Transition rates for interatomic Auger processes

Theoretical estimates in the LCAO approximation reveal that matrix elements for interatomic Auger processes are very strongly energy dependent, but that for very low energy Auger transitions they may be comparable with the competing intra-atomic Auger processes. Though inter-atomic Auger mechanisms will not produce appreciable lifetime broadening, they may radically change the low energy Auger spectrum, for example, of metallic elements in the early stages of oxidation, and may mainly control the transition rates of low energy Auger processes in ionic crystals.     

Absorption and photoluminescence studies of the temperature dependence of exciton life time in lattice-matched and strained quantum well systems

We present systematic studies of the temperature dependence of linewidths and lifetimes of excitonic transitions in quantum wells grown by molecular beam epitaxy using both photoluminescence(PL) and optical absorption. The temperature ranged from 6K to room temperature. Samples under investigation were lattice-matched GaAs/AlGaAs and InGaAs/InAlAs, and strained InGaAs/GaAs and InGaAs/AlGaAs quantum wellssystems. In addition, the effects of well-size variations in GaAs/AlGaAs quantum wells were measured and analyzed. In all cases we were able to observe the excitonic transitions up to room temperature. By a careful fitting of the experimental data we separated the exciton transitions from band-to-band transitions. By deconvoluting the excitonic transitions we obtained the homogeneous and inhomogeneous linewidths. The homogeneous linewidths were used to calculate the exciton lifetimes as a function of temperature using the Heisenberg uncertainty principle. We found the lifetime decreases significantly with temperature and increases with increasing well size. These results are interpreted in terms of the exciton-phonon interaction and are expected to be very useful for the design of semiconductor optical devices operating at different temperatures.     

UV-photoelectron spectroscopy at highest resolution —direct access to lifetime effects in solids?

Lineshapes and linewidth in angle-resolved photoemission spectra from solid surfaces contain a wealth of contributions from e.g. the photohole lifetime, the lifetime of the final state electron, and from their respective interactions with phonons and lattice imperfections. In addition, finite energy and angular resolution contribute to the experimentally observed linewidths. Using photoelectron spectra from bulk and surface state transitions on copper as an example, we discuss to which extent the various contributions may be distinguished experimentally. The results indicate that relevant spectroscopic information can be directly derived from such studies at very high resolution. This will lead beyond the kinematical analysis of photoelectron data in terms of band structures and may enable us to extract quantities which refer to the dynamical properties of the many-electron system.     

First direct observation of coulomb explosion during the formation of exotic atoms

A Doppler broadening of x-ray transitions from pionic nitrogen and muonic oxygen, which is attributed to Coulomb explosion of the molecules, has been observed by using a crystal spectrometer. Large linewidths indicate fast ionization of the molecules and a charge of (3-4)e for the accelerated fragments.     

Study of theK α satellites and hypersatellites in the x-ray and Auger spectra of neon

We report in this paper our calculations of the energies of the satellite and hypersatellite lines in the x-ray and Auger spectra of neon, using screened hydrogenic wavefunctions. Our calculations have enabled us to draw an energy level diagram for the neon atom in various defect-electron configurations. It is possible to show the transitions giving rise to the various x-ray and Auger satellites and hypersatellites using this diagram. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion     

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.     

Laser-induced isomeric transitions and gamma-ray laser

Laser-induced nuclear transitions between isomeric states were calculated by means of a parametrization approach, taking laser linewidths into account. With the calculated transition probabilities, the optical laser-stimulated conversion from long to shorter lifetime isomers were studied to explore possibilities for changing the lifetime of the long-lived isomer in terms of the induced two-step cascade and for developing a gamma-ray laser. Numerical estimates show that the laser intensity needed to generate coherent gamma radiation is higher than that available with current technology.     

Disorder broadening of alloy Auger spectra

Auger spectroscopy promises the means to separate initial and final state contributions to the disorder broadening of core XPS spectra in disordered alloys. Auger disorder broadening, deduced from recent ab initio results, is predicted to be greater than XPS disorder broadening for Cu₅₀Pd₅₀ and Ag₅₀Pd₅₀ alloys. Simulations are used to assess whether this effect is observable experimentally despite the greater lifetime broadening of Auger spectra. A number of cases where narrow core–core–core Auger transitions should allow clear experimental identification of this effect are identified. The prospects for determining environment-resolved Auger spectra using APECS have been investigated.     

Books received

The inhomogeneous broadening of optical transitions in solids is of fundamental importance in understanding the interaction of optical centres with their environment. In perfect crystals optical linewidths are determined by excited state lifetime and other d-phasing mechanisms such as the electron-phonon interaction. However, in real crystals random differences of impurities in crystallographically equivalent sites result in overlapping transitions and inhomogeneously broadened Gaussian lines. The suppression of inhomogeneous broadening of solid-state spectra using optical hole-burning and fluorescence line-narrowing techniques is discussed in terms of the determination of the homogeneous widths, multi-site geometries and the distributions of spectroscopic parameters that accompany disorder in a number of laser crystals and glasses.     

