Function Of Incident Electron Energy Research Articles

High-resolution electron impact excitation functions of metastable states of neon, argon, krypton and xenon

Electron impact excitation of the lowest-lying metastable states of neon, argon, krypton and xenon have been studied as a function of incident electron energy over the range from threshold to the 2P1/2 ionisation potential and at an energy resolution of 20 meV or better. A wealth of structure is observed. Several of the features are very narrow and are coincident in energy with excited states having the classification np5n'p(J=0), while several other features correspond to pairs of resonances that have almost the same energy splitting as that of the corresponding grandparent ion doublet. The interpretation of the structures and classification of the resonances are discussed.

Experimental test of virtual photon theory

An isochromat of the E1 virtual photon spectrum has been measured by counting the number of ground-state protons emitted by the 16.28 MeV isobaric analog state in /sup 90/Zr as a function of incident electron energy in the range 17--105 MeV. The experimental results reproduce well the distorted wave Born approximation spectra for a point Zr nucleus for electron energies up to 30 MeV. A radiator was used for electron energies of 60--100 MeV to measure the photodisintegration plus electrodisintegration cross section. These results showed that the Davies-Bethe-Maximon bremsstrahlung cross section magnitude yields the same result as the electrodisintegration results below 30 MeV where size corrections for the finite extent of the nucleus are minimal. As E/sub 0/ increases to 105 MeV the need for such corrections becomes manifest. Several such corrections are discussed. As by-products of this study the quantity GAMMA/sub ..gamma../GAMMA0/GAMMA was determined to be 63.8 +- 1.9 eV or 66.1 +- 2.0 eV depending on the virtual photon spectrum used in analysis. The GAMMA/sub p//sub 2//GAMMA0 ratio was also determined to be 0.58 +- 0.02. Combining these results yields for GAMMA/sub ..gamma../ 100.8 +- 5.0 eV or 104.4 +- 5.2 eV.

Variation of the threshold energies for core‐electron excitation in electron energy‐loss spectra as a function of incident electron energy

Measurements have been made of the 3p, 3s, and 2p core‐electron energy‐loss spectra of Ti and Ni as a function of incident electron energy. Attention was focused on the threshold energy for core‐level ionization to investigate possible changes in the threshold energies as a function of excitation conditions. Measured threshold energies decreased by between 0.1 and 1.5 eV, depending on the specific core level, as the incident electron energy was lowered from about 1500 to about 20–50 eV above the core threshold. The measured changes in threshold energies are due in part to different core‐electron binding energies for bulk and surface atoms, to varying populations of final states, and to dynamical screening effects. These factors can account for differences in binding energies measured by x‐ray photoelectron spectroscopy and by appearance potential spectroscopy.

Electron stimulated desorption of H − and H + from hydrogen covered tungsten

Electron stimulated desorption from a hydrogen covered polycrystalline tungsten surface has been investigated. The H − and H + desorption current as a function of incident electron energy was measured in both the low and high energy region. Some threshold-like structures appear in the curve of the desorption current versus incident electron energy; however, no threshold structures have been observed for H − desorption in the high energy region. The experimental results obtained and the desorption process are discussed in some detail, including initial excitation, charge transfer and ultimate desorption.

Electron stimulated desorption of excited alkali atoms from alkali halide surfaces

We have measured the alkali resonance photon intensity due to electron stimulated desorption of excited particles from NaCl and KCl targets as a function of incident electron energy (20 to 1000 eV) and sample temperature (room temperature to 300 °C). The energy dependence data suggest a correlation between excited Na and K desorption thresholds and substrate core levels. These are the first experimental correlations of the desorption of neutral atoms, specifically excited neutral atoms, with the creation of core holes. The intensity of emitted radiation is observed to rise sharply and then saturate with increasing temperature.

Studies of the desorption mechanism of O − by electron impact from oxygen covered metal surfaces

Electron-stimulated desorption of negative ions from oxygen covered tungsten and molybdenum has been investigated. The O − desorption current as a function of incident electron energy was measured. The following results were observed: (1) the threshold of negative ion desorption is much lower than that of positive ions (20–25 eV for O +; 4–5 eV for O −); (2) some resonant-like structures appear in the curves of desorption current as a function of electron energy in the low energy regions; these structures do not appear in the high energy regions; (3) these resonant-like structures are basically the same for O 2-W and O 2-Mo systems. Experimental results seem to indicate that the desorption of negative ions are essentially a feature of the absorbate and can be most readily explained based on potential energy curves for some electronic states of oxygen molecules and ions, and within the framework of the Franck-Condon principle.

Dose enhancement from backscattered radiation at tissue-metal interfaces irradiated with high energy electrons.

Irradiation of a tissue-metal interface with 13 MeV to 20 MeV electrons results in an increased dose to the tissue on the entrance side of the metal. Ionization measurements were made with a thin-window parallel-plate chamber to determine the magnitude of the dose enhancement as a function of incident electron energy, thickness and atomic number of metals introduced into the electron beam. The presence of a metal resulted in a dose ranging from 6% to approximately 50% greater than that measured with no metal in the beam. Most of this increase in dose may be eliminated by the addition of 1-2 g/cm2 of low Z material between tissue and metal.

