Thermally Stimulated Exoelectron Emission Research Articles

Thermally stimulated exo-electron emission and desorption from Lu2O3:Eu3+ surfaces

Thermally stimulated exo-electron emission (TSEE) and thermally stimulated desorption (TSD) methods were employed to characterize interactions in the surface of thin films of Lu2O3 doped by Eu3+ fabricated by pulsed laser deposition. Samples with 1 and 3% doped europium were selected. Pulsed laser annealing (PLA) was performed to modify surfaces. TSEE maxima are shown at temperature ranges T ∼120, 270 °C and T ∼250, 350 °C for 1 and 3% europium content, respectively. The TSEE maxima correspond with TSD maxima of LuO and EuO observed at higher temperatures. At lower temperatures the release of atomic oxygen Oat and OH radicals is taking place at the points, edges, and other imperfections of the crystals. The presence of Eu3+ contributes to the incipient material decomposition in dependence on Eu concentration. Desorption of individual atoms and radicals (Oat, OH, Lu, Eu, LuO, and EuO) during decomposition into components reveals a coincidence with the emission of negative ions. This can be explained by the fact that electrons are simultaneously emitted when particles are released. Activation energy and frequency factors were calculated to precise description of the surface processes.

Explosive desorption induced by radical–radical interaction in methane-doped Ar matrices

Delayed explosive desorption of particles from Ar matrices highly doped with CH4 stimulated by an electron irradiation was studied using emission spectroscopy methods. The recorded cathodoluminescence (CL) spectra of CH4-containig Ar matrices revealed following products of radiation-induced CH4 transformation: H, CH and C. Three series of experiments were performed with different irradiation and heating modes: (i) continuous irradiation at low temperature with accumulation of radiolysis products, (ii) heating of the pre-irradiated films with measurements of relaxation emissions, specifically thermally stimulated exoelectron emission (TSEE) and (iii) external heating of the pre-irradiated films under beam. These measurments of the, so called, nonstationary luminescenve (NsL) at the selected wavelenghs and nonstationary desorption (NsD) in combination with TSEE recorded provided an information on reactions of charged and neutral species of interest. Taking into account that the CH radical can be considered as a signature of the CH3 species [27] we obtained the information on a role of H atoms and CH3 radicals in stimulation of explosive delayed desorption of particles from Ar matrices doped with CH4. Two bursts of long-period self-oscillations together with short-period ones with a limited number of periods were observeed during stationary irradiation of all CH4-doped Ar matrices at low temperatures. Processes leading to the self-oscillations of particle yield are discussed.

Quartz defect pair model for exo-electron emission

We present a defect pair model analysis of the physical processes that underlie both thermally stimulated exo-electron emission (TSEE) and optically stimulated exo-electron emission (OSEE) from quartz. We commence our study by examining the sources of electrons within the defect pair model. We then proceed to review the relevant literature on exo-electron emission and note that the electron affinity is the exo-electron parameter best suited for application within our study. Following a brief description of the early literature on quartz TSEE we examine the more recent TSEE and OSEE experimental studies in relation to the underlying electron-generating defect reactions, revealing that in both cases the observed temperature dependencies associated with exo-electron emission can be curve fitted by readily derived parametric expressions. In particular, we find that in each case the electron affinity appears within an appropriate Arrhenius factor. We also find from our analytical curve fitting procedures that TSEE and OSEE are further proportional to the temperature dependencies associated respectively with TL and OSL.