L-shell ionization of atoms and their subsequent decay by radiative and non-radiative transitions

The study of the ionization of atoms resulting in vacancies in their inner shells and the subsequent decay of the atomic-vacancy states by x-ray and Auger transitions continue to be an active area of interest. A rapid survey of the theoretical efforts to calculate the transition probabilities involvingL-subshells in the high-Z atoms is presented. A complete review of theL ₁-subshell yields for single-vacancy atomic states obtained by various experimental techniques is included. The production of multiple vacancies in theL shell and the role of the spectator vacancies in the decay process is discussed. A detailed case study of determining experimentally the number of multiple vacancies produced, and the x-ray fluorescence yields during ionization by heavy-ion bombardment is presented. It is established that the effect of spectator vacancies is to increase the x-ray fluorescence yields substantially.     

Coherent ultrafast core-hole correlation spectroscopy: x-ray analogues of multidimensional NMR

We propose two-dimensional x-ray coherent correlation spectroscopy for the study of interactions between core-electron and valence transitions. This technique may find experimental applications in the future when very high intensity x-ray sources become available. Spectra obtained by varying two delay periods between pulses show off-diagonal crosspeaks induced by coupling of core transitions of two different types. Calculations of the N1s and O1s signals of aminophenol isomers illustrate how novel information about many-body effects in electronic structure and excitations of molecules can be extracted from these spectra.     

X-ray absorption measured in the resonant Auger scattering mode

We report both experimental and theoretical studies on x-ray absorption measured in the resonant Auger scattering mode of gas phase carbon monoxide near the O1s-->2pi region. Both experiment and theory display a crucial difference between the x-ray absorption profiles obtained in the conventional and resonant scattering modes. Lifetime vibrational interference is the main source of the difference. It is demonstrated that such interference, which arises from a coherent excitation to overlapping intermediate levels, ruins the idea for obtaining x-ray absorption spectra in a lifetime broadening free regime.     

The lattice anharmonicity-induced broadenings of defect spectral lines

Summary The effect of the anharmonicity of the host lattice to the defect optical absorption and emission is studied. Anharmonicity contributes to the defect optical linewidths via phonon lifetime effect and defect-phonon scattering mediated by a second phonon. Numerical estimates of these contributions indicate that the anharmonic broadenings may not be negligible in some insulating crystalline hosts.     

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.     

A polynomial energy-gap model for molecular linewidths

A simple phenomenological model for predicting high-temperature linewidths is described. The rotationally inelastic relaxation time is calculated by using a polynomial inverse-energy-gap model. From this time, the linewidths may be calculated and ambient pressure linewidths may then be used to determine the polynomial coefficients and predict high-temperature linewidths. A detailed comparison with known nitrogen linewidths is given. The relaxation rates obtainable from the model may also be used to calculate pressure-narrowing effects for vibrational Q branches.     

Observation of selection rules and restricted valence band self convolution for the KVV Auger transition of alkali graphite intercalation compounds

The Auger spectrum of the KVV transition in alkali graphite intercalation compounds (AGIC's) gives unambiguous evidence that the electron states from the alkali metals in the valence band of the compound form a narrow structure in the KVV Auger spectrum which corresponds to the self convolution of these states and that they are not convoluted with the whole valence band. This observation shows that strong matrix element effects or selection rules for KVV Auger transitions have to be taken into account.     

Bremsstrahlung of the free electrons arising in an irradiated specimen

A model of the bremsstrahlung produced by photoelectrons, Auger electrons, and Compton electrons arising in an irradiated specimen is proposed. Calculations have shown that the contribution of the Compton electron bremsstrahlung shows up for monochromatic primary radiation of high-energy photons. For the primary radiation of x-ray tubes, only the bremsstrahlung spectrum of photoelectrons and Auger electrons is significant. The factors affecting the proportions between these components are considered. The bremsstrahlung spectral distribution of the mentioned electrons shows considerable deviations from that predicted by the Kramers theory that are due to the large depth of their occurrence and to the ambiguity of their energy. The region of the spectrum has been determined where the intensity of the electron bremsstrahlung is greater than the intensity of the x-ray tube polychromatic primary radiation scattered by the irradiated object.     

Angle resolved auger surface spectroscopy

The angular distribution of electrons ejected in core-valence-valence Auger transitions of atoms chemisorbed on metal surfaces is considered theoretically. Since the valence electrons participating in the Auger transition are also involved in chemical bonding to the surface, these initial states contain information pertaining to the chemisorption bonding geometry. The role of the initial state symmetry in determining the angle resolved Auger surface spectrum (ARASS) is investigated through model calculations and is found to be small. Thus the ARASS is expected to be a smoothly varying function of angle with ? ± 15% modulations due to diffraction effects, in agreement with recent experimental results for S adsorbed on Ni(100).     

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.     

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.