Angular anisotropy and dichroism in molecular Auger spectroscopy following photo-absorption

This paper shows that angular distribution of spin-unresolved Auger electrons emitted in the decay of a vacancy created by the absorption of a photon in a certain class of free, unpolarized and gaseous non-linear molecules is completely characterized by three, rather than by two well-known parameters. The presence of this additional third parameter gives rise to circular dichroic effect which varies as cosine of the angle made by the departing Auger electron with the direction of incidence of the circularly polarized radiation. Linear dichroic effect varies as the square of sine of the angle made by the direction of emission of Auger electron and the polar axis of the space frame. Linear and circular dichroism in the angular correlation between theE ₁ photoelectron and Auger electron emitted sequentially from a molecule belonging to one of the 32 point groups has also been investigated.     

Angular studies of potential electron emission in the interaction of slow ions with Al surfaces

We report energy distributions of electrons emitted from Al surfaces under impact by 1 keV Ar+ and 1-5 keV Ne+ ions. The variation of the energy distributions with the angle of incidence is different for both ions and provides information on the mechanism responsible for electron emission. For Ar+ electron emission results mainly from Auger neutralization, while for Ne+ an important emission mechanism is the decay of plasmon excitations. We find a transition between surface and bulk plasmon excitations as the energy of the ion is increased.     

Characterization of electronic structure in dielectric materials by making use of the secondary electron emission

The methodology of characterizing electronic structure in dielectric materials will be presented in detail. Energy distribution of the electrons emitted from dielectric materials by the Auger neutralization of ions is measured and rescaled for Auger self-convolution, which is restructured from the energy distribution of the emitted electrons. The Fourier transform is very effective for obtaining the density of states from the Auger self-convolution. The MgO layer is tested as an example of this new measurement scheme. The density of states in the valence band of the MgO layer is studied by measuring the energy distribution of the emitted electrons for MgO crystal with three different orientations of (111), (100) and (110). The characteristic energy of ?₀ corresponding to the peak density of the states in the band is determined, showing that the (111) orientation has a shallow characteristic energy ?₀ = 7.4 eV, whereas the (110) orientation has a deep characteristic energy ?₀ = 9.6 eV, consistent with the observed coefficient γ of the secondary electron emission for MgO crystal. Electronic structure in new functional nano-films spayed over MgO layer is also characterized. It is therefore demonstrated that secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials. This method simultaneously determines the valence band structure and the coefficient γ of the secondary electron emission, which plays the most important role in the electrical breakdown phenomena.     

The role of primary and secondary electrons in electron induced desorption and dissociation: CO on Pt(111)

The electron impact desorption and dissociation of CO on Pt(111) has been studied under conditions which are fairly typical in Auger Electron Spectroscopy. It was found that desorption was about twenty times faster than dissociation, and the apparent cross sections for these processes were measured. By studying the dependence of these cross sections on electron beam incidence angle, it was shown that most of the disruption of the CO adlayer was caused by the relatively slow backscattered electrons rather than the fast primary beam. The Auger spectrum of adsorbed CO was measured and the relative Auger yields from carbon present as adsorbed CO and as surface carbide were obtained.     

Low-energy ion-induced electron emission from a MgO(100) thin film: the role of the MgO-substrate interface

We present a detailed study of the electron emission from a thin MgO(100) film on a Mo substrate, bombarded with slow He+, Ne+, and Ar+ ions. Neither the high absolute number of emitted electrons per incoming ion nor the electron spectra can be due to Auger neutralization of the incoming ions at the MgO surface alone. Therefore, an additional mechanism is proposed: holes created in the MgO film are transported to the MgO-substrate interface where they give rise to an Auger neutralization process involving two electrons from the metal substrate conduction band.     

Plasmon-enhanced electron acceleration in intense laser metal-cluster interactions

We have measured the energy and angular-resolved electron emission from medium-sized silver clusters (N approximately 500-2000) exposed to dual laser pulses of moderate intensity (I approximately (10(13-14) W/cm2). When the second pulse excites the plasmon resonantly, we observe enhanced emission along the laser polarization axis. The asymmetry of the electron spectrum is strongly increasing with electron energy. Semiclassical simulations reveal the following mechanism: Electrons bound in highly excited states can leave, return to, and traverse the cluster. Those electrons that return at zero plasmon deflection and traverse the cluster during a favorable plasmon half-cycle can experience maximum acceleration by the evolving polarization field. As a result of these constraints energetic electrons are emitted in direction of the laser polarization axis in subcycle bursts.     

