Ion excited auger spectra of aluminum

An Auger spectrum from aluminum has been detected for the first time using argon ion bombardment as the method for excitation. The Auger spectral line shape differs from that obtained using electron excitation.     

Experimentally derived Auger transition probabilities in X-ray excited Auger electron spectroscopy (XAES)

The capability of Auger transition probabilities experimentally derived from X-ray excited Auger electron spectra in XPS were tested. The relative sensitivity factor (RSF) method has been employed in the quantification by AES (Electron excited Auger electron spectroscopy). However, the difference between experimentally derived RSF and theoretically calculated ones has been found in some reports. One of the great reason of the difference may be caused by the calculated values of the Auger transition yield which has been commonly employed without the consideration of the allotment of coupling scheme in the transition selected in the quantification, for instance, the allotment of each six coupling KL₁L₁, KL₁L₂, KL₁L₃, KL₂L₂, KL₂L₃, and KL₃L₃ in KLL transition. The employment of derived Auger transition probabilities reduce the difference between theoretically calculated RSF and experimentally derived one.     

Observation of resonance recombination lines in electron excited auger spectra of Gd

Combined measurements of electron excited N_4,5 Auger spectra and photoelectron emission on clean and oxidized Gd lead to a distinction between Auger lines originating from 4d → continuum and 4d → 4? resonance excitations. Several Auger structures are identified as due to the direct recombination of 4d⁹4?⁸ states with the 4f and valence electrons. The shape of the most prominent Auger line for oxidized Gd agrees perfectly with the Fano profile of the 4? photoemission intensity.     

Vibrations spectra of oxygen chemisorbed on nickel (110) surfaces

Using a simple tight-binding scheme to describe the nickel d states and the oxygen p states, we calculate the positions and the vibration frequencies of chemisorbed oxygen atoms on nickel (110) surfaces. The comparison between our results and the high resolution electron loss measurements suggests a longbridge chemisorption site at low and high oxygen coverages on the nickel surface.     

Vibrational spectra of ammonia chemisorbed on platinum (111): I. Identification of chemisorbed states

Ammonia chemisorption on Pt(111) has been studied with high resolution electron energy loss spectroscopy (EELS), combined with thermal desorption spectroscopy (TDS). We detect two distinct molecular states of ammonia at different coverages. Near saturation monolayer coverage, ammonia is weakly chemisorbed (ΔH_ads ～ 9 kcal mol^?1) and coordinated to the metal surface via the nitrogen atom. The vibrational frequencies are shifted from the gas phase values, but not as strongly perturbed as in stable platinum-ammine complexes. Below 40% saturation coverage, a new molecular ammonia state is detected, which has a distinct vibrational and thermal desorption spectrum. This state has a considerably reduced ν(PtN) intensity, and the other frequencies are closer to those in solid ammonia, indicating a weaker interaction with the Pt surface. The thermal desorption spectrum of this lower coverage state is broad, from 160 to 450 K, and coverage dependent. Conversion between the two molecular states appears to be only a function of coverage. We propose that the two molecular states have different adsorption sites, and convert from one form to the other as the coverage is changed. No evidence is found for significant dissociation of ammonia on the Pt surface. At very low temperatures (100 K), solid ammonia multilayers may be grown.     

Three-electron auger process from beam-foil excited multiply charged ions

Electron emission from collisions of C3+ ions (22.7 A MeV) with carbon foils (21, 49 and 90 microg/cm(2)) was studied by the time-of-flight method. Two prominent emission patterns can be readily identified as "binary encounter" electrons and "cusp" electrons. With the thinnest target only, a third structure is visible at slightly lower time-of-flight (thus slightly higher energy) than the cusp electrons. The energy of these electrons would correspond to 647(+116)/(-104) eV if they were emitted from the projectile frame of reference. A possible explanation is a rare three-electron-Auger K(2)L(2)L(1) process.     

Removal of X-ray satellites from Mg Kα excited photoelectron spectra

A numerical method is described to remove X-ray satellite lines from photoelectron spectra excited with Mg Kα radiation. The satellite intensity     

X-ray photoelectron (XPS) and Auger (XAES) spectroscopy study of rhodium dispersed on carbon substrate

Photoelectron 3d_5/2 binding and Auger M₅VV kinetic energies of dispersed rhodium have been measured. For the smallest deposition of Rh the shifts 0·6 eV and -2·5 eV of 3d_5/2 and M₅VV lines respectively have been found. Electron core level energy, Auger kinetic energy and Fermi edge shifts have been interpreted by a model based on electrostatic relaxation mechanism in metal particles and initial state contribution of surface atoms. Intensity and background height analysis indicate only surface distribution of Rh.I wish to express my gratitude to Dr. Zdenk Bastl for helpful discussions.     