Surface processes on thin layers of black aluminum in ultra-high vacuum

Thin films of black aluminum (B–Al) were prepared by pulsed DC magnetron sputtering in a mixture of argon + nitrogen (Ar + N2) gases with concentration of nitrogen 6%. At this concentration the process of anti-reflective surface (UV to near IR diffuse reflectance below 4%) formation is favored. The properties of such layers have been investigated using methods allowing to follow near-surface processes by means of thermally stimulated desorption (TSD) and thermally stimulated exo-electron emission (TSEE) in ultra-high vacuum conditions (p < 2.10−7 Pa). It is shown that at certain temperatures some adsorbed particles are desorbed and at the same time negative charges are emitted. In some cases, the emission sites are common. Comparison of TSEE measurements at three different excitations (α, proton irradiation and UV illumination) was performed. Common measurements together with calculations of the activation energies of desorption as well as electron emission sufficiently describe the centers responsible for these processes. In particular, the atomic oxygen Oat, the AlOH radical and partly the atomic nitrogen Nat together with the AlN radical are shown to play a significant role in this surface process.

Surface processes on lutetium oxide thin films doped with europium at different concentrations

The surface properties of Lu2O3 thin films doped with different concentrations of Eu3+ prepared by PLD technique were investigated. Thermally stimulated desorption (TSD) and thermally stimulated exoelectron emission (TSEE) methods were used to investigate the surface properties of this material. A comparison was also made between the as prepared material and the same material processed by pulsed laser annealing. The most pronounced TSEE maximum is shown at T ∼120 °C and also at T ∼270 °C. This is due to the release of atomic Oat at the edges, apexes and other imperfections of the crystallites for the lower temperature. At the higher temperature the decomposition of the material itself already starting - especially at the locations where Eu3+ is present. These TSD maxima correspond to the maxima of Eu and EuO in particular. In order to describe the processes of desorption, emission of electrons and negative ions as accurately as possible, the measured curves were decomposed using an inhouse software developed by our group.

Influence of MeV Gamma Photons on Thermally Stimulated Exoelectron Emission from MgO Films

The effect of 6 MeV gamma photons on thermally stimulated exoelectron emission (TSEE) spectra of MgO films was studied. The films were fabricated on Si/SiO2 substrates using the extraction-pyrolytic method. The crystalline structure and surface morphology of the films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). TSEE spectra of MgO films had emission peaks at about 450 oC and 525 oC. The area under the TSEE peaks increased after repeated TSEE measurements. In the case of gamma-irradiated films, the percentage increase in the area depended on the radiation dose, decreasing linearly with an increase in the radiation dose from 0 to 80 Gy. The results suggest that gamma radiation reduced the density of trapped electrons present in the as-grown MgO films or created competing hole traps that inhibited TSEE from the films.

Bulk and surface processes in KBr single crystals examined by thermostimulated luminescence and exo-electron emission

This article belongs to a three-part series dedicated to the simultaneous measurements of thermally stimulated exo-electron emission (TSEE), desorption (TSD) and luminescence (TSL). These works aim is to verify the ability of the applied experimental setup to characterize and analyze at the same time all the three above-mentioned stimulated processes. In agreement with the two preceding publications that deal TSD and TSEE, this article proves the consistency of the results of the individual modes of joint measurements of TSL and TSEE (as well as bulk and surface, the corresponding sources of emission, the recombination processes and their interactions). The whole experimental apparatus was tested using a well-known material, high-purity monocrystalline KBr, in the temperature range ΔT = −196 °C–430 °C (ΔT = 77 K–703 K) and in the chamber with base pressure p < 1 × 10−6 Pa to obtain a better comparison of results. Our in-house developed software for decomposing of the measured curves into the individual components and the subsequent calculation of the corresponding activation energies and frequency factors clearly confirms these results.

Surface processes on KBr single crystals examined by thermostimulated exo-electron emission and desorption

Potassium bromide (KBr) is a material with many potential uses in various branches of physics and technology (in scintillators, for a Fourier spectrometer, electro-optical elements such as IR laser windows, sensors and optical fibers). We conducted a series of combined measurements of thermally stimulated exo-electron emission (TSEE) and thermally stimulated desorption (TSD). At low doses of UV irradiation, the most prominent process is the combined emission of negative charges with desorption of the components of O, H2O and mainly K, KOH and Br, as described below. At higher irradiation doses, the KBr F-center plays the most important role (Tm = 160 °C) and at temperatures above +200 °C also the decomposition of chemically bonded atoms and other components occurs. This process is accompanied by the corresponding emission of electrons and at the same time some of the already chemically bonded structures such as HBr3 are released. TSD maxima are defined and the same temperature maxima for TSD and TSEE are shown, mainly at the already mentioned higher temperatures. These maxima are linked to the defects and interactions in the surface layer. The obtained results fully confirm the applicability of our apparatus for combined measurements of TSD, TSEE, thermally stimulated luminescence (TSL) and possibly also thermally stimulated conductivity (TSC).

