苝四甲酸二酐薄膜电子结构的同步辐射共振光电子能谱研究

利用基于同步辐射的近边X射线吸收精细结构谱(NEXAFS)和共振光电子谱(RPES)研究了苝四甲酸二酐分子(PTCDA)薄膜的电子结构.碳K边NEXAFS谱中能量小于290 eV的四个峰对应于PTCDA分子不同化学环境碳原子1s电子到未占据分子轨道的共振跃迁.RPES谱中观察到共振光电子发射和共振俄歇电子发射导致的共振峰结构,以及二次谐波激发的碳1s信号.根据电子动能对入射光能量的依赖性分别对三类峰结构进行了归属.同时,发现PTCDA分子轨道共振光电子峰的强度具有光子能量依赖性.这种能量选择性共振增强效应是由于PTCDA分子轨道空间分布差异导致的.共振俄歇峰主要源于高结合能(4.1 eV)分子轨道能级电子参与的退激发过程.明确RPES实验谱图中各个峰结构的起源有助于准确利用基于RPES的芯能级空穴时钟谱技术定量估算有机分子/电极异质界面处电子从分子未占据轨道到电极导带的超快转移时间.     

Korrelationseffekte in den Auger-Spektren von Edelgasen

The possibility of studying correlation effects through Auger electron spectrometry has been shown recently by Krause, Carlson and Moddeman in the case of theK Auger spectrum of neon. As a further example we have measured theM _4,5 Auger spectrum of krypton with high resolution. Correlation effects have been found via the strong deviations of relative intensities of Auger diagram lines (e.g.M _4,5 N ₁ N _2,3(¹ p ₁)) from theoretical values and via the occurence of double Auger transitions, where one electron is emitted and another is excited. A critical examination of high resolution Auger spectra of noble gases, which has been measured so far, has shown that several non diagram lines can be assigned to double Auger transitions of the above kind.     

Spin polarization and angular distribution of Auger electrons formed as a result of decay of the 3<Emphasis Type="Italic">d</Emphasis><Superscript>−1</Superscript>5<Emphasis Type="Italic">p</Emphasis> state in the Kr atom

The results of numerical calculations of the energies of Auger transitions, as well as the angular distribution (α₂) and spin polarization (β₂) anisotropy parameters, are presented for transitions in a photoexcited Kr* atom with two open shells. Matrix elements are calculated by the multiconfigurational Fock-Dirac relativistic method using an intermediate type of coupling. The wavefunctions of the initial and final states of the Auger transition are calculated with allowance for relaxation effects. The one-electron wavefunction of the continuous spectrum for an Auger electron is obtained using the single-configuration Fock-Dirac method. The results are compared with experiment and a new experiment is proposed for identifying the Auger state not only from the energy, but also from the total angular momentum of the Auger state.     

KLL Auger resonant Raman transition in metallic Cu and Ni

High energy resolution KL₂₃L₂₃ Auger spectra of polycrystalline Cu and Ni were measured using photon energies up to about 50 eV above the K-absorption edge and down to 5 eV (Cu KLL) and 4 eV (Ni KLL) below threshold. The spectra show strong satellite structures varying considerably as a function of the photon energy. In the sub-threshold region the linear dispersion of the diagram line energy positions and a distortion of the line shape as a function of photon energy, attributable to the Auger resonant Raman process, is clearly observed, indicating the one-step nature of the Auger emission. These changes in the resonant spectra are interpreted using a simple model based on resonant scattering theory in combination with partial density of states obtained from cluster molecular orbital (DV-Xα) calculations.     

Spectres Auger sous bombardement d'ions Ar+ de 60 keV de quelques éléments purs et de composés binaires

Auger spectra of different elements, alloys and compounds excited by 60 keV argon ions have been obtained. These spectra show a detailed structure with, in particular, a sharp Auger peak. This peak is interpreted as a superposition of two kinds of Auger transitions: the first one occurs with two electrons of the electronic band, the second, takes place from electrons situated on “quasi-atomic” outer shells. On the low energy side of Auger line, two equidistant peaks attributed to Auger electrons having suffered energy losses by ionization of electronic outer shell of target-atoms were observed. From Mg, Al, Si, P and S Auger transitions due to initial double L_ii,iii level ionization were detected. The interpretation of the creation of a vacancy in electronic inner shell at the origin of Auger effect by ionic bombardment is given from the electronic promotion model of Fano and Lichten.     