Auger spectra of carbon monoxide chemisorbed on Pt(111) and Cu(111)

The Auger spectra of carbon monoxide adsorbed on Pt(111) and Cu(111) are compared. The characteristic features now regarded as a fingerprint of this adsorbed species are observed, even for the weakly adsorbed CO on copper which gives complex X-ray photoelectron spectra. No coverage dependence of the spectra was observed on either substrate. The C lsVV spectrum of CO/Cu(111) is dominated by transitions involving the “screening” electron in the 2π orbital.     

High energy (~ 107 eV) Si peak in Ar excited auger emission from silicon and silicides

The Ar⁺ excited electron emission from Si and Si-Ni compounds (from NiSi₂ to Ni₃Si) was studied, with emphasis on the high energy peak in the Si (L₂₃-related) spectrum. This peak is associated with the decay of Si atoms having two 2p holes (Si²*); therefore it originates in asymmetric (Ar-Si) collisions only. It has been investigated to determine the occurrence of these collisions with respect to the symmetric (Si-Si) ones and their relative weight.The threshold energy for the Si^2* related Auger emission was found to be about 2.9 keV in Si and silicides, significantly lower than previously reported. Above this threshold, the relative weight of the asymmetric collisions increases with ion energy and depends on the target stoichiometry, being greater in metal-rich silicides. However in the investigated ion energy range (1 to 5 keV) the total Auger yield was found to be mainly related to the symmetric collisions.We also investigated the dependence of the high energy Si peak on the excitation and acceptance geometry. The results indicate that asymmetric collisions are mainly “surface events”, resulting in the ejection of an anisotropic flux of energetic Si atoms.     

Binding energies and K-edge spectra of NO chemisorbed on Ru(0001)

The electronic structures, adsorption energies and equilibrium geometries of chemisorbed NO on the Ru(0001) surface for several adsorption sites were determined using cluster models and employing the first principles, local density theory. Ground state one-electron eigenvalues and wave functions were obtained using the discrete variational method with numerical atomic bases. K-edge X-ray absorption spectra of both nitrogen and oxygen for each geometry were obtained using a scattered wave method. The effects of the surface on these spectra were studied by comparison with results from an isolated NO molecule and with the XPS and UPS experiments of Umbach et al. Both the NO and the NO-substrate vibration frequencies were calculated. Comparison between these theoretical results, recent electron energy loss spectra of Conrad et al. and early experiments of Thomas et al. suggests that NO is adsorbed only at threefold and top sites.     

X-ray excited photoelectron spectra of LiNbO3: A quantitative analysis

X-ray excited photoelectron spectra (XPS) using monochromatized Al — K radiation ( =1,486.7 eV) have been analysed quantitatively. The intensities of the various core lines (Li(1s), O(1s), O(2s), Nb(3s), Nb(3p), Nb(3d), Nb(4s), Nb(4p)) normalized to the intensity of the Nb (3d _5/2)-line, show good agreement with those calculated with theoretical values for the photoabsorption cross section of the free atoms and the theoretical estimates of the dependance of the electron escape depth on the electron kinetic energy. From the energy loss spectra observed at the high binding energy side of the XPS core lines the energy loss function Im {–1/()} is calculated and compared to published optical data. This comparison yields directly the total electron escape depth as function of the electron energy over a large energy range.     

Stimulated desorption from CO chemisorbed on Cr(110)

Electron stimulated desorption (ESD) experiments using a time-of-flight pulse counting method are reported for molecular CO chemisorbed on the Cr(110) surface at 90 K. Consistent with previous qualitative observations, negligible CO⁺ and O⁺ desorption signals were measured from the ₁CO overlayer which saturates at 1/4 monolayer. For θ_CO > 0.25, a terminally-bonded (₂CO) binding mode is populated in addition to the existing ∝₁CO binding mode and the ion yield increases sharply. For ₂CO, both O⁺ and CO⁺ ions are observed; the CO⁺ ions desorb with characteristically lower kinetic energies than O⁺ ions. Near saturation coverages of CO(ads), an observed decrease in the O⁺ yield is attributed to adsorbate-adsorbate interactions which reduce the ion desorption probability, as seen in ESD studies of terminally-bonded CO on other metals. These results are considered in the context of two possible models proposed for the ₁CO binding state and related ESD observations for CO chemisorbed on Fe(001) and potassium-promoted Ru(001).     