Surface structure from angular dependence of auger electron emission

Auger electrons emitted from atoms in the vicinity of a solid surface can show strong angular dependence in their emission due to elastic scattering from the surrounding ion cores of the solid. Model calculations for the emission of Auger electrons from sulphur adsorbed on a Ni (100) surface show that these angular features are very sensitive to the adsorption site of the emitting atoms, and that a study of these effects should produce a method of surface structure analysis for the location of preferred surface adsorption sites. This approach would appear to be an improvement over LEED because of its high sensitivity to structural changes and because it does not require long range order in the adsorbate layer.     

On the anisotropy of electron backscattering and of the sticking coefficient of gases on single crystal surfaces

The influence of the anisotropy of backscattering of electrons with energies above the core hole ionization energy of selected Auger processes on the anisotropy of the Auger electron emission is studied using a cylindrical W single crystal. After the determination of the anisotropy of the elastic backscattering its contribution to the anisotropy of Auger electron emission from adsorption layers is obtained by postulating that all other anisotropy causes are negligible. When applied to an adsorbate with orientation-independent coverage such as Fe this leads to an orientation-dependent backscattering correction term which may be applied to adsorbates with orientation-dependent sticking coefficients such as O₂. In this manner Auger signals may be converted into true relative coverages (corrected for backscattering anisotropy). In the case of oxygen adsorption this correction is significant and leads to an orientation-independent initial sticking coefficient (of 1) over most of the 〈110〉 zone except in the neighbourhood of the {110} planes where it reaches its minimum value of 0.28.     

A secondary electron emission correction for quantitative auger yield measurements

The Auger electron yield from the de-excitation of L_2,3 core holes of Cu is determined, in the approximation of isotropic emission, from the integral of the characteristic (elastic) Auger emission. The characteristic emission function is obtained from the secondary electron spectrum in two steps: First, the cascade background at a given Auger line is characterized accurately by the method of cascade linearization and subtracted. In the second step the resulting line is debroadened by an approximation to deconvolution debroadening. The concept of modulation stability is introduced as a criterion for credibility of the results of this analysis.     

A detailed experimental investigation of the KMM + KMN + KNN Auger electron spectrum of Cd from the EC-decay of In

KMM, KMN and KNN Auger electron spectra of ¹¹¹Cd emitted in EC-decay of ¹¹¹In were analyzed at instrumental resolutions of 14 and 21 eV using a combined electrostatic spectrometer. Energies and relative intensities of 26 resolved components were determined and compared with theoretical predictions. For the first time, the predicted intermediate coupling structure of some KMM Auger lines was observed. A structure of the KNN Auger group for Z < 54 was resolved for the first time. Relative intensities of the KMM, KMN and KNN Auger groups were also determined.     

Angle-resolved electron spectroscopy of laser-assisted Auger decay induced by a few-femtosecond x-ray pulse

Two-color (x-ray+infrared) electron spectroscopy is used for investigating laser-assisted KLL Auger decay following 1s photoionization of atomic Ne with few-femtosecond x-ray pulses from the Linac Coherent Light Source. In an angle-resolved experiment, the overall width of the laser-modified Auger-electron spectrum and its structure change significantly as a function of the emission angle. The spectra are characterized by a strong intensity variation of the sidebands revealing a gross structure. This variation is caused, as predicted by theory, by the interference of electrons emitted at different times within the duration of one optical cycle of the infrared dressing laser, which almost coincides with the lifetime of the Ne 1s vacancy.     

A comparison of ion-induced electron emission and secondary ion yields

Ion-induced electron emission from solid surfaces is studied using a beam of caesium ions. Features of the spectra obtained during depth profiling of layered structures suggest a novel technique for investigating ion-induced Auger processes. Depth profiles are presented in terms of measured secondary ion signals, electron-induced Auger emission, and the intensities of features in the ion-induced electron spectra. It is shown that changes in features of the ion-induced electron spectra can be related to changes of chemical composition and sputtering probability. These help in the interpretation of variations in secondary-ion yields with matrix composition during depth profiling.     

Temperature dependent peaks in secondary electron emission spectra

Peaks in the secondary emission spectra not associated with Auger effects have been observed in various metals (particularly copper and cobalt). These peaks are suppressed at elevated temperature and appear to have amplitudes which are unique functions of temperature. They are characteristic of well ordered, and, particularly, clean surfaces, and are interpreted in terms of the diffraction of the emitted secondary electrons.     

