Escape probability of O 1s photoelectrons leaving copper oxide

The escape probability of O 1s photoelectrons as a function of depth of origin in CuO has been studied experimentally and by the Monte Carlo technique. It has been proved experimentally that the escape probability in the direction of X-ray propagation is no longer described by a simple exponential law but is a complex function with a maximum at a depth of 4–10 Å. The mean escape depth is derived from the experimental data and is compared with the results of theory; satisfactory agreement is obtained.     

Angular distribution of photoelectrons emitted from solids

The angular distributions of photoelectrons emitted from Cu, Ag and Au foils irradiated by Al Kα X-rays along the surface normal have been measured over a wide range of polar emission angles for different photoelectron lines. The experimental results are compared with analytical predictions based on a solution of the kinetic equation in the transport approximation, and satisfactory agreement is observed. The angular dependence of the photoelectron line intensity has been found to be less anisotropic than that of the differential photoelectric cross section. This effect is due to the elastic collisions that photoelectrons suffer on their way out of the target, and is more pronounced for samples with a high atomic number.     

Inverse photoemission from solids: Theoretical aspects and applications

The current status of inverse photoemission from clean crystals is reviewed with special emphasis on theoretical aspects and applications. The article focuses on general photoemission theory, the effective barrier potential at metal surfaces and the calculation of inverse photoemission spectra for both bulk and surface electronic states. In particular the information originating from inverse photoemission on Shockley and image-potential surface states, effective masses of mage-potential states and spin-split surface states at ferromagnets is discussed.     

Escape of photoelectrons and Compton‐scattered photons from an HPGe detector

The response function of a planar HPGe detector due to escape of photoelectrons and Compton‐scattered photons was studied for a point source with 59.5 keV energy. It was shown that both mechanisms, in addition to Ge x‐ray escape, leading to partial deposition of energy, could be observed in the same experiment. A Monte Carlo program was used to investigate these components of the response function. The results indicate that although the escape of scattered photons and Ge x‐rays are of the same magnitude, the escape of photoelectrons plays a more important role in the efficiency of a detector. Copyright © 2003 John Wiley & Sons, Ltd.     

Observation of high energy photoelectrons from solids at moderate laser intensity

We investigate the photoemission for a set of wide band-gap crystals irradiated by femtosecond Ti-Sapphire laser pulses at intensities varying from 0.5 to 6 TW/cm² (below the optical breakdown threshold). The measured total electron yield increases linearly with the laser intensity in this intensity range. An intense and wide plateau of high energy electrons appears in the photoelectron spectra at excitation intensities larger than 1 TW/cm². The exponential cut-off of this plateau reaches 40 eV at maximal applied intensities. In order to explain such a behavior, we propose a mechanism where the heating is due to a sequence of direct interbranch one- and multi-photon transitions in the conduction band. PACS 79.60.-i; 79.20. Ds; 42.50. Hz     

Optical and transport properties of defective non-crystalline solids

A method is proposed to analyze the optical absorption and electrical conductivity of non-crystalline materials having a high concentration of localized mid-gap states. The method is applied to dual ion beam deposited silicon nitride films containing various N contents. In this method, the optical absorption spectrum is fitted by using hypothetical functions of the density of states (DOS). The contribution from electron transitions between localized states is taken into account. The DOS at the Fermi level (E_f) was found to be very high (10¹⁹–10²⁰ eV^-1cm^-3), so that hopping of charge carriers around E_f dominates the transport properties. Combined with the data of electrical conductivity, the hopping distance and the spread of the electron wave function of the charge carriers at E_f are calculated. Furthermore, the temperature dependence of the mobility of the charge carriers at E_f is also deduced. PACS 72.20.-i; 72.80.-r; 73.61.Jc; 78.30.Ly     

On the problem of medium-range ordering in non-crystalline solids

The problem of medium-range ordering (MRO) in non-crystalline solids (NCS) is formulated on the basis of local (network) dimensionality ascribed to regions (clusters) not greater then 30 to 50 Å in diameter. We propose therefore to distinguish two MRO processes: The first one is MRO of the 1st kind, i.e. of basic motives/units, the second one is MRO of the 2nd kind, i.e. of clusters. Only the second process gives rise to scattering features in the medium(intermediate)-angle range. In several examples it is shown how it is possible to contribute to a better understanding of MRO by a direct calculation of the intensity of scattered X-rays (neutrons) and-or using 2D-models of nonperiodic structures.     

Characteristic energy loss structure of solids from x-ray photoemission spectra

Energy loss peaks in x-ray photoemission spectra of nine metals are presented. No strong evidence for intrinsic plasmon structure was observed. Spectra from the free electron-like metals, Al, Li, and Na under ultra-high vacuum (10^?11 torr) conditions show intense bulk plasmons and surface plasmons. Systematic variations in the characteristic energy losses are reported for the series Ag to Te. In addition to losses that may be attributed to plasma oscillations of the 5s5p bands, with N = 1 and 2 electrons, respectively, both Ag and Cd show additional high-energy losses that may arise through 4d-shell participation, with N = 11 and 12.     