Magnetic circularly polarized X-ray excited optical luminescence (MCP-XEOL)

Abstract

We present recent experimental results of magnetically induced circular polarization of X-ray excited optical luminescence of paramagnetic rare-earth ions. X-ray excitation offers not only the valuable advantage of a high quantum yield but also eliminates the contamination of the luminescence signal with scattered excitation light. These combined advantages allowed us to record MCP-XEOL spectra from a thin layer of powdered samples whereas all previous MCPL experiments were inherently restricted to single crystalline samples or homogeneous liquid solutions.     

Angle-resolved photoemission from CO chemisorbed on Rh(110)

He(II)-excited UPS spectra of CO chemisorbed on Rh(110) have been acquired in symmetry-equivalent directions relative to the substrate. In these spectra a marked anisotropy was noted in the emission from the 4σ level. Comparison of the experimental spectra with calculations of the differential photoionization cross-sections suggests that the CO molecule is chemisorbed in an end-on configuration with the carbon end nearer to the surface.     

Exafs like oscillations in X-ray excited autoionization spectra assisted by compton process

Oscillating features have been observed in the background of the inelastic electrons excited by high energy monochromatic X-ray photons at kinetic energy above the Cr M_2,3VV Auger transition. These structures are absent when X-ray photons of are used. We suggest that these extended features are due to an autoionization process experienced by the 3p core electron assisted by a Compton scattering with the X-ray photons. Through the inelastic Compton process the incoming photons excite the core electrons towards empty states above the Fermi level in a continuous manner up to a maximum energy, which strongly depends on the incident photon energy. These excited states are modulated in energy by the crystalline structure as it has been shown by means of the EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy. These oscillating structures are compared with those obtained through electron excitation for the same Auger transition in the Extended Fine Auger Structure (EXFAS) spectroscopy.     

High energy (˜ 107 eV) Si peak in Ar excited auger emission from silicon and silicides

The Ar⁺ excited electron emission from Si and Si-Ni compounds (from NiSi₂ to Ni₃Si) was studied, with emphasis on the high energy peak in the Si (L₂₃-related) spectrum. This peak is associated with the decay of Si atoms having two 2p holes (Si²*); therefore it originates in asymmetric (Ar-Si) collisions only. It has been investigated to determine the occurrence of these collisions with respect to the symmetric (Si-Si) ones and their relative weight.

The threshold energy for the Si^2* related Auger emission was found to be about 2.9 keV in Si and silicides, significantly lower than previously reported. Above this threshold, the relative weight of the asymmetric collisions increases with ion energy and depends on the target stoichiometry, being greater in metal-rich silicides. However in the investigated ion energy range (1 to 5 keV) the total Auger yield was found to be mainly related to the symmetric collisions.

We also investigated the dependence of the high energy Si peak on the excitation and acceptance geometry. The results indicate that asymmetric collisions are mainly “surface events”, resulting in the ejection of an anisotropic flux of energetic Si atoms.     

Chemical shifts in auger electron spectra from silicon in silicon nitride

The chemisorption of nitric oxide on (110) nickel has been investigated by Auger electron spectroscopy, LEED and thermal desorption. The NO adsorbed irreversibly at 300 K and a faint (2 × 3) structure was observed. At 500 K this pattern intensified, the nitrogen Auger signal increased and the oxygen signal decreased. This is interpreted as the dissociation of NO which had been bound via nitrogen to the surface. By measuring the rate of the decomposition as a function of temperature the dissociation energy is calculated at 125 kJ mol^?1. At ～860 K nitrogen desorbs. The rate of this desorption has been measured by AES and by quantitative thermal desorption. It is shown that the desorption of N₂ is first order and that the binding energy is 213 kJ mol^?1. The small increase in desorption temperature with increasing coverage is interpreted as due to an attractive interaction between adsorbed molecules of ～14 kJ mol^?1 for a monolayer. The (2 × 3) LEED pattern which persists from 500–800 K is shown to be associated with nitrogen only. The same pattern is obtained on a carbon contaminated crystal from which oxygen has desorbed as CO and CO₂. The (2 × 3) pattern has spots split along the (0.1) direction as $\text{(m,}\text{n}\text{3}\text{)}$ and $\text{(}\text{m}\text{2, n}\text{)}$. This is interpreted as domains of (2 × 3) structures separated by boundaries which give phase differences of $\text{2π}\text{3}$ and π. The split spots coalesce as the nitrogen starts to desorb. A (2 × 1) pattern due to adsorbed oxygen was then observed to 1100 K when the oxygen dissolved in the crystal leaving the nickel (110) pattern.