The structure of theL 2,3 VV-auger line of silicon and silicon compounds

TheL _2,3 VV Auger transitions of Si, SiO₂, and SiC have been measured and compared with the self-fold electron density of states. The data indicate that Auger matrix effects must be included to explain the structure of the Auger lines. A comparison with soft X-ray measurements of Wiech shows, that the measured Auger line shape is nearly identical with the self-foldK _β emission band. The selection rules for X-ray emission lead then to the conclusion that mostlyp-like valence electrons are involved in the Auger transition. This result indicates the relative importance ofs andp states in Auger transitions which is in accordance with theoretical calculations of Feibelman et al.     

Changes in Auger spectra of Mg and Fe due to oxidation

Auger electron spectra of clean Mg and Fe surfaces have been investigated under UHV conditions. The main Auger peaks in the low energy Auger spectra of these elements are identified as due to L_2,3VV and M_2,3VV transitions for Mg and Fe respectively. Changes in the low energy spectra of these clean surfaces of Mg and Fe due to chemisorption of residual oxygen in the UHV system, were also studied. The results indicate that for each oxidised surface new larger Auger peaks appear at energies lower than the original main peaks in the clean spectra. The changes in the spectra are believed to be due to the energy shifts of inner energy levels and valence bands involved in the Auger transitions as an oxide is formed.     

Comments on Gallon's method for determining the ionization cross-section and the backscattering factor

The energy dependence of the ionization cross-section and the backscattering factor resulting from the experimental method of Gallon are shown to be affected by simplifying the definition of the backscattering factor. In the case of Auger transitions following the ionization of the L₃ shell the errors in the ionization cross-section and the backscattering factor reach 10% and 4%, respectively. Larger deviations are probable for Auger transitions involving higher shells.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

LEED and Auger investigations of Cu (111) surface

After ultrahigh vacuum bake-out, electropolished Cu (111) surfaces were shown by Auger analysis to be contaminated by C, N, O, S and Cl. Other than C and S, which were contained in the bulk, the impurities were introduced by surface preparation; but all were easily removed by light Ar ion bombardment. Heating to ≈ 750°C caused diffusion of C and S from the bulk to the extent that a clear diffraction pattern corresponding to a √7 × √7 structure was produced by S on the surface. At ≈ b 900°C evaporation of Cu occurred to an observable degree, and S and C could no longer be detected on the surface. Auger analysis of clean Cu surfaces showed many details of the LMM and MMM types of transitions. Kinetic energies of all observed Auger electrons were in excellent agreement with calculated values. Also, the ≈ 62 eV MMM peak was resolved into two components related to the small differences in the M₂ and M₃ energy levels. The LMM transitions were classified according to their intensities, which could be rationalized on the basis of Coster-Kronig transitions and transition probabilities, as L₃MM > L₂MM > L₁MM.     

Strong valence electron excitations due to internal conversion or K-capture

Excitations of valence electrons in Auger transitions of atoms with a vacancy in internal shells, produced by internal conversion or K-capture processes, are studied theoretically. It is demonstrated that this effect is large and the excitation spectrum is very sensitive to recombination processes. A resonant behaviour of the excitation process as a function of temperature is predicted.     

Study of adsorption phenomena on aluminium by interatomic Auger transition spectroscopy: II. CO adsorption onto Al

Adsorption of carbon monoxide on aluminium is studied by the combination of in situ evaporation of Al and the Auger electron spectroscopy. New Auger signals which are attributed to interatomic Auger transitions in Al₄C₃ type AlC bonds are found. It is revealed that two adsorption stages exist in the adsorption process, i.e., that after the formation of the AlC bond takes place, the adsorbed layer consisting of AlC and AlO bonds is formed.     

Interfacial auger transitions in oxidised sodium and magnesium

A study of the oxidation of sodium and magnesium under precisely controlled U.H.V. conditions using Auger electron spectroscopy has revealed the presence of Auger transitions which may be interpreted as arising from the interface between the oxide and the underlying metal. These Auger peaks have been assigned transitions of the type: M¹(L_2,3) M(V) O¹(L) where M¹ and O¹ are ionic levels present in the oxide and one of the final state holes M(V) is in the conduction band. The experimentally observed Auger energies agree well with those calculated for the proposed transitions. It is expected that transitions of this type might occur in other metal—oxide systems.     

De-excitation processes near the surface of ion bombarded SiO2 and Si

Ion bombardment induced secondary ion and optical excitation from oxide coated Si has been studied as the oxide layer is eroded away. By comparing ion and photon yields, a model for the role of resonant and Auger electron transitions in the neutralisation and de-excitation of the sputtered atoms and ions has been suggested.     

