectra of gas phase alkanes from CH₄ through C₆H₁₄. The O(KVV) spectrum of poly(ethylene oxide) is similar to that of methyl ether, consistent with similar nearest neighbor environments for the oxygen atoms in the two materials. In contrast, the C(KVV) spectrum of poly(ethylene oxide), a material which contains C-C bonds, is better approximated by the spectrum of ethane (H₃C-CH₃). A comparison of the polyethylene Auger spectrum with the spectra of the normal alkanes and with a self-fold of its X-ray excited valence band photoemission (single hole) spectrum indicates the presence of correlated two-hole final states in the case of polyethylene.     

Core-hole screening effects in the initial state of Auger emission across the transition metal series

Supercell method is used to study the relaxation and screening effects on the initial state of the Auger transition in metals. Our consideration is based on the assumption that when a core-hole exists long enough before the Auger transition occurs, the occupied valence states relax to screen the core-hole which results in a redistribution of the valence electrons, in particular within the atom that contains the core-hole. In order to make the interaction between the core-holes sites at different atoms negligible, the real metal is simulated by supercells repeated periodically. In each supercell one atom is considered to have a core-hole and many others not to have one. The electronic states concerned by the Auger transition are calculated by the self-consistent full-potential linearized augmented plane wave (FLAPW) method. Different responses of the local valence band on the site of the core-hole have been shown depending on whether the d-bands are partially or completely filled. According to the final state rule, the screening to the two holes in the local valence band after the Auger transition has also been considered, as examples, for Ni and Cu metals. The result shows that, with the existence of two holes in them, the states of the local valence band of Cu relax to atomic-like impurity states, while the local valence band of Ni changes to a much narrow band at the bottom of the original band. As examples, L₃VV and M₁VV Auger spectral profiles of Cu have been calculated in reasonably good agreement with the experiment.     

Quasi-atomic fine structure in the Auger spectra of solid silver and indium

High resolution M₄,₅N_4,5N_4,5 Auger spectra of Ag and In reveal fine structure, which may be interpreted in terms of spin-orbit splitting of the initial state and multiplet structure in the final state. Interaction between d holes in Ag is an important factor in determining the shape of the Auger spectrum, and so the Auger profile is not related in any simple way to the one-particle density of states.     

Cs Ⅳ离子4d内壳层激发态衰变过程的相对论理论研究

在相对论多组态Dirac-Fock理论方法基础上,通过系统考 虑电子关联效应和由于内壳层电子激发而导致的电子自旋-轨道波函数的弛豫效应,详细研究了Cs Ⅳ离子的4d内壳层电子激发组态4d95s25p5、辐射末态4d105s25p4及Auger末态4d105s25p3和4d105s15p4的能级结构及各种可能的辐射和Auger衰变过程.获得了与已有的实验结果和相关的半经 验准相对论组态相互作用计算结果相符的辐射跃迁能、振子强度以及线宽,预言了4d95s25p5态的以Auger衰变为主的 Auger电子谱的特 关键词： 内壳层激发态 辐射衰变 Auger衰变     

Spin polarization and angular distribution of Auger electrons formed as a result of decay of the 3<Emphasis Type="Italic">d</Emphasis><Superscript>−1</Superscript>5<Emphasis Type="Italic">p</Emphasis> state in the Kr atom

The results of numerical calculations of the energies of Auger transitions, as well as the angular distribution (α₂) and spin polarization (β₂) anisotropy parameters, are presented for transitions in a photoexcited Kr* atom with two open shells. Matrix elements are calculated by the multiconfigurational Fock-Dirac relativistic method using an intermediate type of coupling. The wavefunctions of the initial and final states of the Auger transition are calculated with allowance for relaxation effects. The one-electron wavefunction of the continuous spectrum for an Auger electron is obtained using the single-configuration Fock-Dirac method. The results are compared with experiment and a new experiment is proposed for identifying the Auger state not only from the energy, but also from the total angular momentum of the Auger state.     

Unveiling residual molecular binding in triply charged hydrogen bromide

We present an experimental and theoretical study of triply charged hydrogen bromide ions formed by photoionization of the inner 3d shell of Br. The experimental results, obtained by detecting the 3d photoelectron in coincidence with the two subsequent Auger electrons, are analyzed using calculated potential energy curves of HBr3+. The competition between the short-range chemical binding potential and the Coulomb repulsion in the dissociative process is shown. Two different mechanisms are observed for double Auger decay: one, a direct process with simultaneous ejection of two Auger electrons to final HBr3+ ionic states and the other, a cascade process involving double Auger decay characterized by the autoionization of Br*+ ion subsequent to the HBr2+ fragmentation.     

Chemical effects in the N7 VV Auger spectra from W(100) and W(110) surfaces

Fine structure in the N₇ VV Auger spectra from clean W(100) and W(110) surfaces has been interpreted by Lander's band model for the doubly ionized final state. It is shown that the energies of the prominent emissions in the spectra are similar for the two surfaces and furthermore are consistent with the self convolution of a bulk density of states for tungsten. An additional feature in the spectrum from the W(100) surface has been attributed to emission from an intrinsic surface state at ?0.4 eV. The localization of this state at the surface was confirmed by its sensitivity to adsorbates (H₂, CO, O₂ and I₂). During the interaction of these gases with the surface the Auger spectra always retained the features attributed to the bulk density of states which were modified only by a shift in the background intensity profile. New emission features in this part of the spectra were not seen except for the example of hydrogen adsorption when a single new emission could be seen on each of the two tungsten surfaces. However, each adsorbate produce either one (H₂) or two (CO, O₂ and I₂) new emissions at lower energies which were attributed to emissions from new adsorbate derived levels which reside at energies below the prominent features of the tungsten valence band. The location of these new adsorbate levels is compared and contrasted with the equivalent ultraviolet photoelectron spectroscopic determinations.     

O 1s --> sigma* resonance in O2: inadequacy of only two exchange-split components

The O 1s-->sigma* transition below the O K-edge in O2 has been investigated by absorption, constant ionic state (CIS) experiments, and extensive configuration interaction calculations. CIS scans of the three lowest-lying final states reached in resonant Auger decay provide a wealth of information on energy range, symmetry, and spin multiplicity of the intermediate states with sigma* character. We conclude that the identification of only two exchange-split components is inadequate because a complex manifold of states with sigma* character exists with no unique energy difference between related states.