Auger Deexcitation Process Research Articles

Ion neutralization and high-energy electron emission in He+scattering by a highly oriented pyrolitic graphite surface

Fil: Iglesias Garcia, Adalberto de Jesus. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Santa Fe. Instituto de Fisica del Litoral; Argentina

Theoretical modeling of excitation and de-excitation processes of Er in SiO 2 with Si nanocrystals

We construct the theory of carriers confined in Si quantum dots with finite energy barriers for electrons and holes in the framework of the multiband effective mass theory. We apply this theory for theoretical modeling of the excitation of erbium inside and outside of Si nanocrystals in SiO 2 matrix due to the Auger process induced by the recombination of a confined electron–hole pair as well as the intraband transitions of “hot” confined carriers. Auger de-excitation processes of the Er 3+ ion leading to the quenching of erbium luminescence are discussed as well.

Microscopic theory of erbium ion de-excitation processes in silicon

Abstract There are some evidences that Er related optically active centers in silicon form donors with binding energy about 150–250 meV, and the excitation of erbium occurs in a result of Auger recombination process of the electron bound to the donor and a free hole or bound exciton. It is supposed that there are two effective de-excitation processes: (i) back transfer process to recombination of electron–hole pair and (ii) collision with free carriers. Both of them are being theoretically studied in this paper. The probability of back transfer increases dramatically with temperature rising. The collision with free carriers is the dominating de-excitation process at low temperature. It was shown that the perturbation of wave function of free carriers by the ionized Coulomb donor center associated with Er ion increases the probability of the Er-ion de-excitation by free electrons dramatically and practically supresses the de-excitation by holes. The Auger de-excitation process, in which the energy from Er ion is consumed by electron going from Er related donor to the conduction band, has been investigated as well.

Influences of the incident He* velocity on metastable de-excitation processes at a Cs-covered Si(1 0 0) surface

A Si(1 0 0) surface covered with cesium at saturation coverage were exposed to helium metastable-atom beams. The spectra of the electrons induced by metastable-atom de-excitation were detected as a function of the velocity of the incident He* in the velocity range 1–5 km/s. We discuss the influence of the incident He* velocity on the metastable de-excitation processes that take place at the Cs/Si(1 0 0) surface of very low work function. With increasing He* velocity, the relative weight of the peak from the decay of He* −(1s2s 2) in the spectrum was decreased, which means the auto-detachment process becomes more important than the Auger de-excitation process. The low-energy threshold of the spectrum shifted to higher energies with higher He* velocities. This implies that the surface electronic states are distorted by incident atoms, and, consequently, the surface potential barrier is increased.

Interaction between Er ions and shallow impurities in Si nanocrystals withinSiO2

The interaction between ${\mathrm{Er}}^{3+}$ and shallow impurities in Si nanocrystals (nc-Si) is studied for $\mathrm{Si}{\mathrm{O}}_{2}$ films containing Er and nc-Si $(\mathrm{Er}:\mathrm{nc}\text{\ensuremath{-}}\mathrm{Si}:\mathrm{Si}{\mathrm{O}}_{2})$. The luminescence property of ${\mathrm{Er}}^{3+}$ is strongly modified by shallow impurities in nc-Si. The formation of excess carriers in nc-Si by P or B doping results in the quenching of infrared photoluminescence (PL) of ${\mathrm{Er}}^{3+}$ and the shortening of the lifetime. When P and B are doped simultaneously and carriers are compensated, the intensity and the lifetime are recovered. It is shown that the mechanism of the interaction is Auger de-excitation of excited ${\mathrm{Er}}^{3+}$ with the interaction of electrons or holes in nc-Si. The estimated Auger coefficient is found to be two orders of magnitude smaller than that of Er doped bulk Si at low temperatures where carriers are bound to donor or acceptor ions, and four orders of magnitude smaller than that at room temperature. This small Auger coefficient makes nc-Si immune from the impurity Auger de-excitation process compared to Er doped bulk Si and is considered to be responsible for temperature independent efficient PL of $\mathrm{Er}:\mathrm{nc}\text{\ensuremath{-}}\mathrm{Si}:\mathrm{Si}{\mathrm{O}}_{2}$ systems.

