Theoretical fundamentals of high resolution Auger electron spectroscopy

A method is discussed for obtaining narrow Auger electron lines of atoms, molecules and solids with a halfwidth much smaller than the natural width Γ_n of a respective intermediate X-ray excited state |n>. The method provides a possibility of resolving diverse fine structures in Auger spectra.     

An interpretation of the O2 Auger electron spectrum

The Auger electron spectrum of O₂ is interpreted by comparing with other spectra, such as the photoelectron spectrum, the electron impact mass spectrum, and the double charge transfer spectrum. Each of these four spectra obeys its own selection rule; the difference in the selection rules plays a key role in our interpretation. Auger decays following a core shake-up excitation are identified in the O₂ spectrum for the first time. Many of the previous assignments are revised.     

Theoretical investigation of the sulphur K-LL auger energies and chemical shifts in a series of sulphur compounds

Two series of investigations are reported on LL double-hole states of molecules containing sulphur. First the results on the LL double ionization potentials and K-LL Auger energies of H₂S and SO₂ show that the use of a frozen-orbital approximation is preferable to SCF methods for calculation of chemical shifts in K-LL Auger energies and LL ionization potentials. Secondly chemical shifts of K-LL Auger energies of a series of model molecules H₂S, H₂SO₄, H₄SO₂ and H₂SO₂ are correlated with the formal oxidation state of sulphur. This correlation gives a shift of 12 eV in Auger energy per formal charge on sulphur.     

Quantitative Auger electron spectroscopic analysis of carbides,nitrides and carbonitrides

A novel method for the quantitative evaluation of Auger electron spectra based on peak areas is presented. Sample and reference spectra in integral mode are filtered by an area conserving digital filter. This transforms the peak shapes influenced by chemical effects into standard peak shapes. After filtering a linear combination of reference spectra, differentiated spectra accounting for peak shifts and some low order polynomials to account for variations in the background is fitted to the sample spectrum by a least squares method. The need to approximate the spectrum of the secondary electron background explicitly for direct calculation of peak areas is thus eliminated. Filters of different widths are applied to reduce errors by chemical effects. The composition of the sample is computed from the composition of the reference samples and the coefficients obtained from the fit.To demonstrate the validity of this technique it has been applied to both, Gaussian model peaks and spectra of titanium carbonitrides. A further test on an alloy series is under investigation. The results show that the method works as predicted and gives accurate quantification.     

Vibrational and lifetime line broadenings in ESCA

The line profile of the narrow, symmetric 1s line from neon, recorded with the new ESCA instrument with X-ray monochromatization, is analyzed. The natural linewidth of this line is found to be 0.23 ± 0.02 eV, in good agreement with theoretical calculations of the oscillator strengths for Auger transitions and X-ray emission. Spectra from molecules show frequently asymmetric core electron lines under high resolution. This rules out previous explanations based on a chemical influence on the natural lifetime. Contrary to earlier assumptions, vibrational excitations are shown to be important in core electron spectra. For methane, the vibrational energy spacing is large enough to allow the vibrational lines to be partly resolved. Recent results from accurate PNO CI calculations on methane agree well with the experimental findings. The Franck-Condon transitions in the C1s and N1s lines from CO and N₂ are shown to be well described in the harmonic approximation and approximating the potential curves of the highly excited core hole states with the potential curve for the ground state of NO⁺, X¹ Σ⁺. Knowledge of vibrational excitations in core electron spectra is shown to be valuable in the analysis of high resolution X-ray emission spectra of free molecules.     

SCF Xα SW calculations of chemical shifts of X-ray and electron emission lines and relaxation properties of SiX4 molecules

SCF Xα SW calculations of the 1s and 2p binding energies, KLL Auger energies and Kα transition energies for the molecules SiH₄, SiCl₄ and SiF₄ and the corresponding atomic Xα calculations for charged free silicon ions have been carried out. The results provide information about relaxation properties and anomalous chemical Kα shifts in hydrides.     

