Autoionization Emission Research Articles

Resonant photoemission of TiN films

The bonding and electronic structure of TiN thin films grown by sputtering have been characterized by means of resonant photoemission spectroscopy using synchrotron radiation. Specifically we found a complex resonance profile that exhibits a maximum at 45 eV followed by a second structure at 50 eV. The intensity enhancement observed at 45 and 50 eV is consistent with the resonant photoemission of the Ti $3d$ states involved in the valence band of TiN and the multiplet configuration of the $[\mathrm{Ti}{3p}^{5}{3d}^{2}{]}^{*}$ excited states. The autoionizing character of the $[\mathrm{Ti}{3p}^{5}{3d}^{2}{]}^{*}$ states could also be confirmed by observation of the corresponding autoionization emission. The resonance is used to determine the Ti $3d$ contribution to the valence band. The results are in good agreement with calculated Ti $3d$ partial density of states.

Ion-excited low-energy Auger electron emission from Ti and TiNi

Two distinct Auger transition features appear in the spectrum of electrons emitted from titanium surfaces by kiloelectronvolt argon ion bombardment. The main feature, corresponding to the electron-excited M23VV transition and originating with Ti-Ti collisions inside the solid, is accompanied by a very narrow atomic-like line at a slightly lower energy attributed to electron emission from Ti atoms sputter ejected from the surface. Ion bombardment does not stimulate any spectral feature corresponding to the characteristic electron-excited Ti autoionization emission, 3p53d23d* to 3p63d, which is severely attenuated upon simultaneous ion and electron excitations. The Ar+-excited emission from the Ti target is compared to that measured for the intermetallic compound TiNi.

Ion bombardment influence on the Cr Auger autoionization structure

The effect of fast Ar + ion bombardment (1÷5 keV) on the Cr autoionization processes has been investigated by AES and EELS. The autoionization satellite intensity in the e --excited Cr MVV Auger emission spectrum decreases showing a marked dependence on the ion energy and on the density of both ion and electron beam. Such a behaviour has been assigned to the activation of different deexcitation paths associated to the bombarding charged particles, while the excitation mechanism preceding the autoionization emission is substantially unaffected, as shown by the M 23 absorption edge in the loss spectra. The dependence on the ion energy suggests that the kinematics of the Ar +-induced collisions at the surface play a role in the quenching of autoionization structure in Auger spectra.

Photoelectron and resonance P( LVV) Auger electron spectra of gas-phase PF 3 excited by synchrotron radiation

The photoelectron spectra and the resonance P( LVV) Auger spectra of PF 3 were measured with monochromatized synchrotron radiation. The binding energies and the relative intensities of the molecular orbitals were obtained by curve fitting of the spectra. In addition the spin-orbit splitting of the P2p level in PF 3 has been determined. The relative intensities of the 2a 1 and 4a 1 molecular orbitals show clearly the autoionization emission excited by a photon energy of 136.6 eV. Furthermore, the 2a 1 photoelectron line shifts about 0.1 eV to a higher binding energy at this photon energy, and a weak spectral structure appears on the high binding energy side of the 4a 1 photoelectron line. These lines are caused by the very fast dissociation of the molecule followed by autoionization of the excited state in fragments. The kinetic energies of the P( LVV) Auger lines in the resonant excitation are found to be about 8 eV larger than the energies of the spectral lines in the normal Auger spectrum. The photon energy 136.6 eV causes the excitation of the electron from the P2p 1 2 level to the unoccupied σ * orbital. The resonantly excited state seems to decay mainly through the spectator Auger process or through the autoionization emission.

O2.3 core-electron excitations and their decay via Auger and autoionization emissions in clean and oxidised hafnium

Electron energy loss spectra associated with the excitation of the O2 and O3 core levels and secondary electron emission spectra arround the O2VV and O3-VV Auger transitions have been studied for clean and oxygen exposed hafnium. The excitation spectra are interpreted in terms of highly localized quasi-atomic excitations (i.e. 5p55d3 multiplets). This is based on the observation of autoionization features in the emission spectra which clearly resemble in shape the excitation process (i.e. EELS spectra). Therefore, it is concluded that in spite of the more extended nature of the 5d wavefunctions as compared to the 4d and 3d the requirements of localization to explain autoionization are also fulfilled in clean and partially oxidised hafnium.

O 2,3 core-electron excitations and their decay via Auger and autoionization emissions in clean and oxidised hafnium

O 2,3 core-electron excitations and their decay via Auger and autoionization emissions in clean and oxidised hafnium

On the Autoionization Emission for X-Ray Excited Auger Spectra

Autoionization electron emission spectra coexisting with the M 2,3 -VV Auger line by X-ray excitation are discussed theoretically. The present study is motivated by an experiment of Brener et al. (Phys. Rev. B 37 (1987) 1387) on metallic Ti, V and Cr by the use of X-rays of Ti K α(4.511 keV) and Al K α(1.487 keV), where autoionization electron emission spectra coexisting with the M 2,3 -VV line are possible only when the X-rays are Ti K α. By a model in which autoionization electron emission spectra are generated by the scattering of X-rays, while M 2,3 -VV spectra are generated by a resonant electron emission following the X-ray absorption, the increase of autoionization emission relative to M 2,3 -VV spectra with increasing energies of the incident X-rays is obtained for metallic Ti, in agreement qualitatively with the corresponding experimental result of Brener et al .

