Exoelectron Research Articles

Normalization of kinetic equations for thermostimulated processes

The article introduces a new approach to construction of kinetic equations where the system variable—concentration of reactants—is substituted by relative dimensionless value. It offers the canonical form of kinetic equations and their approximated expressions revealing some general characteristics of thermally actuated processes. Necessity and expediency of building kinetic equations on the basis of relative dimensionless variables have been proved theoretically and also confirmed with original experimental research. Some aspects of experimental investigation of thermally stimulated exoelectron emission phenomenon also are described.

Triggering of relaxation cascades in pre-irradiated RGS by chemiluminescent reactions

The relaxation processes in oxygen-containing Rare Gas Solids (RGS) pre-irradiated by a low energy electron beam are studied with the focus on the role of diffusion-controlled atom-atom recombination reaction in the relaxation cascades. The results of correlated in real time measurements of thermally stimulated phenomena are presented. The experiments were performed using activation spectroscopy methods–spectrally resolved thermally stimulated luminescence (TSL) and exoelectron emission (TSEE). The suggested mechanism provides a long-range energy transfer and demonstrates the existence of a new relaxation channel–triggering of electronic relaxation processes by diffusion-controlled atom-atom recombination reaction followed by the emission of photons in the visible range which is converted due to the relaxation process into a vacuum ultraviolet (VUV) emission.

Electronic energy states in Si-doped MgO for exoelectron emission

A generalized analytical method to determine the density of energy states of electron emission source (EES) is devised by using a thermal excitation and emission model for an exoelectron in the MgO layer and the emission time constants of the exoelectron extracted from experimental stochastic distributions of discharge delay time. When applied to Si-doped MgO, the emission time constant of the exoelectron from the Si EES becomes shorter at high temperature and at short time intervals due to thermal excitation. The density of energy states of the Si EES DSi(E) shows the main peak at 736 meV, a satellite peak at 601 meV, and broad energy structures over the range of 586–896 meV. The effective number of Si EES is 5.5 times larger than that in purified MgO. The excitation energy in a Si-doped MgO cluster with a crystal structure is obtained to be 0.83 eV by using the symmetry-adapted-cluster configuration interaction method and the Si EES contributes to exoelectron emission. The thermal excitation is governed by the transition from the Si–O bound state and the Mg edge state to the antisymmetrical edge states and the extended surface state. The excitation energy in an MgO cluster with a Si-doped atom inside and a nearest oxygen vacancy taking account of structural relaxation is calculated to be 0.75 eV, which shows good agreement with the main peak in DSi(E). The excitation energies of 0.64, 0.73, and 0.78 eV are also obtained in an MgO cluster with a Si-doped atom at the surface and a nearest oxygen vacancy. The first excitation energy corresponds with the satellite peak. The broad energy structures of DSi(E) are caused by the dependence of excitation energy on the position of Si-doped atoms inside and at the surface of the MgO cluster, and on the interatomic distance of Si–O due to structural relaxation. The energy structures can be also attributed to the thermal excitation to the various symmetrical Mg edge states and the surface states. When the number of complex structures of the Si EES with adjacent oxygen vacancies increases, oxygen vacancies are generated from the complex structures and the increase in the electron traps degrades electron emission rate. Therefore, the number of complex structures has an optimum value that leads to the maximum effective number of Si EES.

Relaxation channels and transfer of energy stored by pre-irradiated rare gas solids

The processes of energy relaxation in rare gas solids pre-irradiated with an electron beam are discussed. The emission of exoelectrons and photons from rare gas solids is studied. Investigations are performed by a set of activation spectroscopy methods applied simultaneously to each sample. Photon-stimulated exoelectron emission from solid Ne is observed for the first time.

Optically stimulated exoelectron emission processes in quartz: comparison of experiment and theory

Recent experiments have demonstrated that it is possible to measure optically stimulated exoelectron emission (OSE) signals simultaneously with optically stimulated luminescence (OSL) from quartz samples. These experiments provide valuable information on the charge movement in quartz grains. Two specific experiments measured the temperature dependence of the OSL and OSE signals on preheat and stimulation temperature. This paper provides a quantitative description of these experiments by using a previously published theoretical model for photostimulated exoelectron emission (PSEE). The experimental data yield a value of χ∼1.2 eV for the work function of quartz. The experimental temperature dependence of the OSE signals is interpreted on the basis of a photo-thermostimulated (PTSEE) process involving the main OSL trap at ∼320 °C; this process takes place with a thermal assistance energy estimated at W∼(0.29±0.02) eV. Good quantitative agreement is obtained between theory and experiment by assuming a thermal broadening of the thermal depletion factor for the OSL traps, described by a Gaussian distribution of energies.

Electronically induced modification of atomic solids and their relaxation probed by luminescence methods

A role of the electronic excitations in a modification of the structural properties of atomic solids is considered by the example of solid Ar. The studies were performed employing a wide array of luminescence methods: cathodoluminescence (CL), photoluminescence (PL), measurement of excitation spectra and thermally stimulated luminescence (TSL) in combination with thermally stimulated exoelectron emission (TSEE). Analysis of the luminescence band shape, their excitation spectra and temporal evolution under excitation evidenced the formation and accumulation of the permanent lattice defects (Frenkel pairs). A key part of the exciton self-trapping into molecular states in the defect formation is demonstrated and model of electronically induced lattice defects is discussed.