Electronic structure of N,N-dimethylnitramine and N,N-dimethylnitrosamine from X-ray and UV electron spectroscopy

The electronic structure of N,N-dimethylnitranine. (CH₃)₂NNO₂, and N,N,-dimethylnitrosamine, (CH₃)₂NNO, has been investigated through the use of electron spectroscopy and quantum chemical methods. Experimentally, the electron spectra of (CH₃)₂NNO₂ and (CH₃)₂NNO were measured in the valence region on the gas phase molecules using UV (HeI and He II) radiation and in the valence and core regions on the solid materials (frozen at 170 K) using X-radiation (AlK_α). Theoretically, the results of INDO and MWH Extended Hucket MO calculations are used for spectral interpretation. The semi-empirical potential model of Ellison is used for predicting is chemical shifts. The results of this investigation exemplify how the UV and X-ray electron spectroscopy techniques are symbiotic and how application of the two techniques along with the use of quantum chemical methods provide unique information about electronic structures.     

X-ray photoelectron emission studies of mixed selenides AgGaSe2 and Ag9GaSe6

Auger and direct electron specta from crystalline AgGaSe₂ and Ag₉GaSe₆ have been studied with X-ray photoelectron spectroscopy. It is shown that the AgM₅N_4,5N_4,5 and M₄N_4,5N_4,5 Auger spectra are more sensitive to the chemical environment than the Ag 3d direct photoelectron spectra. Furthermore the Auger parameter as defined by Wagner is used in order to characterize the chemical state of these compounds. Last, the XPS spectra of the valence-band region are investigated and chalcogen s and p and noble-metal d bands are clearly identified. The electronic structure of these two selenides does not seem to be determined predominantly by the crystal structure. As a whole, the spectral features are discussed in connection with the character of the chemical bonding and the physical properties of these compounds.     

Coincidence electron spectrometer for studying electron–atom collisions

The details of our recently developed coincidence electron spectrometer system with the block diagram and characteristics of the coincidence circuit are presented. As a first application we measured the continuous energy spectra of the scattered electron projectiles producing an L_2,3 inner-shell ionization/excitation of argon. The studied energy region was the neighbourhood of the ionization edge, where nearly the total excess energy is taken away by the scattered projectile. Above the edge we identify the peaks of the scattered electrons that produced 2p_3/2→4s or 2p_3/2→3d excitation.     

Characterization of aluminum–organic‐stabilized platinum–colloid networks with electron and photon spectroscopies

We have measured and interpreted the ultraviolet (HeI) and X‐ray photoelectron spectra and the metastable impact electron spectra (MIES) from aluminum–organic‐stabilized platinum–colloids and colloid networks, deposited on silicon substrates and characterized by X‐ray photoelectron spectroscopy, scanning Auger electron microscopy and transmission electron microscopy. MIES, in particular, gives information on the electronic structure of the spacer molecules interconnecting the colloids. In addition, changes in the electronic structure of the platinum clusters that are induced by different spacer molecules were identified by means of X‐ray absorption near‐edge structure measurements at the platinum L_III‐edge of these materials. This combination of techniques was also employed to follow the chemical changes that occur upon heating of the network in situ. It turns out that the thermal decomposition of the network is driven by the disintegration of the spacer molecules. Moreover, less sintering of the colloidal particles occurs in the networked systems than in unconnected particles. Most of the networked platinum–particles are still present in their original shape even after the destruction of spacer molecules. This observation could be linked to the encapsulation of these platinum particles into an (Al? O) protecting shell. Copyright © 2003 John Wiley & Sons, Ltd.     