Theory of autoionization emission for X-ray excited Auger spectra

Variation with X-ray excitation energies of spectral shapes and intensities of autoionization electron emission coexisting with the M2,3-VV Auger line is discussed theoretically. Autoionization spectra are obtained as a one-step process of X-ray scattering, while M2,3-VV spectra are obtained as a two-step process of a resonant electron emission following the X-ray absorption. The present theoretical result agrees qualitatively with the experimental result of Brener et al. [Phys. Rev. B37, 1387 (1987)].

Interference effects in recombination from coupled states.

The dc-field-induced interference effects in recombination in a model system consisting of two close-lying autoionizing states are studied. The two states are strongly coupled by a dc field and can decay via both autoionization and spontaneous emission. All possible channels of radiative decay are included for the model system. In special cases the isomorphism of the recombination and photoionization studies is established. The total integrated recombination probability is calculated and its behavior as a function of dc field is presented. Our density-matrix calculations lead to the known results in the limiting case of no dc field.

Autoionization emission for x-ray-excited Auger spectra in the Compton-scattering process.

Autoionization emission in the 3d transition-metal series is observed for the x-ray-excited Auger electrons in the Compton-scattering process. Kinematical analysis of Compton scattering shows that such emission becomes possible only when the x rays have sufficiently high energy to lift the bound 3p electrons into the conduction band. With the tuning capabilities of synchrotron sources, this should open a new channel in autoionization experiments.

N 23-core losses and autoionization emissions of clean and oxygen exposed zirconium

The electron energy loss spectra associated with N 23-excitation and the low energy N 23VV Auger emission have been studied for both the clean and oxygen exposed zirconium. In the high energy side of the N 23VV Auger spectrum, autoionization emission of electrons of the valence band due to the decay of 4p electrons excited to states ≈9eV above the Fermi level has been identified. The excitation process can be also observed in the electron energy loss spectra. This is the first time that an autoionization feature is observed in a electron excited Auger spectrum of a 4d transition metal.

Oxygen induced surface segregation of Fe in Fe–18Cr–3Mo(100)

Variations in the composition of the outermost few atomic layers in Fe–18Cr–3Mo (100), caused by exposure to O2 (up to 100 L) at ambient temperature, were studied by monitoring the low-energy Auger (MVV) and autoionization emissions of Fe and Cr, and the MNV emission of Mo. Under these conditions neither Cr–oxide nor Mo–oxide were formed. At∼5-L O2, the Cr0 line disappeared completely, and at ∼10-L O2 enhancement of the Fe autoionization emission at ∼51 eV indicated the onset of Fe–oxide formation. The low-energy region of the spectrum recorded after 100-L O2 exposure exhibited striking resemblance to the spectrum recorded for an oxidized elemental iron sample. Severe relative attenuation of the Mo lines indicated that this constituent, too, does not participate in the initial surface segregation and oxidation, but remains in the subsurface region. The observed O chemisorption induced surface segregation of iron and the formation of surface Fe–oxide rather than the more stable Cr–oxide (or Mo–oxide) is attributed to kinetic effects.

Extended fine structures in autoionization emission spectra of bulk chromium.

Extended fine structures have been observed in autoionization emission spectra of chromium. Their origin has been determined by a one-to-one comparison with the extended energy-loss fine structures above the ${M}_{23}$ energy absorption process. Similarly to x-ray and electron-induced absorption spectra, both near-edge and extended fine features can be observed and distinguished in autoionization spectra.

Electronic excitations and their relaxations in clean and oxidized zirconium surfaces measured in eels

Electron energy loss Spectroscopy on clean and oxidized Zr surfaces shows features related to collective and single electron excitations. The 4 p–4 d transition resembles that encountered in isolated atoms. On oxidized surfaces, the interatomic transitions dominate. At the low kinetic energy side, spectra contain contributions due to the decay of plasmons, inter- and intra-atomic Auger processes, and autoionization emission.

