Electronic Decay Processes Research Articles

Photoluminescence properties of trace amounts of Pr and Tb in yttria-stabilized zirconia

We have investigated the photoluminescence (PL) properties of trace amounts of Pr and Tb in single-crystal samples of yttria-stabilized zironia (YSZ), and found that Pr of the order of 10 −6 mass% and Tb of the order of 10 −5 mass% in YSZ can be detected by the PL spectroscopy. The PL spectra of the YSZ samples for the 280 nm excitation were comprised of several peaks and a broad emission. The peaks were attributed to transitions of Pr 3+ and Tb 3+ in the YSZ samples, whereas the broad emission seemed to be attributed to the yttria-associated oxygen vacancies. The peak intensities corresponded to the amounts of Pr and Tb in the YSZ samples, the amounts of which were analyzed by glow discharge mass spectrometry. In the PL excitation spectra, but not in the photoabsorption spectra, small peaks at 376 and 381 nm were observed, and were attributed to the transitions of Tb 3+ in the YSZ samples. The results of the PL excitation spectra corresponding to the Pr 3+ line emissions suggest that the charge transfer occurs between the YSZ and Pr ion in it. The trace amounts of these lanthanoids in YSZ would disturb the decay process of the photoinduced electrons to the yttria-associated oxygen vacancies.

Impact of interatomic electronic decay processes on Xe 4d hole decay in the xenon fluorides

A hole in a 4d orbital of atomic xenon relaxes through Auger decay after a lifetime of 3 fs. Adding electronegative fluorine ligands to form xenon fluoride molecules, results in withdrawal of valence-electron density from Xe. Thus, within the one-center picture of Auger decay, a lowered Xe 4d Auger width would be expected, in contradiction, however, with experiment. Employing extensive ab initio calculations within the framework of many-body Green’s functions, we determine all available decay channels in XeFn and characterize these channels by means of a two-hole population analysis. We derive a relation between two-hole population numbers and partial Auger widths. On this basis, interatomic electronic decay processes are demonstrated to be so strong in the xenon fluorides that they overcompensate the reduction in intra-atomic Auger width and lead to the experimentally observed trend. The nature of the relevant processes is discussed. These processes presumably underlie Auger decay in a variety of systems.

Electronic decay in weakly bound heteroclusters: Energy transfer versus electron transfer

Inner-valence ionized states of weakly bound systems like van der Waals clusters can efficiently decay by electron emission. The mechanism of the decay, which does not occur in the isolated monomer units constituting the clusters has recently been shown to be of intermolecular/interatomic nature. This intermolecular/interatomic Coulombic decay (ICD) mechanism prevails in many systems ranging from hydrogen-bonded molecular clusters to atomic rare gas clusters. In the present paper we extend our previous studies to weakly bound heteroclusters built up of monomer units of largely differing energetics. It is shown that, as soon as the double ionization potential of a monomer unit is lower in energy than the ionization potential of the initially created inner-valence vacancy on a neighboring monomer unit, an additional electronic decay process can take place. In contrast to the ICD mechanism, which involves an efficient energy transfer between the monomer units, this second process is essentially based on an electron transfer process. It is therefore termed electron-transfer mediated decay (ETMD). We have analyzed the mechanisms of the electronic decay processes taking place following inner-valence ionization in weakly bound heteroclusters in an exemplary study of the NeAr dimer. The involved electronic states have been calculated using ab initio Green’s function techniques. The lifetime of the inner-valence Ne(2s−1)Ar vacancy has been estimated and partitioned according to the contributions of the two decay channels based on a perturbation-theoretical description of the decay process. As a result, the lifetime of the inner-valence resonance state is estimated to be of the order of 10–100 fs, the specific value strongly depending on the internuclear separation of the monomers. The ICD process is shown to be by far the dominant decay channel at distances corresponding to bound states of the dimer. With decreasing internuclear separation the ratio of the ETMD and ICD decay widths quickly increases over several orders of magnitude.