Aspects of Angular Dependent Auger Spectroscopy (ADAS)

Calculational results are presented which pertain to recent observations that considerable structure exists in the angular distributions of 62 eV M_2,3VV Auger electrons emitted from copper surfaces. A direct interpretation of these results demonstrates that both electron diffraction and anisotropic Auger emission must contribute significantly to the observed angular distributions.     

Auger electron spectroscopy - a local probe for solid surfaces

The Auger electron transition in solids is discussed under the aspect of a local excitation due to the strongly localized primary hole in an inner atomic core level. In first approximation the solid is represented by a cluster model, consisting of the excited atom and its neighbors. Using this simple model it is possible to describe the Auger electron energies, intensities and line shapes of transitions in solids in a satisfactory way. Only for the angular dependent Auger emission, characteristic long-range crystalline order has to be taken into account. It is the aim of this introductory review to point out that Auger spectra bear more information about the solid surface and particularly on its chemical bonds as has yet been exploited by surface spectroscopists.     

Semi-empirical elemental sensitivities for quantitative auger electron spectroscopic analysis

A semi-empirical equation is presented which may be used to calculate the relative sensitivities of the elements for Auger electron spectroscopic analysis. The equation involves the product of a large number of theoretically or empirically derived terms, including ionization cross section. Coster—Kronig transitions, backscattered electron contribution, individual Auger transition probabilities, atomic density, electron mean free path, analyser transmission, modulation effects, detector efficiency, and peak asymmetry. The calculated sensitivities are compared with experimental relative elemental sensitivities from the Handbook of Auger Electron Spectroscopy. The close agreement obtained with experiment suggests that the semi-empirical theory is now capable of providing elemental sensitivities for analysis, in a predictive way, where reliable empirical sensitivities are missing.     

A study of the charging and dissociation of SiO2 Surfaces by AES

AES is used to determine the initial spectrum of a vacuum-broken SiO₂ surface and to follow its dissociation under the electron beam probe. Both Auger peaks heights and energies are affected by the irradiation. The change in stoichiometry is accompanied by a decrease of the surface charge by 5–8 V. The relation between stoichiometry and charge is explained by the influence of radiation-induced defects on secondary electron emission. The reduction of SiO₂ is characterized in terms of irradiation dose, dissociation cross-section and electron impact efficiency. Resistance to radiation damage is increased by surface carbon contamination. The chemical contribution to the Auger peak energy can be distinguished from the charging effect leading to a shift between element and compound of 12 eV for the silicon peak.     

高入射能量下的金属二次电子发射模型

基于高入射能量电子产生二次电子发射的物理过程, 分别对高入射能量电子产生的真二次电子和背散射电子的概率进行理论分析与建模. 利用Bethe能量损失模型和内二次电子逸出概率分布, 推导出高入射能量电子产生有效真二次电子发射的系数与入射能量的关系式; 根据高入射能量电子在材料内部被吸收的规律, 推导出高入射能量电子产生背散射电子的系数与入射能量之间的关系式. 结合两者得到高入射能量下金属的二次电子发射模型. 利用该模型计算得到典型金属材料Au, Ag, Cu, Al的二次电子发射系数, 理论计算结果与采用Casino软件模拟金属内部散射过程得到的数值模拟结果相符. 关键词： 二次电子发射 高入射能量 金属表面 散射过程     

Energy-resolved analysis of ferroelectric electron emission from TGS using emission electron microscopy

Ferroelectric electron emission arises when the spontaneous polarization of a ferroelectric is switched due to the application of an electric field. In order to study the origin of emission and the related emission mechanism, space-resolved emission electron microscopy has been employed. The integral energy distribution of the emitted electrons from triglycine-sulfate surfaces has been investigated using a cylindrical sector analyzer and an imaging retarding field analyzer. Space-resolved emission photography and energy distribution measurements were obtained, revealing the effect of ferroelectric switching on the electric field distribution and hence on the emission process. Evidence of secondary electron emission from the metal electrodes has been found.     

Circular polarization of ion fluorescence completing the analysis of resonant Xe* 4d(-1)(5/2)6p Auger decay

The relative contributions of the partial electron waves emitted in the Auger decay of the Xe* 4d(-1)(5/2)6p(J(*)=1) resonance have been determined by fluorescence polarimetry after excitation with circularly polarized synchrotron radiation. The analysis of circularly polarized fluorescence of the photoion leads to an independent determination of the orientation parameters for all states of the Xe II 5p(4)6p multiplet. The present study provides, in combination with data on the angular distribution and spin polarization of the Auger electrons, complete quantum mechanical information on the resonant Auger decay, i.e., branching ratios and relative phases of the Auger decay amplitudes.