Simultaneous measurements of thermostimulated exo-electron emission, luminescence, and desorption from a KBr single crystal

Abstract The results of simultaneous measurements of three thermally stimulated phenomena (TSP) are presented as functions of temperature: thermally stimulated exo-electron emission (TSEE), thermally stimulated luminescence (TSL) and thermally stimulated desorption (TSD). These phenomena can be described by similar mathematical relationships. The measurements have been carried out in the temperature range ΔT = −196 to 430 °C (ΔT = 77–700 K) under conditions of ultrahigh vacuum (p

Nonstationary processes in matrix-isolated methane probed by optical and current emission spectroscopy

Nonstationary processes in the Ar matrix doped with CH4 were studied with a focus on the behavior of radiolysis products – H atoms and CH radicals. Samples of 25 μm thickness were irradiated with 1.5 keV electron beam. It has been shown that because of the small penetration depth of electrons the bulk of the matrix is excited preferentially by photons with an energy of 9.8 eV, the most intense emission band of the matrix (excitons self-trapped into the configuration of Ar2∗), In other words “internal photolysis” occurs. The spectrally resolved emission of H atoms and CH radicals, along with the total yield of desorbing particles, were monitored in a correlated manner under the concurrent action of photons and external heating. These dependences, called nonstationary luminescence (NsL) and nonstationary desorption (NsD), were compared with the measured yield of thermally stimulated exoelectron emission (TSEE), that enabled us to reveal the appearance of the CH radical in the neutralization reaction CH3+ + e− → CH3∗ followed by dissociation of CH3∗. Thus, the CH radical was assumed to be a signature of the CH3∗ species. The antibate behavior of the CH NsL and the NsD in the temperature range 37–43 K, where diffusion in the Ar matrix occurs, was interpreted as a result of the recombination of CH3 radicals with the formation of C2H6 and the energy release spent for desorption. Self-oscillations (with τ of about 10 s) of increasing amplitude were detected, indicating self-oscillating processes in the bulk of the Ar matrix doped with 5% CH4.

Explosive desorption from solid methane and methane-containing argon matrices

The total yield of emitted particles from solid CH4 and Ar matrices doped with CH4 was studied under irradiation with electrons. The explosive desorption was detected for these systems. Strong bursts of particles were delayed with respect to the start of irradiation indicating the need for accumulation of radiolysis products. The study was performed using optical and current spectroscopy – cathodoluminescence (CL) together with thermally stimulated exoelectron emission (TSEE) and thermally stimulated post-desorption (TSpD). In the CL spectra, the products of radiation-induced transformation of CH4 molecules were recorded: H, CH, C. Dose dependencies of the optical emissions were measured in a correlated manner with the total yield of particles. It was found that the particles burst correlates with an increase in the concentration of H atoms. Scenario suggested for the delayed particle emission incorporates the recombination of CH3 radicals appearing simultaneously with H atoms in the reaction CH4 → CH3 + H.