Experimental evidence of above-threshold photoemission in solids

Nonlinear photoemission from a silver single crystal is investigated by femtosecond laser pulses in a perturbative regime. A clear observation of above-threshold photoemission in solids is reported for the first time. The ratio between the three-photon above-threshold and the two-photon Fermi edges is found to be 10(-4). This value constitutes the only available benchmark for theories aimed at understanding the mechanism responsible for above-threshold photoemission in solids.     

Calculation of inelastic mean free path of photoelectrons in some solids

The method of independent atomic centre approximation for the calculation of inelastic mean free paths of electrons in solids is described. To this end, the atomic ground and exited wave functions and the total cross sections of electron scattering on C, O, Al, Si, and Ge atoms were calculated in Hartree-Fock approximation and first Born approximation, respectively. Results for the application of the method to the six solid media amorphous carbon, silicon, germanium, Al₂O₃, SiO₂, and GeO₂; are presented. Agreement between calculated mean free paths and experimental attenuation lengths indicates that the approximation of independent atomic centres provides a useful method in the electron energy range from 200 to 4000 eV.     

Superconducting gap anisotropy in Nd1.85Ce0.15CuO4: results from photoemission

We have performed angle resolved photoelectron spectroscopy on the electron doped cuprate superconductor Nd1.85Ce0.15CuO4. A comparison of the leading edge midpoints between the superconducting and normal states reveals a small, but finite shift of 1.5-2 meV near the ( pi,0) position, but no observable shift along the zone diagonal near ( pi/2, pi/2). This is interpreted as evidence for an anisotropic superconducting gap in the electron doped materials, which is consistent with the presence of d-wave superconducting order in this cuprate superconductor.     

Cooper minima in the photoemission spectra of solids

Variations of the photoionization cross section of valence states as a function of interatomic distance are studied by means of atomic and solid-state density functional approaches and compared with photoemission data. In contrast to the free atom case, a series of Cooper minima is found for 4d, 5d, and 5f states in Pd, Ag, Au, and U metals. The discovered fundamental phenomenon is of high importance for the correct interpretation of photoemission data.     

Semiclassical approximation for the specific heat of non-crystalline solids at intermediate temperatures

The origin of the broad maximum in the specific heat (c _P) of glassy systems in the c _P/T ³ representation, in correspondence with the boson peak, has been described from a semiclassical perspective. A new model, based in the Debye approximation, taking into account the anharmonic vibrations of correlated atoms, is proposed. The theoretical predictions are shown to be in good agreement with the experimental data obtained in several glass samples in the temperature range 2–80 K.     

Angular resolved photoemission and the band structure of solids

Current models used to described angular resolved photoemission data are discussed. Emphasis is placed on the energies of structure in the spectrum and its relationship to the bulk band structure. Taking the lead salts (PbS, PbSe, PbTe) as an example a detailed comparison between measured and calculated spectra is given using time reversed LEED functions to describe the final state. On the basis of this comparison criteria are discussed that allow the application of simpler models for the interpretation of angular resolved photoemission. Such models include the direct, k-conserving model and the model of complete neglect of k conservation as well as the use of free electron like final states.     

Influence of octupole photoionization transitions on the angular distribution of photoelectrons from solids with account for elastic scattering

Using the transport theory approach, equations are derived for the angular dependence of XPS intensities from semi-infinite solids taking into account elastic scattering of photoelectrons and non-dipolar transitions up the second-order corrections. The unpolarized excitation source is considered. The elastic scattering is the main term smearing the non-dipolar contribution in complete analogy with smearing of dipole contribution. Some additional terms are shown to be negligible in comparison with the main term for complete sets of emission and azimuth angles, three representative single scattering albedo values and several representative sets of the second-order non-dipolar parameters. The photoelectron angular distributions are calculated for Al 2s and Ag 3d lines using transport theory and Monte-Carlo simulation. The Monte-Carlo calculations are in agreement with transport theory approach.     

Doppler effect in resonant photoemission from SF6: correlation between Doppler profile and Auger emission anisotropy

Fragmentation of the SF6 molecule upon F 1s excitation has been studied by resonant photoemission. The F atomiclike Auger line exhibits the characteristic Doppler profile that depends on the direction of the photoelectron momentum relative to the polarization vector of the radiation as well as on the photon energy. The measured Doppler profiles are analyzed by the model simulation that takes account of the anisotropy of the Auger emission in the molecular frame. The Auger anisotropy extracted from the data decreases with an increase in the F-SF5 internuclear distance.     

Magnetic properties of solids with small random anisotropy

Equilibrium state and magnetic excitations are studied within a phenomenological model for disordered magnetic systems with ferromagnetic exchange and weak random anisotropies. These systems exhibit the new type of magnetic behaviour characterized by finite ferromagnetic correlation length at zero temperature. Effects of coherent anisotropy and external magnetic field are considered.