Core-Excited Molecules by Resonant Intense X-Ray Pulses Involving Electron-Rotation Coupling

It has been reported that electron-rotation coupling plays a significant role in diatomic nuclear dynamics induced by intense VUV pulses [Phys. Rev. A 102(2020) 033114; Phys. Rev. Res. 2(2020) 043348]. As a further step, we present here investigations of the electron-rotation coupling effect in the presence of Auger decay channel for core-excited molecules, based on theoretical modeling of the total electron yield(TEY), resonant Auger scattering(RAS) and x-ray absorption spectra(XAS) for two showcases of CO and CH~+ molecules excited by resonant intense x-ray pulses. The Wigner D-functions and the universal transition dipole operators are introduced to include the electron-rotation coupling for the core-excitation process. It is shown that with the pulse intensity up to 10~(16) W/cm~2, no sufficient influence of the electron-rotation coupling on the TEY and RAS spectra can be observed. This can be explained by a suppression of the induced electron-rotation dynamics due to the fast Auger decay channel, which does not allow for effective Rabi cycling even at extreme field intensities,contrary to transitions in optical or VUV range. For the case of XAS, however, relative errors of about 10% and 30% are observed for the case of CO and CH~+, respectively, when the electron-rotation coupling is neglected.It is concluded that conventional treatment of the photoexcitation, neglecting the electron-rotation coupling,can be safely and efficiently employed to study dynamics at the x-ray transitions by means of electron emission spectroscopy, yet the approximation breaks down for nonlinear processes as stimulated emission, especially for systems with light atoms.     

Fast-electron-impact study on excitations of 4d electron of xenon

The electron energy loss spectrum of the 4d excitations of xenon was measured at an incident electron energy of1500 e V and a scattering angle of 6?. Besides the optically allowed transitions of 4d-15/2np and 4d-13/2np, the optically forbidden transitions of 4d-15/2ns, 4d-15/2nd, 4d-13/2ns, and 4d-13/2nd were observed. The measured features are assigned with the help of the calculation by the Cowan Code. The line profile parameters of both optically allowed transitions and optically forbidden ones were determined and compared with the previous available data. It is found that the natural widths of both dipole-allowed and dipole-forbidden excitations are approximately identical, which means the spectator transitions dominate the resonant Auger effect for both dipole-allowed and dipole-forbidden transitions.     

Auger and electron energy loss spectroscopic study of surfaces of iron-sulfur alloy,Fe7S8 and FeS2 cleaved in ultra high vacuum

The surfaces of an iron-210 at. ppm sulfur alloy, Fe₇S₈ and FeS₂ cleaved in an ultra high vacuum were studied by Auger (AES) and electron energy loss Spectroscopy (EELS). The S LVV Auger transition for the intergranular fracture plane of the alloy indicates that the sulfur is bonded to the surface as though it were adsorbed. The loss energies of the transition from valence to conduction bands for the surface are identical to those for the transgranular fracture planes. The trans- and intergranular fracture planes have very similar fine structure in the Fe MVV Auger transition profile. This indicates that the interaction between iron and sulfur is too weak to perturb the electronic structure at the fracture surface. The spectral features of the electron transitions having kinetic energies between approximately 40 and 50 eV are explained by a normal Fe MVV Auger transition and an autoionization process after excitation of Fe 3p electrons. The low spin ferrous ion in FeS₂ results in triplet peaks for the Fe MVV transition and doublet peaks for the autoionization event, but similar transitions for Fe₇S₈ exhibit singlet peak for each process.     

Method for observing Doppler-free hot-band transitions

A novel spectroscopic method for highly sensitive detection of hot-band transitions based on infrared-infrared double resonance in a molecular beam was successfully demonstrated. The intensity of the infrared transitions from a vibrationally excited state is enhanced by two orders of magnitude by resonant laser pumping. The hot-band transitions are identified in connection with the assigned transitions in the fundamental band. Doppler-free spectral lines are observed. The experiment was carried out on the 2v ₃←v ₃ transitions of CH₃F with CO₂ laser pumping and probing.     

Interlayer-exciton mediated three-hole-Auger-decay in the π⁎-band of highly oriented pyrolytic graphite

Resonant photoemission spectroscopy (resPES) is used to probe the occupied π- and unoccupied π^⁎-bands of carbon thin films with particular focus on the Auger decay. Highly Oriented Pyrolytic Graphite (HOPG) is studied at the C1s edge. We find strong resonant features at 285.5 eV and 292 eV in the resPES diagram. The normal two-hole Auger decay proceeds under constant kinetic energy (45°) only in the σ^⁎-region. In the π^⁎-region, however, it proceeds under 67.5° in terms of a E_bind(ħω) diagram. We attribute this to a multiple Auger decay with a net three hole final state. For this novel decay process we propose a model. We discuss the long lifetime of the first resonant excitation and conclude that it arises from the strong excitonic character of the first resonant state. We use HOPG as a reference system and suggest that this novel process is a tool to identify interlayer–substrate interaction of the carbon layers involved.