Effects induced by electron beam in methane ices

The radiation effects and relaxation processes in solid CH4 irradiated by an electron beam were studied with a focus on the beam-induced particle emission. The study was performed using cathodoluminescence (CL), monitoring of the total yield of particle emission and applying activation spectroscopy technique – thermally stimulated exoelectron emission (TSEE). Emissions of photons and particles under beam were monitored in a correlated style. Upon irradiation by 1 keV electron beam of 3 mA cm−2 current density we observed for the first time explosive particle emission which occurred with the delay of an hour with respect to the start of irradiation. Simultaneously with the particles outburst a CL flash was detected. Delay in the giant outburst of particles indicates the need for the accumulation of radiolysis products for this phenomenon. The critical dose for this effect is 100 eV per methane molecule. The outburst of particles is preceded by oscillations of particles emission with increasing amplitude. The period of these oscillations depends on the intensity of the irradiation, it decreases with an increase in the beam current density. Possible mechanisms of low-temperature explosive particle emission are discussed.

Relaxation processes in solid methane pre-irradiated with an electron beam

Abstract The relaxation processes in CH4 solids irradiated by an electron beam were studied with a focus on post-irradiation phenomena. Because of absence of thermally stimulated luminescence (TSL) from pure solid methane we applied current activation spectroscopy method – thermally stimulated exoelectron emission (TSEE). Two broad features were detected near 18 and 43 K. Their relative intensity appeared to depend on irradiation time. The experiments performed with a variable voltage applied to an electrode for current detection first revealed significant accumulation of uncompensated negative charge in pre-irradiated methane films. The TSEE current was detected at negative voltages up to −35 V. Concurrently with thermally stimulated yield of electrons the total yield of thermally stimulated post-desorption (TSpD) from pre-irradiated solid methane film was detected. Correlation of TSEE and TSpD yields is discussed and compared with the temperature behavior of solid methane subjected to a neutron flux.

Evaluation of the thermally and optically stimulated response of an Italian Obsidian irradiated in 60Co beams

From Obsidian silicate, in its natural form, pellets were manufactured adding Teflon to the material in powder. These pellets were exposed to the gamma radiation beams of a 60Co source in a dose interval from 10 Gy to 10 kGy. The Obsidian in powder was investigated in relation to its physical and chemical characteristics, by means of the X-ray diffraction, scanning electronic microscopy and energy-dispersive X-ray spectroscopy techniques. After the irradiation procedure, the signal of the samples was evaluated by means of three techniques: thermoluminescence (TL), optically stimulated luminescence (OSL) and thermally stimulated exoelectron emission (TSEE). The objective of this work was to verify the TL, OSL and TSEE response of the Obsidian+Teflon pellets after exposure to high doses of gamma radiation. Different response characterization tests were performed, as TL and TSEE emission curves, OSL signal decay, reproducibility, dose-response curves, lower detection limits and fading. From these results, it can be observed that the Obsidian+Teflon pellets present good conditions to be used as high dose gamma radiation detectors; the results obtained with the OSL technique were the most adequate in terms of response in function of absorbed dose. The fading of the signals, varying from about 26% (TSEE) to 44% (OSL), in a time interval of 360 h post-irradiation, also showed favorable results for the use of this material as radiation detector, when the measurements are taken after a defined time interval.

Applying the TSEE technique to Spectrolite and Opal pellets irradiated with high doses of gamma radiation

Spectrolite + Teflon and Opal + Teflon pellets were studied in this work in relation to their dosimetric properties, using the thermally stimulated exoelectron emission (TSEE) phenomenon. The purpose of this work was to study these materials to be used in high-dose dosimetry of 60Co irradiators, which are employed for several industrial applications. The basic physical principle of this technique is the emission of low energy electrons from the surface of different crystals. For this reason, it is very employed in work with specially radiation sources of low penetrating power, as alpha and beta radiation, but also with gamma sources. Both materials had already their dosimetric chatacteristics verified in previous works after exposure to high doses of a 60Co source and measurements by means of thermoluminescence (TL) and optically stimulated luminescence (OSL) techniques. The TSEE response was investigated in terms of the following tests: TSEE emission curves, reproducibility, minimum detectable doses and dose-response curves.