Theory of metastable deexcitation spectroscopy on simple metals

In this work we investigate the Auger deexcitation process of a slow metastable helium atom $({\mathrm{He}}^{*})$ in its triplet state on a Na surface. The electronic properties and the Auger rates of the interacting atom-metal system are calculated self-consistently at any distance of ${\mathrm{He}}^{*}$ from the surface in the framework of the local spin-density approximation using the embedding Green's function method. The energy distribution of the emitted electrons is obtained by integrating the Auger rates along the classical trajectory in the adiabatic approximation. The analysis of the spin-resolved local density of states shows that the excited atomic states broaden into resonances already at large atom-surface distances. The calculated atom deexcitation rates are in good agreement with experimental results.

Cesium-induced transient effects on the Si + and Si − secondary ion emissions from Si and SiO 2

Measurements of the concentration and speciation of Cs implanted into Si and SiO 2 thin film surfaces as a result of 1 keV Cs + ion impact have been carried out using X-ray photoelectron spectroscopy. In most cases, the presence of Cs resulted in enhanced Si − and quenched Si + secondary ion intensities. These intensities could be correlated to the Cs concentration via exponential functions over and following the transient regions noted in secondary ion mass spectrometry. These trends are predicted by the electron tunneling model in which a resonance charge transfer mechanism is assumed to dominate. An exception arises in the case of the Si + secondary ions from SiO 2; these initially rose in intensity with increasing Cs concentration. This exception is believed to result from the introduction of an Auger de-excitation process that is active in the excited sputtered neutral Si population some distance from the surface. The Cs retained in the Si substrate was found to exist as a mixture of Cs 2O with some form of CsSi complex in Si. Cs in SiO 2 existed primarily as CsO 2.

Angle-resolved auger electron emission from ion-bombarded NaCl surface

Characteristic electron emission spectra obtained from ion-bombarded sodium chloride surface are distinctly different and more complex than such spectra from metallic sodium. The spectra for sodium halides have been explained recently in a context of the collisional deexcitation model in which the core excited sodium ion colliding with a halogen anion captures an electron which activates autoionizing deexcitation channels. However, a controversy has arisen whether the Auger deexcitation process takes place inside the solid or in vacuum above the surface. In the present work we report on the angular dependence of the Na Auger electron line positions and intensities. The source atom energies and basic characteristics of their angular distributions are derived from the experimental data. An interpretation of the observed angular-dependent line shifts is given in terms of the Doppler effect. The results indicate strongly that the Auger deexcitation processes occur on sodium atoms ejected directionally above the surface due to direct knock-off collisions with primary ions.

H 2 + scattered off Al surfaces: The role of the negative ion resonance Σu+2

A theoretical study of the electronic processes occurring in collisions of H2+ molecular ions on an Al surface is presented, with an emphasis on the molecular dissociation induced by electronic transitions. The H2+ neutralization proceeds through two different channels: the H2 (b 3Σu+) dissociative state (resonant process) and the H2 (X 1Σg+) ground state (Auger process). The Auger deexcitation process of the b state, mediated by the H2− (2Σu) resonant state is studied in detail and shown to result in an efficient quenching of the excited state and a reduction in the molecular dissociation probability. Its efficiency depends on the competition between the molecular dissociation and the electronic process. The importance of this process can explain the experimental observation of significant amounts of bound H2 molecules surviving the electronic processes.

The N −2( 2Π g) shape resonance in slow [formula omitted] collisions with metallic surfaces

Metastable N 2( A 3Σ + u) molecules colliding with a metal surface can be de-excited via an Auger process in which the excitation energy of the molecule is given to an electron of the metal. We report on a theoretical study of this process for N 2( A 3Σ + u) molecule collisions on a free electron metal, Al. This de-excitation process is efficiently mediated by the formation of a transient negative molecular ion, in the present case, the well-known resonance N − 2 of 2Π g symmetry. A modelling of the electron–N 2 molecule interaction in the intermediate 2Π g symmetry allows the computation of the Auger de-excitation process characteristics: transition rate, emitted electron spectra. The present results are used to discuss the experimental results on the N 2( A 3Σ + u) de-excitation in thermal collisions on metal surfaces by Stracke et al. [Surf. Sci. 396 (1998) 212] who first proposed this de-excitation mechanism. We find this resonance mediated process to be very efficient.