Electron-hole spectra created by adsorption on metals from density functional theory

Nonadiabaticity in adsorption on metal surfaces gives rise to a number of measurable effects such as chemicurrents and exoelectron emission. Here we present a quantitative theory of chemicurrents on the basis of ground-state density functional theory (DFT) calculations of the effective electronic potential and the Kohn-Sham band structure. Excitation probabilities are calculated both for electron-hole pairs and for electrons and holes separately from first-order time-dependent perturbation theory. This is accomplished by evaluating the matrix elements (between Kohn-Sham states) of the rate of change of the effective electronic potential between subsequent (static) DFT calculations. Our approach is related to the theory of electronic friction, but allows for direct access to the excitation spectra. The method is applied to adsorption of atomic hydrogen isotopes on the Al(111) surface. The results are compatible with the available experimental data (for noble metal surfaces); in particular, the observed isotope effect in H versus D adsorption is described by the present theory. Moreover, the results are in qualitative agreement with computationally elaborate calculations of the full dynamics within time-dependent density functional theory, with the notable exception of effects due to the spin dynamics. Being a perturbational approach, the method proposed here is simple enough to be applied to a wide class of adsorbates and surfaces, while at the same time allowing us to extract system-specific information.

Electron traps in solid Xe

Correlated real-time measurements of thermally stimulated luminescence and exoelectron emission from solid Xe pre-irradiated with an electron beam are performed. The study enables us to distinguish between surface and bulk traps in solid Xe and to identify a peak related to electronically induced defects. The activation energy corresponding to annihilation of these defects is estimated by the following methods: the method of different heating rates, the initial-rise method, and the curve cleaning technique with fitting of the thermally stimulated luminescence glow curve.

Effects of exo-electron emission from MgO thin film on statistical delay of glow discharge of ac-PDP

Relationship between exo-electron emission currents and statistical delay of glow discharges of ac-PDP was investigated by measuring exo-electron currents. The currents were measured as a function of time and doping elements. The measured results indicated that the exo-electron currents decrease exponentially with time, while the statistical delay increases with time. The results demonstrate that the statistical discharge delay is inversely proportional to the currents of exo-electron emission from MgO film in a time interval of 1 to 10 ms.

Modification of atomic solids via electronic subsystem probed by spectroscopic methods

The role of electronic excitations in the modification of surface properties of atomic solids (AS) is considered using solid Kr as an example. The study is focused on the novel phenomenon found recently in AS: anomalous low temperature “post-desorption” (ALTpD) observed after irradiation of AS with low-energy electron beam. The experiments were performed employing cathodoluminescence (CL) method in combination with correlated in real time measurements of thermally stimulated luminescence (TSL), thermally stimulated exoelectron emission (TSEE) and detection of the “post-desorption” yield. Analysis of the yields correlation enabled us to verify a two-stage mechanism of the ALTpD: (i) thermally assisted release of electrons from their traps followed by (ii) intrinsic charge recombination resulting in “excimer dissociation” via radiative transition to a repulsive part of the ground state potential curve.

Measurement of exo‐electron emission from MgO thin film of ACPDPs

Exo‐electron emission from MgO thin film was measured by attaching a high‐precision current sensor to the address electrode of the rear plate of an ACPDP test panel. The measured results revealed that the exo‐electron emission currents can vary very sensitively with the type of doping elements used in MgO film and the measuring temperature. The activation energy of the exo‐electron emission estimated from the emission curves indicated that the trap levels are between 0.05 and 0.32 eV below the bottom of its conduction band. This suggests that shallow electron‐trap levels within MgO film are mainly responsible for the exo‐electron emission.

Inserted defects in bone after radiotherapy using exoelectron emission technique

This article deals with the estimation of inserted defects in bone material by photons used for radiation therapy, for instance, to treat carcinogenic diseases including bone cancer. However, there are side effects by radiation on structures of bones. Structure alters electron density of states that, in time, influences biomedical reactions on bone life condition. Exoelectron emission (EEE) phenomenon underlies a new electron spectroscopy to explore alteration of the electronic structurally dependence properties of bone material. The influence of photons with different energies and the dose rate on the bone structure is discussed. The bone EEE response is not linear on the dose rate and probably is connected with interaction of radiation induced centers responsible for EEE process. EEE structural changes have varied with energy variety of doses used in radiotherapy.

Comparative study of electronically induced desorption from pre-irradiated solid Ar and Ar excited with an electron beam

Relaxation processes in films of solid Ar irradiated with low-energy electrons were studied with emphasis on the recently found phenomenon – anomalous low temperature desorption (ALTD) of rare gas atoms from pre-irradiated rare gas solids. A combination of optical and current activation spectroscopy methods were used; thermally stimulated luminescence, thermally stimulated exoelectron emission as well as measurement of the total desorption yield. The measurements were performed simultaneously in real time. Analysis of the data provides solid evidence of a two-stage mechanism for ALTD: (i) thermally assisted release of electrons from their traps followed by (ii) intrinsic charge recombination. The dose behavior and efficiency of the ALTD in comparison with desorption induced by electronic transition under an electron beam is discussed.