Anomalous low-temperature “post-desorption” from solid nitrogen

Anomalous low-temperature post-desorption (ALTpD) from the surface of nominally pure solid nitrogen preliminary irradiated by an electron beam was detected for the first time. The study was performed using a combination of activation spectroscopy methods—thermally stimulated exoelectron emission (TSEE) and spectrally resolved thermally stimulated luminescence (TSL)—with detection of the ALTpD yield. Charge recombination reactions are considered to be the stimulating factor for the desorption from pre-irradiated α-phase solid nitrogen.

Charging effect and relaxation processes in electron bombarded cryogenic solids

Charging effect and relaxation processes in cryogenic solids pre-irradiated by an electron beam are discussed with the example of solid Xe. The experiments were performed employing combination of the cathodoluminescence method with activation spectroscopy techniques: thermally stimulated luminescence (TSL) and thermally stimulated exoelectron emission (TSEE). Relaxation emissions of photons and electrons were detected in the time correlated manner. A long-lasting “afteremission” of electrons and afterglow of VUV photons were observed on completing irradiation. The experiments performed with a variable voltage applied to the Faraday plate enabled us to demonstrate appreciable accumulation of electrons and ascertain the spatial distribution of the charge centers. The uncompensated negative charge is found to be localized preferentially near the surface. Xe layers close to the sample-substrate interface are shown to be positively charged. The part played by pre-existing and radiation-induced defects as well as dopants is considered and the temperature range of the electron traps’ stability is elucidated.

Relaxation of charged and neutral particles in doped atomic solids: TSL, OSL, TSEE, OSEE and their interconnection

Relaxation processes in nitrogen-containing Ar solids pre-irradiated by an electron beam are studied with the focus on the interconnection of relaxation channels in system of charged and neutral species. The results of correlated in real time measurements of thermally stimulated phenomena are presented. The experiments have been performed using activation spectroscopy methods – thermally stimulated exoelectron emission (TSEE), spectrally resolved thermally stimulated luminescence (TSL) measured in visible, NUV and VUV ranges. Solid evidence of the charge relaxation cascades triggering via radiative recombination of neutral species is presented.

Radiation effects in atomic cryogenic solids

Radiation effects and relaxation processes in atomic solids are discussed with the example of solid Xe preliminarily irradiated by an electron beam. The study was performed employing concurrently the combination of current and optical “activation spectroscopy” methods. Three relaxation processes were monitored simultaneously upon controlled warming-up of pre-irradiated solids: thermally stimulated exoelectron emission (TSEE), thermally stimulated luminescence (TSL) in the VUV range and the total desorption yield by pressure measuring above the sample. Anomalous strong low-temperature “post-desorption” (ALTpD) of own atoms from pre-irradiated Xe solids was observed for the first time. The data obtained demonstrated a clear correlation between the yields of exoelectrons, photons of recombination luminescence and neutral particles pointing to the common origin of the phenomena. It was shown that the key primary process of the relaxation cascade including the ALTpD is a thermally stimulated electron detrapping, promoting electrons into the conduction band. Subsequent branching of the relaxation paths results in the relaxation emissions observed. An accumulation of charges of both signs as well as excess electrons under exposure to an electron beam was found.

Formation of (Xe2H)* centers in solid Xe via recombination: nonstationary luminescence and internal electron emission

The formation of excimers (Xe2H)* in solid Xe doped with molecular hydrogen under irradiation by an electron beam is studied using the original two-stage technique of nonstationary (NS) cathodoluminescence (CL) in combination with current activation spectroscopy—thermally stimulated exoelectron emission (TSEE). Charged species are generated using a high-density electron beam. The species produced are probed with a low-density beam with sample gradually heated. The near-UV emission of the (Xe2H)* is used to monitor the neutralization process. It is found that the temperature behavior of the NS CL band of (Xe2H)* clearly correlates with the yield of TSEE measured after identical pre-irradiation of the sample. The fingerprints of the thermally stimulated detrapping of electrons—«internal electron emission»—-in the spectrum of NS CL point to the essential role of the neutralization reaction in the stability of protons solvated by rare-gas atoms.