An Auger study of the adsorption of oxygen on TiNi, TiFe and Ti 4Fe 2O

The room temperature adsorption of oxygen on clean TiNi, TiFe and Ti4Fe2O was studied using Auger spectroscopy. On initial exposure the Ti(M23VV) signal rapidly decreases and is replaced by a peak at 23 eV which arises from the bonding of incorporated oxygen to titanium. At ~ 2 L an autoionization emission feature characteristic of TiO2 emerges, and the 23 eV feature begins to decrease. We attribute this behaviour to the initial incorporation of oxygen to a critical concentration followed by the nucleation of a stoichiometric titanium oxide. In all three compounds only the titanium component is oxidized, the Auger features of the partner elements are simply attenuated. At ~ 70 L exposure the attenuation of M23VV and LVV peaks of Ni in TiNi indicates that effectively three monolayers of TiO2 are formed, and that a nickel rich region precipitates beneath the oxide. Ti(LMV) lineshape changes at high exposure are related mainly to the removal of defects in the oxide layer. Heating an oxide covered TiNi sample to 800 K leads to the complete dissolution of the oxide. At intermediate annealing temperatures, ~ 650 K, the data are consistent with the formation of a surface ternary phase, such as Ti4Ni2O.

An auger study of the adsorption of oxygen on TiNi, TiFe and Ti 4Fe 2O

The room temperature adsorption of oxygen on clean TiNi, TiFe and Ti 4Fe 2O was studied using Auger spectroscopy. On initial exposure the Ti(M 23VV) signal rapidly decreases and is replaced by a peak at 23 eV which arises from the bonding of incorporated oxygen to titanium. At ~ 2 L an autoionization emission feature characteristic of TiO 2 emerges, and the 23 eV feature begins to decrease. We attribute this behaviour to the initial incorporation of oxygen to a critical concentration followed by the nucleation of a stoichiometric titanium oxide. In all three compounds only the titanium component is oxidized, the Auger features of the partner elements are simply attenuated. At ~ 70 L exposure the attenuation of M 23VV and LVV peaks of Ni in TiNi indicates that effectively three monolayers of TiO 2 are formed, and that a nickel rich region precipitates beneath the oxide. Ti(LMV) lineshape changes at high exposure are related mainly to the removal of defects in the oxide layer. Heating an oxide covered TiNi sample to 800 K leads to the complete dissolution of the oxide. At intermediate annealing temperatures, ~ 650 K, the data are consistent with the formation of a surface ternary phase, such as Ti 4Ni 2O.

Spin polarized Auger spectroscopy from Fe(100)

The spin polarization of the M 23M 45M 45 Auger line of Fe(100) is shown to reveal two gain satellites at 4 and 17 eV, respectively, above the M 23 threshold. The lower one was previously identified as autoionization emission, whereas the higher one is a newly observed feature with a strikingly large positive spin polarization of > 80%. The spin polarization of the M 1M 45M 45 line is also reported.

Spin polarized Auger spectroscopy from Fe(100)

The spin polarization of the M 23M 45M 45 Auger line of Fe(100) is shown to reveal two gain satellites at 4 and 17 eV, respectively, above the M 23 threshold. The lower one was previously identified as autoionization emission, whereas the higher one is a newly observed feature with a strikingly large positive spin polarization of > 80%. The spin polarization of the M 1M 45M 45 line is also reported.

Low-energy Auger and autoionization emissions of clean and O 2-exposed FeTi and Fe 2Ti 4O

Three distinct features in the low-energy Auger spectrum of the intermetallics FeTi and Fe 2Ti 4O, namely the Ti(M 23VV) transition, a transition involving titanium-oxygen bonding electrons, Ti(M 23V)TiO(V), and gain satellites recently identified as being due to autoionization emission, were found to furnish detailed information concerning chemical bonding at the surface of clean and oxygen exposed samples. Marked changes in these spectral features upon O 2 exposure indicate selective interaction of oxygen with the Ti atoms from the early stages of adsorption. The spectral changes at low oxygen exposure are discussed in terms of the population of interstitial oxygen sites to a critical local concentration, and the subsequent nucleation and growth of a stoichiometric surface oxide. At ∼ 40 L O 2 exposure apparent saturation is reached with a TiO 2 overlayer formed on both of the compounds at 300 K. Subtle differences between the autoionization emission feature in pure and in alloyed Fe are noted and discussed.

Low-energy Auger and autoionization emissions of clean and O 2-exposed FeTi and Fe 2Ti 4O

Three distinct features in the low-energy Auger spectrum of the intermetallics FeTi and Fe 2Ti 4O, namely the Ti(M 23VV) transition, a transition involving titanium-oxygen bonding electrons, Ti(M 23V)TiO(V), and gain satellites recently identified as being due to autoionization emission, were found to furnish detailed information concerning chemical bonding at the surface of clean and oxygen exposed samples. Marked changes in these spectral features upon O 2 exposure indicate selective interaction of oxygen with the Ti atoms from the early stages of adsorption. The spectral changes at low oxygen exposure are discussed in terms of the population of interstitial oxygen sites to a critical local concentration, and the subsequent nucleation and growth of a stoichiometric surface oxide. At ∼ 40 L O 2 exposure apparent saturation is reached with a TiO 2 overlayer formed on both of the compounds at 300 K. Subtle differences between the autoionization emission feature in pure and in alloyed Fe are noted and discussed.