Electronic structure, electronic decay, and desorption processes of molecular solid SiCl4 following core-level excitation.

We report high-resolution Si ${\mathit{L}}_{23}$-edge total-electron-yield (TEY) measurements from solid ${\mathrm{SiCl}}_{4}$, and a comparison with the gas-phase photoabsorption data to characterize the Si(2p) pre-edge spectral features. By applying resonant photoemission spectroscopy, the spectator Auger process is found to be the major decay channel for the resonantly excited Si(2p) core hole of condensed ${\mathrm{SiCl}}_{4}$. Participant Auger decay makes a notable contribution to the resonant Auger processes for core-to-valence excitation. The close resemblance of the photon-stimulated desorption ion spectra and the TEY spectrum was interpreted in terms of the Auger-initiated desorption or Knotek-Feibelman mechanism. \textcopyright{} 1996 The American Physical Society.

Core-hole Electronic Structure Studies on Molecules using Synchrotron Radiation

Some recent developments in the study of core-hole photoabsorption, photoionisation, X-ray emission, Auger electron decay and photofragmentation processes for molecules are discussed. The emphasis is on the interpretation of experimental data obtained from synchrotron radiation studies. New insights which are being obtained through theoretical investigations into the electronic and nuclear properties of excited and ionised states of simple molecules are reviewed.

Electronic excitation, decay and photochemical processes in rare gas clusters

An overview of processes of electronic excitation and the subsequent following decay in clusters is presented. Pure rare gas clusters (Xe-Ne) are chosen as model systems for non-metallic clusters. Selective photo-excitation with synchrotron radiation offers the possibility to study a variety of different decay and relaxation processes (exciton trapping, desorption of electronically excited species) depending both on the cluster size and of the excitation energy. Special emphasis is given to the role of energy transfer, and in particular, vibrational energy flow, radiative and non-radiative electronic relaxation. Some recent results on the decay processes in He clusters are given in the last part of the article. Decay processes in He clusters differ remarkably from that observed in the heavier rare gas clusters.

Electronic decay processes following the resonance excitation of the B 1s core electron in BF3

Electron spectroscopy has been carried out to investigate spectator and participant resonance Auger decay processes following the B 1s→2a2″ excitation in gaseous BF3 molecules using monochromatized synchrotron radiation. The resonance-enhanced satellite bands corresponding to the spectator Auger electron emission form six broad peaks showing good correspondence with the normal Auger bands. Resonance enhancement of the photolines corresponding to the participant Auger electron emission occurs only for the bonding orbitals having B character.

Electronic decay processes of photoexcited 2p resonances of atomic Ar, K, and Ca.

An experimental and theoretical study is presented of the 2p\ensuremath{\rightarrow}ms,nd inner-shell excitations in the elements Ar, K, and Ca with special emphasis on the 2p\ensuremath{\rightarrow}3d excitations. The present work is mainly concerned with investigations of K and Ca. An interpretation is given of the resonant Auger spectra in which it is shown that the difference in collapse of the 3d wave function in the presence of holes in the 2p shell compared to holes in the 3p shell leads to qualitative differences between the three atoms. In Ar, shake effects are very strong between the initial state with one 2p hole and the final state with two holes in the 3p shell due to the fact that the 3d orbital collapses between the two. However, in K and Ca the 3d orbital is collapsed in both states and shake effects are found to be much smaller for this excitation. In contrast, for K and Ca it is predicted that the 4d orbital will show strong shake effects. The collapse of the 3d orbital in the final state of all three atoms leads to resonant Auger spectra which are very different from those following direct ionization in the 2p shell. The details of the resonant Auger spectra depend on the degree of collapse; we discuss the differences in the observed spectra in terms of the relative strengths of the electrostatic 3p\ensuremath{\leftrightarrows}3p and 3p\ensuremath{\leftrightarrows}3d interactions in the three elements. The second-step Auger decays in K and Ca are studied both experimentally and theoretically.