Decay modes for highly excited states of He-

The yield of ejected electrons from the decay of helium autoionising states excited by low-energy electron impact has been analysed as a function of incident electron energy in the region 61-65 eV. Small structures are visible in these spectra and it is shown that at least some of these are caused by the decay of high-lying negative-ion states into the autoionising-state channel, and thence into the ionisation continuum.

Electropion production inHe3determined by measuring the triton recoil cross section

The $^{3}\mathrm{He}(e, t){\ensuremath{\pi}}^{+}$, ${e}^{\ensuremath{'}}$ cross section has been measured as a function of incident electron energy, triton recoil energy, and angle. The experiment corresponds to excitation energies near 20 MeV above pion threshold. Comparison is made with a theory including all partial waves using Gaussian and Irving-Gunn ground states for $^{3}\mathrm{He}$. The forward peaking approximation is also discussed.NUCLEAR REACTIONS $^{3}\mathrm{He}(e, t){\ensuremath{\pi}}^{+}$, ${e}^{\ensuremath{'}}$, measured $\ensuremath{\sigma}({E}_{t}, {\ensuremath{\theta}}_{t}, {E}_{0})$ compared with electropion production theory.

Internuclear dependence of the polarizability ofN2

A study is made of the internuclear dependence of the polarizability of N2. Specifically, the dependencies of parallel and perpendicular polarizabilities on internuclear separation is found for the N2 molecule in its ground state. Results are presented for vibrationally elastic cross sections as functions of incident electron energy in an eight-state calculation and for the excitation of the first vibrational state.

Elastic and inelastic contributions to the auger electron appearance potential spectrum of titanium

The energy distribution of electrons contributing to the L-shell Auger electron appearance potential spectrum of a polycrystalline titanium surface has been measured. The Auger electron appearance potential spectrum is obtained by differentiating the total secondary electron yield of an electron bombarded sample as a function of incident electron energy. At the threshold for scattering from a core level the secondary yield increases. Most of the electrons contributing to this increase have energies below 30 eV, and result from secondary processes following Auger recombination of the core hole. The elastic yield decreases at the threshold, however, due to opening a new channel for inelastic scattering. A comparison of the elastic yield spectrum (DAPS), the total yield spectrum (AEAPS) and the soft X-ray yield spectrum (SXAPS), shows very similar line shapes, but differences in the relative strengths of the lines.

Electron-impact excitation of helium: Cross sections,n, andldistributions of high Rydberg states

Helium atoms in highly excited Rydberg states are produced by electron impact and are detected by electric field ionization. The principal quantum numbers $n$ are determined to lie in the range $20\ensuremath{\lesssim}n\ensuremath{\lesssim}80$ by ionizing the excited He atoms in a uniform, swept electric field. The absolute excitation cross section for 100-eV incident electrons is found to be $(9\ifmmode\pm\else\textpm\fi{}5)\ifmmode\times\else\texttimes\fi{}\frac{{10}^{\ensuremath{-}17}}{{n}^{3}}$ ${\mathrm{cm}}^{2}$. This expression predicts a value for $n=4$ that agrees with the optically measured $n=4$ cross section. The 108-\ensuremath{\mu}sec time of flight requires that the high Rydberg He atoms have lifetimes much longer than those calculated for low-angular-momentum ($l\ensuremath{\le}2$) states, those expected to be populated by electron impact. The proposed explanation is that the excited He atoms suffer collisions with electrons and/or ground-state He atoms causing longer-lived higher $l$ states to be populated. This explanation is supported by measurements of signal as a function of incident-electron energy, electron current, and He pressure. The cross section for $l$-changing collisions between electrons and high Rydberg helium atoms is given by ${\ensuremath{\sigma}}_{\ensuremath{\Delta}l}^{e}\ensuremath{\cong}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}(15\ifmmode\pm\else\textpm\fi{}0.3)}\frac{\mathrm{ln}(100E{n}^{2}){n}^{4}}{E}$ ${\mathrm{cm}}^{2}$.

Elastic-scattering resonances observed in very low energy electron reflection at oxygenated Ni(001) surfaces

Observations of elastic-scattering resonances in electron reflection at Ni(001), Ni(001) p(2 × 2) O and Ni(001) c(2 × 2) O surfaces are reported. The total reflection coefficient R (secondary yield) was determined from the net current to the crystal as a function of incident electron energy E in the range 0.5 < E < 25 eV. Weak features in the R(E) plot were accentuated by differentiating the signal. Resonance features were identified by their correlation in energy with beam thresholds under varying angles of incidence. Resonances associated with every beam threshold in the energy range of the experiment were observed. For incidence angles θ ≳ 3° from normal incidence, each resonance was observed to have a nearly constant binding energy relative to the corresponding beam threshold. Splitting of resonances of like symmetry was observed for smaller θ. Observed binding energies and splittings are reported. They are of the order of 1 eV in all cases. A phenomenological description which accounts qualitatively for all observations is offered.

