Molecular Auger Spectra Research Articles

Molecular MNN auger spectra of gaseous tetramethyltin and hexamethylditin

Photoelectron and Auger (MNN) spectra of tetramethyltin and hexamethylditin have been measured and the results compared to those of metallic tin. Energies and intensities of the Auger lines are in agreement with theory based on an Intermediate Coupling scheme since LS coupling theory was inadequate. Relaxation contributions have been evaluated.

Qualitative propagator theory of AX4 Auger spectra

Nonempirical, parametrized two-electron propagator theory is employed in the characterization of molecular Auger spectra. Based on the Mulliken approximation for many-center Coulomb integrals, the model Hamiltonian requires three parameters for each valence atomic orbital: an exponent for a Slater function, an orbital energy, and an electron–electron repulsion integral. All of these quantities are taken from atomic calculations. Two-electron propagators are derived using the superoperator formalism. The working equations yield some popular expressions that have been used in interpreting molecular Auger spectra as special cases. Partitioning technique reveals qualitative factors that govern configuration mixing in the final state. Electron interaction can play a qualitatively dominant role in some of the final states. Some final states display localization of the holes on the same halogen atom, while others place holes on adjacent halogen atoms. This effect is especially pronounced for valence s orbital holes, but it is also observed for A-X bond orbitals and X lone pair orbitals. Trends in electronegativity differences and orbital sizes determine how much final state localization occurs. Propgator theory permits intensity borrowing between configurations leading to richer predicted spectra for the carbon Auger. The importance of using a theory that qualitatively supersedes molecular orbital theory is emphasized.

Angle-Dependent Changes of Auger Line Shapes from Adsorbed Molecules

Strong polar-angle dependence has been observed in $\mathrm{KVV}$-Auger spectra of adsorbed molecules. These angular effects are caused by the anisotropy of the Auger matrix element and can be explained by simple symmetry arguments. It is shown with the example CO on Ni(100) that molecular Auger spectra can be largely understood with use of nothing but the present angular results and experimental one-electron binding energies. Information about the adsorbate orientation as well as about the screening mechanism can also be obtained.

Semiempirical Solid State Shifts in the Auger-and Photoelectron Energies

Kinetic energy shifts of the Auger and photoelectrons between free atoms and their solid samples are studied for three series of consecutive elements: Na to Si, Cu to Se and Pd to Ba. The free atom binding and Auger energies are evaluated semiempirically for the elements for which experimental atomic data are not available. The obtained energies are also compared to the corresponding values from other calculation methods. With the aid of calculated atomic Auger energies and measured molecular Auger spectra also the molecular Auger shifts are obtained for several molecules. Experimental values for solid state shifts are obtained either from direct simultaneous vapour-solid measurements or from comparison between calculated or measured atomic energies and separate solid state measurements. The solid state Auger shifts are also calculated by applying the thermochemical model to the Auger process. Good agreement between calculated and experimental shift values are found through the studied series.

On the interpretation of molecular valence Auger spectra

The interpretation of molecular valence Auger spectra by means of ab initio computational methods is discussed. As alternatives to self-consistent-field optimizations of the full Auger spectrum, the feasibility of a procedure based on single ionization potentials and of CI calculations using frozen orbitals is tested. Numerical calculations are performed for the CO, N2, NO, and CO2 molecules, which show especially structure-rich Auger spectra. These spectra are analyzed in detail and they are found to contain three nonoverlapping regions of transitions corresponding to final state vacancies in outer–outer, outer–inner, and inner–inner valence orbitals. It is investigated whether this division of the transitions also is relevant with respect to the magnitude of dynamical relaxation errors in the single ionization potential procedure or with respect to the choice of molecular orbital basis in the CI calculations. CI effects are found to be important for intensities and energies of the Auger transitions in the major energy interval of the spectra. These effects are mainly due to near degeneracies between the main double hole states or between these states and configuration states formed by internal–external double excitations. In particular, configuration states involving π–π excitations are found to give important contributions. The one-center intensity model for Auger transitions is evaluated in the case of an open-shell molecule.

Localized states in molecular auger spectra

The influence of the degree of localization of the two final holes on the Auger spectra of a simple diatomic molecule is analysed in detail and cases where such effects may be observed experimentally are discussed. The results are compared with studies of the valence band Auger spectra of solids.

Initial-state relaxation effects in molecular Auger spectra

The initial-state valence relaxation of the molecular orbitals to the presence of the core hole is found to have a significant effect upon the core-valence-valence Auger decay probabilities and hence upon the Auger line shape. Comparisons are made of experimental and theoretical line shapes of ammonia and ethane. The best theoretical amplitudes were calculated by using self-consistent initial-state molecular orbitals which were projected upon the final states. The use of explicit initial-state orbitals leads to significant improvement over a simpler approach consisting of the use of minimum basis set ground-state orbitals and atomic Auger matrix elements.

The calculation of molecular and cluster auger spectra

The work of Asaad and Burhop and of Shirley on atoms is Shown to form the basis of a useful theory for the analysis of molecular core-valence-valence Auger spectra. If the static relaxation term of Shirley is neglected, the theory requires only a single SCF calculation on the neutral molecule. Since this approximation seems justified for the class of itinerant holes, the theory not only pemits the interpretation of the spectra of molecules considerably larger than previously analyzed, but potentially the spectra of chemisorbed molecules from cluster calculations. Theory and experiment are compared for the carbon KW spectra ofCH 4, C 2H 2, C 2H 4 and CH 3OH.

Direct measurement of the kinetic energy shift between the molecular and atomic M4.5N4.5N4.5 Auger spectra of iodine

The M4.5N4.5N4.5 Auger electron spectrum of iodine has been measured from molecular and atomic iodine vapours. The energy shift of 3.25 = 0.10 eV between molecular and atomic Auger spectra has been determined using a least-squares fitting procedure to spectra containing both molecular and atomic contributions. Different initial and final state splittings for atomic and molecular spectra, as well as the extra-atomic relaxation energy, are discussed.