Coherent excitation of vibrational wave functions observed in core hole decay spectra of O 2, N 2 and CO

High resolution studies of the electron emission generated in the decay of neutral core to bound state excitations in small molecules, reflect the dynamics of the time evolution of the coherently excited vibronic wavefunctions in the core excited states. This coherent excitation gives rise to interference effects observed in the electronic decay processes. Since the lifetime of the core excited states in O 2, N 2 and CO is only a few fs, the study of these interference phenomena offers a stringent test of the quantum mechanical description of the time evolution of the excited states. Here we demonstrate that the resulting interference effects are not only observed when the lifetime broadening of the core excited state approaches the vibrational spacing but also for systems where the vibrational levels are clearly resolved in the excitation spectrum.

Electron mean free paths in the alkali metals.

Photoemission data in which the signal from the first atomic layer is well resolved from that of the bulk are used to determine accurately the kinetic-energy dependence of the inelastic-electron mean free path in the alkali metals. At the higher kinetic energies, the data are in very good agreement with the theory of Penn. Below about 10 eV, the mean free path in the heavier alkali metals drops markedly below the theoretical values. This is attributed to electron decay processes involving the unoccupied d bands

Direct Observation of Photoinduced Charge Separation Dynamics in Solid Poly(N-vinylcarbazole) Powders by Diffuse Reflectance Laser Photolysis

Abstract Transient absorption spectra of poly(N-vinylcarbazole) powders doped with some electron acceptors are composed of the bands of acceptor anion and carbazole cation, indicating directly charge separation from the excited state. Electronic structure and decay processes of polymer cation are considered and compared with transparent film.

Femtosecond time-resolved ionization spectroscopy of ultrafast internal-conversion dynamics in polyatomic molecules: Theory and computational studies

A framework for the theoretical description of two-pulse time-resolved ionization spectroscopy of ultrafast excited-state dynamics of polyatomic molecules is developed. The radiation–matter interaction as well as intramolecular couplings in the excited-state manifold are treated nonperturbatively by solving the time-dependent Schrödinger equation. The numerical solution is based on a discretization of the ionization continua which becomes particularly efficient for ultrashort laser pulses. With this method converged computations of ionization signals become possible even for complex molecular systems. Computer simulations are performed for a model system representing three-dimensional non-Born–Oppenheimer excited-state dynamics on conically intersecting potential-energy surfaces (the S1 and S2 surfaces of pyrazine). The dependence of the observable time-resolved ionization signals (total ion yield as well as photoelectron spectrum) on the properties of the laser pulses (carrier frequency and pulse duration) is explored. It is demonstrated that ultrafast electronic decay processes as well as coherent vibrational motion in excited states can be monitored by pump–probe ionization with suitable pulses. The dependence of the time-resolved ionization signals on properties of the cation (ionization potentials and potential-energy surfaces) is also discussed.

Far-Infrared Magneto-Optical Study of Photoexcited Indium Antimonide. II. Spin Dependent Properties

Time-resolved measurements of the magneto-optical absorption due to photoexcited electrons have been carried out in order to investigate the relaxation of spin hot electrons in p-type InSb samples. For a sample with an excess carrier concentration of 5.8×1012 cm-3, the spin flipping time from the O- to the O+-Landau subband was found to be as long as 3.5 µs at 1.6–4.2 K. It is primarily determined by donor impurity scattering and is independent of their charge states. The spin property of electrons is important in the recombination between the conduction band and the acceptor. Other electron decay processes are also discussed with the help of experimental results under pulsed photoexcitation and under quasi-steady state excitation.