21.5L: Late‐News Paper: Measurement of Exo‐Electron Emission from MgO Thin Film of AC‐PDP

AbstractExoelectron emission from MgO thin film was measured by attaching a high precision current sensor to the address electrode of rear plate of test panels of AC‐PDP. The measured results revealed that the exoelectron emission currents vary very sensitively with the type of doping elements used and measuring temperatures. The activation energy of the exo‐electron emissions estimated from the emission curves indicated that the trap levels lies between 0.05∼0.32eV below the bottom of its conduction band. This suggests that shallow electron trap levels within MgO film are mainly responsible for the exo‐electron emissions, rather than deep trap levels like F‐type centers.

Exoelectron Emission of a Carbon Nanomaterial

The exoemission properties of a Taunite carbon nanomaterial consisting of nanosized multiwalled nanotubes and nanofibers were investigated by thermally stimulated exoelectron emission (TSEE). The TSEE spectra of the carbon nanomaterial differed from the spectra of pressed graphite. It was assumed that defect—adsorbate complexes were emission-active centers on the surface of the nanomaterial

Optically stimulated desorption of “hot” excimers from pre-irradiated Ar solids

Electronically induced desorption from solid Ar pre-irradiated by a low-energy electron beam is investigated by activation spectroscopy methods —photon-stimulated exoelectron emission and photon-stimulated luminescence in combination with spectrally resolved measurements in the VUV range of the spectrum. Desorption of vibrationally excited argon molecules Ar2*(ν) from the surface of pre-irradiated solid Ar is observed for the first time. It is shown that desorption of “hot” Ar2*(ν) molecules is caused by recombination of self-trapped holes with electrons released from traps by visible-range photons. The possibility of optical stimulation of the phenomenon is evidenced.

Charge movement in grains of quartz studied using exo-electron emission

A flow-through Geiger-Müller pancake electron detector attachment has been fitted to the Risø TL/OSL reader, enabling optically stimulated electrons (OSE) to be measured simultaneously with optically stimulated luminescence (OSL). This detector has been used to provide new insights into charge movement in luminescence phosphors. Here we show that OSE from natural quartz grains gives an easily detectable, reproducible and light sensitive signal, although it is not as intense as OSL. A single sample of natural quartz grains extracted from a sediment is used to investigate the thermal stability and dependence on stimulation temperature of the OSE and OSL signals. The response to dose of these signals is also investigated, and conclusions drawn on the movement of electrons in this natural phosphor.

TSEE response of silicates of the jade family in gamma radiation beams

In the present work some dosimetric characteristics of pellets of Brazilian silicates with Teflon as: actinolite-Teflon, tremolite-Teflon, rhodonite-Teflon and diopside-Teflon were investigated in gamma radiation beams of Co 60 using the thermally stimulated exoelectronic emission (TSEE) technique. The TSEE response reproducibility did not exceed 20%. The TSEE curves are simple, with a peak at 240 ∘ C . The dose response curves show a sublinear behavior between 100 Gy and 20 kGy, and the lower detection limit for the pellets was ( 2.0 ± 0.4 ) Gy . These results may be considered satisfactory for applications at high-dose dosimetry.

Radiation-induced defects and processes on the surface of radiation detector oxides according to exoelectron emission data

Exoelectron emission from materials for high-sensitivity thermoluminescence (TL) and exoelectron emission (EE) dosimeters based on α-Al2O3, BeO, and LiF was examined, a scheme for the formation of TL and EE centers induced by low-penetrating (β, α) radiation was proposed, and their relation to electron and hole (F, V K) centers and to the emergence of dosimetric peaks during dose readout (thermal stimulation) was revealed. A unified approach to the interpretation of the phenomenon of thermally stimulated emission (TSEE) from these materials in terms of valence transformations of oxygen that is weakly bound to the surface was developed. The difficulties in the manufacture of hybrid high-sensitivity TL-TSEE dosimeters are explained from the standpoint of the physicochemical mechanism of exoelectron emission.

Effect of electrolytical hydrogenation on the thermal stability and crystallization kinetics of METGLASS MBF-50

The effect of electrolytical hydrogenation on both the surface and volume crystallization kinetics and thermal stability of amorphous alloy METGLASS MBF-50 has been investigated. The surface crystallization has been investigated by the exoelectron emission (EEE) technique, whereas the volume crystallization has been followed by differential thermal analysis (DTA).It has been found that both the surface and volume crystallization of investigated material occur in two stages. The surface crystallization occurs at temperature lower and with activation energy distinctly smaller than the volume crystallization. Hydrogenation of the investigated metallic glass enhances its thermal stability by increasing the activation energies for both the surface and volume crystallization. The results of DTA measurements indicate that hydrogenation causes an increase in the enthalpy of both stages of volume crystallization.