Computer simulation of image acceleration of highly charged xenon ions in front of a metal surface

Using the time-evolution BC code, DYACOCT, we investigate the image acceleration of Xe q + ions ( q=1–33) with energies 3.7 q keV which are scattered under a grazing angle of incidence from an Al (1 1 1) surface. We solve the coupled rate equations of the population P n of the nth shell of the projectile, where electron capture, electron loss, Auger deexcitation and peeling-off process are explicitly included. For surface scattering of Xe q +, we determine the screening lengths of highly charged Xe ions interacting with surface atoms, based on those electron distributions. The calculated image energies were compared with the experimental works of Winter et al., and we got the reasonable agreement with the experimental image energies for q < 25. The discrepancy between simulation and experiment is found for q > 25. This discrepancy is mainly due to the breakdown of the assumption employed in the experiment that the surface scattering is always the specular reflection.

Development of alkali-induced electronic states at GaAs(0 0 1) surfaces and their electron-transfer interaction with helium metastable atoms

Modifications in the surface local electronic structure by the adsorption of alkali metals and its charge-transfer interaction with excited-state helium atoms have been investigated by metastable deexcitation spectroscopy (MDS) at Cs-adsorbed GaAs(0 0 1)(4×2) surfaces. Energy distributions of electrons ejected by Auger-type decay processes of holes on the incident He* atoms were measured as a function of Cs coverage. At a clean GaAs(0 0 1) surface, He* atoms were deexcited via resonance ionization followed by Auger neutralization (AN), from which the toplayer density of states was extracted. With the adsorption of Cs ( θ<0.25), a new peak relating to the Ga dangling bonds filled by charge transfer from adsorbed Cs appeared by an Auger deexcitation (AD) process. At around θ∼0.5, we observed a development of Cs 6s-induced states at E F. We discuss a local nature in the charge-transfer interaction of the alkali-induced states with the He* atoms at semiconductor surfaces.

Auger deexcitation in a helium atom induced by a metal surface.

A theoretical model has been developed to study the direct Auger deexcitation process between two-electron states. The model is based on linear response theory. Therefore the excitations induced in the surface may be described with the surface response function. This model has been applied to calculate the Auger deexcitation rates when an excited He atom is moving parallel to an aluminum surface. More precisely, the distance and velocity dependence of the transition from the singlet states 2${\mathrm{}}^{1}$S and 2${\mathrm{}}^{1}$P to the ground state is presented. I finally compare these results with the ones obtained by using a one-electron description for the He atom. The differences between both models vanish when effective charges ${\mathit{Z}}_{\mathit{eff}}$=1.69 a.u. and ${\mathit{Z}}_{\mathit{eff}}$=1.57 a.u. are used for the 1s and the 2p states, respectively, in the one-electron model. \textcopyright{} 1996 The American Physical Society.

Electron emission in slow collisions of He projectiles (He∗, He +, He ++) with Li- and LiF-surfaces

We report the spectra of the electrons emitted in slow collisions of He + (50 eV) and He ++ (100 eV) ions with a Li covered W(110) surface and with LiF films grown on Li W(110) . The preparation of the surfaces was made under in situ control of the metastable impact electron spectroscopy (MIES). The interpretation of the spectra is made on the basis of simulated electron spectra: For collisions with Li W(110) the ion induced electron spectra are mainly due to Auger processes of the projectiles after their resonan capture of electrons from the surface. Both Auger deexcitation and intra-atomic Auger processes, such as the autodetachment of He −∗ (1s2s 2) and the autoionization of the “hollow atoms” He∗∗ (2 l2 l′) ( l, l′=0, 1), are observed. For collisions with LiF f neutralization of the projectiles takes place mostly by Auger capture processes involving two electrons from the surface. The resonant capture of an electron by the He ++ into He +∗ ( n=2) state still occurs, but is weak. The formation of “hollow atoms” does not take place in front of dense LiF films. There are some indications that the quasi-resonant electron transfer involving F 2s electrons plays also a role in the interaction of He ++ with the surface.