Kinetic energies to analyze the experimental auger electron spectra by density functional theory calculations

In the Auger electron spectra (AES) simulations, we define theoretical modified kinetic energies of AES in the density functional theory (DFT) calculations. The modified kinetic energies correspond to two final-state holes at the ground state and at the transition-state in DFT calculations, respectively. This method is applied to simulate Auger electron spectra (AES) of 2nd periodic atom (Li, Be, B, C, N, O, F)-involving substances (LiF, beryllium, boron, graphite, GaN, SiO₂, PTFE) by deMon DFT calculations using the model molecules of the unit cell. Experimental KVV (valence band electrons can fill K-shell core holes or be emitted during KVV-type transitions) AES of the (Li, O) atoms in the substances agree considerably well with simulation of AES obtained with the maximum kinetic energies of the atoms, while, for AES of LiF, and PTFE substance, the experimental F KVV AES is almost in accordance with the spectra from the transitionstate kinetic energy calculations.     

Summary of ISO/TC201 standard: ISO/TR 18394:2006—surface chemical analysis—Auger electron spectroscopy—derivation of chemical information

ISO/TR 18394 provides guidance for the identification of chemical effects on x‐ray or electron‐excited Auger electron spectra as well as for applications of these effects in chemical characterization of surface/interface layers of solids. In addition to elemental composition, information can be obtained on the chemical state and the surrounding local electronic structure of the atom with the initial core hole from the changes of Auger electron spectra upon the alteration of its local environment. The methods of identification and use of chemical effects on Auger electron spectra, as described in this Technical Report, are very important for accurate quantitative applications of Auger electron spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.     

Observation of Main‐Group Tricarbonyls [B(CO)3] and [C(CO)3]+ Featuring a Tilted One‐Electron Donor Carbonyl Ligand

A combined experimental and theoretical study on the main‐group tricarbonyls [B(CO)₃] in solid noble‐gas matrices and [C(CO)₃]⁺ in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η¹(μ₁‐CO)‐bonded carbonyl ligand, which serves as an unprecedented one‐electron donor ligand. Thus, the central atoms in both complexes still retain an 8‐electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two‐electron donors that display classical σ‐donation and π‐back‐donation following the Dewar–Chatt–Duncanson model. The tilted CO ligand is a formal one‐electron donor that is bonded by σ‐donation and π‐back‐donation that involves the singly occupied orbital of the radical fragments [B(CO)₂] and [C(CO)₂]⁺.     

Quantitative Auger electron spectroscopy of PtSi and Pd2Si

Summary Quantitative AES analysis has been carried out on PtSi and Pd₂Si using dN(E)/dE spectra. The concentration ratios are determined by elemental sensitivity factors after correcting for matrix and sputter effects. In the case of matrix correction an improved correction procedure has been developed which considers effects created by different atomic densities, electron backscattering, electron attenuation and peak shapes. The concentration ratios X_metal/X_Si deduced from measurements of mechanically cleaned sample surfaces agree very well with the nominal bulk values of the silicides. Ion bombardment creates strong intensity and line shape changes of the Auger signals. Therefore, in case of sputtered silicide surfaces, both matrix and sputter effects must be corrected.     

CO inner-shell excitation studied by electron impact spectroscopy

The inner-shell excitation and decay of the CO molecule have been studied in electron impact experiments. The dipole-forbidden transition (1sσ_c)⁻¹(2pπ) ³Π has been characterized by angular resolved electron energy loss spectroscopy and its decay via the measurement of resonant Auger spectra. The contribution of the (1sσ_c)⁻¹(2pπ) ³Π state to the CO resonant Auger spectrum in the region of the “spectator transitions” has been isolated and the population of CO⁺ quartet final states has been observed.     

CI calculations of electron and x-ray scattering cross sections of non-linear molecules: H2O and NH3

Differential cross sections for high-energy electron and X-ray scattering on H₂O and NH₃ molecules have been calculated in the first Born approximation using approximate Hartree—Fock and configuration-interaction wavefunctions. For non-linear molecules, the first theoretical inelastic and total cross sections with explicit inclusion of electron correlation are presented. A comparison between the theoretical and experimental scattering functions is made. Apart from minor discrepancies which can be due to deficiencies in the theoretical as well as the experimental procedures, agreement between theory and experiment is satisfactory.     