Deexcitation Electron Spectroscopy: A Probe for the Localisation of Valence Wavefunctions in Free and Adsorbed Molecules

Deexcitation electron spectroscopy (DES) is the measurement of the kinetic energy distribution of electrons emitted in the decay of highly excited states of molecules formed by core to bound resonant photon absorption. This spectroscopy has the potential of becoming a new probe of the localised properties of the valence electronic states in molecules as well as furnishing new insights into the dynamics of the electronic decay and screening processes. This paper describes the basic principles of DES by following the evolution of the DES spectra in CO, from the isolated CO molecule in the gas phase, through the transition metal carbonyls, to CO adsorbed on a surface. In all three cases the energetics, intensities and dynamics of DES will be compared with photoelectron spectroscopy (PES) and Auger electron spectroscopy (AES).

Cross section for photoelectron capture by IrBr3−6 in AgBr

The lifetime of photogenerated electrons in AgBr crystals doped with trivalent iridium ions was determined from photoconductivity measurements. A first-order electron decay process occurs at Ir3+ sites, with a room-temperature photoelectron lifetime of 13–150 ps for materials with 89–4 ppm of dopant. A cross section of 26±5 Å2 was deduced for photoelectron capture by IrBr3−6 centers.

Photoemission investigation of auto-ionising Na+-2p core excitons in NaCl

The first sharp structures in the NaCl Na+-2p absorption spectrum were investigated by means of constant final state spectroscopy (CFS). This method allows for a detailed observation of lattice relaxation and electronic decay processes. The analysis of the spectra leads to an identification of the first ten structures in the absorption spectra as being bound or quasi-bound electron-hole states. Five of them overlap with the continuum states above the bottom of the conduction band. The latter ones are short lived and auto-ionise through one-electron valence band excitations.

Electron clouds: A general survey

This paper summarizes the results and analysis of seven rocket experiments designed and performed to study systematically the physics of the generation of artificial electron clouds in the upper atmosphere. The several experiments can be discussed in four phases. For example, the first involved the daytime Aerobee rocket release of potassium at 121 km and served to establish the feasibility of solar photoionization generation of artificial electron clouds. The second phase was the Nike-Cajun night-time release of cesium at 101 km to establish the role of thermo-chemical generation of electron clouds. The third phase was designed to study the correlation of optical and radio-radar data as obtained from a visible, photographic electron cloud. This was accomplished by the morning twilight releases of cesium and sodium at 128 km and 116 km, respectively. The final phase of this series reported here involved three releases of cesium at 91 km, 82 km and 69 km, in order to determine the altitude below which long-lived, dense artificial electron clouds cannot be generated by the techniques employed here. The results of this last phase yield valuable physical data pertinent to several of the electron decay processes dominant in the lower altitudes. The overall analysis of the extensive optical and radar data has yielded information on the following physical parameters: (a) chemical yield of contaminant, (b) thermal ionization efficiency, (c) upper atmospheric wind velocity, (d) wind shear, (e) optical and Gaussian halfwidth of cloud with time, (f) ambipolar diffusion, (g) neutral diffusion, (h) photoionization cross-sections, (i) mutual neutralization processes and (j) rate of chemical consumption of alkali by atmospheric oxygen. Finally, the role that artificial electron clouds can assume for practical utilization for radioradar propagation is emphasized with some discussion indicating the extent of the available potential for this engineering purpose.

Artificial electron clouds—II: General considerations

This is the second in a series of papers which deals with the creation of artificial electron clouds by the ejection of proper contaminants into the upper atmosphere. It presents those general considerations which are common to the several papers to follow. A square-well model is used to describe the distribution of the ejected contaminant immediately after the fast initial expansion. The diffusion equation is applied to this model and the salient features are noted. Basic equations are applied to this simplified model to describe the electron generative and decay processes on a term-by-term basis, in order to ascertain the relaxation times involved for the several interrelated problems. Chemical consumption of the alkali metal contaminant by reaction with the ambient species is found to be negligible above 95 km. It is concluded that “for generation methods similar to those outlined here the practical altitude limits of generation of long lived electron clouds is between about 70 and 200 km; chemical consumption determines the lower while the diffusion process establishes the upper limit.