Substrate mediated autoionization of benzene on graphite

Benzene adsorbed on graphite is found to increase the photoelectric yield even below the ionization energy of benzene. A two step model for this substrate mediated autoionization channel is proposed. After intramolecular π→π ∗ excitation an Auger deexcitation process may pump the electron from the π ∗ level into the vacuum. An adsorbate induced resonance 5.4 eV above the Fermi level is found to promote the yield enhancement. Implications for soot particle characterization are discussed.

Auger deexcitation rates in grazing atom-surface collisions

The interaction of particles with a metal surface may induce different processes which can affect the charge state of the projectile. Here, we present a theoretical study based on linear response theory of the Auger deexcitation processes for ions scattered from a metal surface. Different values of the atomic number, Z, have been considered. Thus, we analyze the dependence of the Auger transition rates on the velocity of the particle for different ion charges. Results are presented for the particular case of deexcitation from the n = 3 to the n = 2 level. The asymptotic behaviour with the distance to the surface is presented for the different deexcitation processes.

Singlet to triplet conversion in low energy metastable helium-metal surface collisions: Auger deexcitation process

Experimental studies of metastable He atom collisions on alkali coated metals revealed that incoming He∗ (2 1S) atoms are very efficiently converted into He∗ (2 3S) atoms far from the surface. Various conversion processes have been proposed. This work concerns the Auger conversion process proposed by Woratschek et al. [Phys. Rev. Lett. 55 (1985) 611]. The non-perturbative coupled angular mode (CAM) method is shown to be able to treat Auger deexcitation processes. The predictions for the Auger conversion are shown to be quite consistent with those previously obtained for the He − intermediate process [Borisov et al., Surf. Sci. 284 (1993) 337]. This is interpreted by comparing the two processes which are shown to have the same physical basis. The conditions of applicability of the two approaches (resonant intermediate and Auger) are discussed. The importance of the singlet to triplet conversion for helium atoms colliding on low work function surfaces is confirmed.

Iodine adsorption on clean and cesiated W(110) studied with UPS and metastable impact electron spectroscopy (MIES)

Photoemission spectra (UPS with HeI) and metastable impact electron spectroscopy (MIES with He∗(1s2s)) are reported for the adsorption of iodine alone, the coadsorption of iodine and cesium, and the adsorption of CsI on W(110) at room temperature. It is demonstrated that the I(5p) ionization is sensitively dependent on the chemical environment of the iodine adsorbate. For iodine in a strong external field, such as for adsorption on clean metals, the photoemission spectra reflect the Stark splitting of the (5p) orbital while the MIES spectra are due to the Auger capture process. When adsorbed near to cesium iodine feels a comparatively weak external field and the emission spectra both for UPS and MIES (due to the Auger deexcitation process) reflect the I(5p) finestructure splitting in the final state after the emission process.

Kinetics of activated nitrogen states in late afterglow by the time-delay method

A long-time decay of activated nitrogen is studied by the time-delay method. The kinetics in late nitrogen afterglow is determined by nitrogen atom recombination. Surface-catalysed excitation of nitrogen atom recombination at the cathode gives a metastable induced electron yield in the inter-electrode space (the Auger de-excitation process) that determines the time delay. The probability of N2(A3 Sigma u+) metastable state formation by recombination at the electrode surface is determined, as are the contribution to electron yield from this mechanism and the contribution from the gas phase metastable species. On the basis of the above-cited mechanism, the time-delay method is found to be very efficient for detection of activated nitrogen.

X-ray and Auger de-excitation processes in resonantly-excited and core-ionized molecules

Some recent developments in the understanding of molecular Auger and X-ray spectra are reviewed. Factors leading to complex molecular Auger spectra are discussed. The importance of correlation satellites in some spectra, such as for the SiF 4 molecule, are reported. The prediction of inner-shell photoelectron linewidths and the discrepancies with experiment for silicon-containing molecules are reported. The strong dependence of resonant X-ray emission spectra on photon excitation energy is discussed in the context of the CO 2 and N 2O molecules.