Quantitative surface analysis by X-ray photoelectron spectroscopy and X-ray excited auger electron spectroscopy

It is shown that X-ray excited KLL Auger electron spectra allow it to describe measured signal strengths similarly to X-ray photoelectron signals, thus offering valuable information on the quantitative surface composition of a solid sample. The principal equation and corresponding fundamental parameters are discussed. As a result Auger spectra of C, N, O, F, and Na can be easily used in a multiline approach for quantitative analysis. LMM and MNN spectra give rise to more problems, due to their more complicated structure, uncertainties with regard to the background and the influence of Coster-Kronig transitions. These problems are overcome by the use of empirical ratios of the strongest lines of 2p/LMM or 3d/MNN. Since these ratios are independent of sample composition, they allow it to transform the Auger signal into the corresponding photoelectron signal, provided that a standard sample has been measured. Thus a true additional information is obtained and moreover difficulties in cases of photoelectron spectra with overlapping lines from other chemical elements can be overcome.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Competition between reactivity and relaxation in free electron attachment to molecules

The reactivity of the C₆F₅X (X=F, Cl, Br, I) molecules following low energy (0–15 eV) electron attachment is studied in the gas phase under single collision conditions, free molecular clusters and condensed molecules by means of crossed beams and surface experiments. All four molecules exhibit a very prominent resonance for low energy electron attachment (<1 eV, attachment cross section >10⁻¹⁴ cm²). Under collision free conditions thermal electron capture generates long lived molecular parent anions C₆F₅X^−*. Along the line Cl, Br, I dissociation into X⁻+C₆F₅ and X+C₆F₅-increasingly competes until for X=1 only chemical fragmentation is observed on the mass spectrometric time scale. In free molecular clusters chemical fragmentation is quantitatively quenched at low energies in favour of associative attachment yielding undissociated, relaxed ions (C₆F₅X)⁻ _n,n≥1. A further dissociative resonance at 6.5 eV in C₆F₅Cl is considerably enhanched in clusters. If these molecules are finally condensed on a solid surface, one observes a prominent Cl⁻ desorption resonance at 6.5 eV. While the quantitative quenching of the chemical reactivity at low energies is due to the additional possibilities of energy dissipation under aggregation, the enhanched reactivity at 6.5 eV is interpreted by the conversion of a core excited open channel resonance in single molecules into a closed channel (Feshbach) resonance when it is coupled to environmental molecules.     

Effects of electron polarization on the auger spectra of lanthanum oxide

The SCF-X scattered-wave method has been used in the quasirelativistic approximation to calculate the energies and relative intensities of the transitions in the M₄, ₅N₄,₅V and N₄,₅VV Auger electron spectra AES of lanthanum in the LaO ₆ ^9– cluster, which is taken as a model for crystalline La₂O₃. A study has been made of the influence of relaxation effects on the AES. The AES intensity distribution is largely determined by the population of the 4f states of La in the initial state of the Auger process. The AES for lanthanum in La₂O₃ are interpreted from the calculations.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 207–210, March–April, 1986.     

Synchrotron radiation photoelectron studies for primary radiation effects using a liquid water jet in vacuum: Total and partial photoelectron yields for liquid water near the oxygen -edge

A new spectroscopy to identify the hydration structure playing important role in liquid-phase radiation damage is in progress using a laminar liquid water jet sample in vacuum in combination with soft X-ray synchrotron radiation. We present the total and partial electron yields for liquid water using a photoelectron spectroscopy. Partial electron yields for the K^-11b₁1b₁ Auger transition are obtained for the first time by measuring the electrostatically dispersed electron kinetic energy spectra as a function of photon energy of synchrotron radiation.