Spectra of Bremsstrahlung Produced in Very Thick Lead Targets by 15-, 20-, and 25-MeV Electrons

The spectra of bremsstrahlung from very thick lead targets bombarded by 15-, 20-, and 25-MeV electrons from a linear accelerator were measured with activation detectors. This activation method has the advantage of being available for measuring the spectrum of a bremsstrahlung burst and being suitable for determining the distribution of the photon energy spectrum in the medium. By use of the most accurate photonuclear cross section data available, the bremsstrahlung spectra were evaluated with the LYRA unfolding code. The evaluated spectra were in good agreement with the calculated spectra, especially at 9 deg for 15- and 20-MeV electrons. It is concluded that the spectrometry of bremsstrahlung radiation by activation detectors can satisfactorily represent the variation of bremsstrahlung spectrum as a function of incident electron energy. The spectra obtained by this method are very much dependent on the photonuclear cross section data of activation detectors. This activation experiment can be used as an integral experiment to evaluate photonuclear cross section data by coupling with the reference calculations. 4 tables, 7 figures. (auth)

Total cross section results for deuterium electrodisintegration

Theoretical total cross sections for deuterium electrodisintegration are presented as a function of incident electron energy. The cross section has been calculated using virtual photon theory with Partovi's photodisintegration calculation for ${E}_{x}&lt;10$ MeV and effective range theory for ${E}_{x}&gt;10$ MeV. Experimental electrodisintegration cross sections integrated over limited ranges of energies have been deduced from some previously reported measurements. Satisfactory agreement is found between experiment and theory. These results are of interest not only for checking virtual photon theory but also in estimating neutron fluxes arising from the $^{2}\mathrm{H}(e,n)$ reaction in Tokamak reactors.NUCLEAR REACTIONS $^{2}\mathrm{H}(e,p)n{e}^{\ensuremath{'}}$; calculated $\ensuremath{\sigma}$.

Mean-square displacements of (100) surface atoms in vanadium determined by low-energy electron diffraction

The effective-surface Debye temperature for the (100) face of single- crystal vanadium has been measured as a function of incident electron energy in the energy range 0 to 400 eV using low-energy-electron diffraction. The ratio of surface-to-bulk atomic mean-square displacements has been estimated from the experimental data and has been investigated theoretically. A lattice-dynamical model consisting of central-potential interactions up to fourth neighbors together with harmonic angle-bending interactions is employed. The calculations have been performed using a set of special wave-vector points in the two- dimensional Brillouin zone which are the most efficient in finding the average of a periodic function over the Brillouin zone. Results have been obtained for both unchanged and changed force constants for the surface atoms. A comparison between the experimental and theoretical results is included. (auth)

Cross sections and threshold effects for electron-impact excitation of the(2s2)S1and(2s2p)P3states of helium

The energy and intensity of scattered electrons resulting from excitation of the (2s$sup 2$) $sup 1$S and (2s2p) $sup 3$P autoionizing states of helium have been measured as a function of incident electron energy. For incident energies only slightly higher than the threshold for (2s$sup 2$) $sup 1$S excitation, post-collision interactions between the scattered and ejected electrons cause the slowly moving scattered electron to lose an amount of energy proportional to E$sup -1$.$sup 25$/sub ex/ (where E/sub ex/ is the energy the scattered electron would have had in the absence of a post-collision interaction). This energy dependence compares well with an energy gain of the ejected electron proportional to E$sup -1$.$sup 2$/sub ex/, obtained by previous workers. Measurements of the intensity of scattered electrons show that both $sup 1$S and $sup 3$P total cross sections peak at threshold and have magnitudes about (2.0 and 4.0) x 10$sup -20$ cm$sup 2$, respectively, then rise to broad maxima of magnitudes about (2.5 and 5) x 10$sup -20$ cm$sup 2$ about 4.0 eV above their thresholds. A subsidiary peak at 59.0 eV in the $sup 3$P cross section is attributed to decay of the previously unidentified (2s2p$sup 2$) $sup 2$S He$sup -$ statemore » into the $sup 3$P channel. The magnitude, energy, and width of this resonant structure in the $sup 3$P channel agrees with theoretical predictions. (AIP)« less

Theory of current characteristics of a thin dielectric film under slow electron impact

A relation between the current transmitted by a thin dielectric film as a function of incident electron energy and the electronic energy structure of the film is derived. These current characteristics have been measured by W. H. Hamill et al. For low energies, 1–4 eV, it is found that the current characteristics are insensitive to the detailed energy structure of the solid. In the moderately high energy range, 4–10 eV, the transmitted current is more sensitive to the energy levels of the solid, but the relation is not straightforward, involving a convolution of a joint density of states and a conduction band density of states. Analysis of the low energy region is shown to give values of parameters, such as energy resolution and energy zero, relevant to a detailed quantitative study of the data.

Dissociative ionization by electron impact. I. Protons from H2

A new apparatus designed to investigate various aspects of the electron impact dissociative ionization process is described in detail. It is shown that the angular distributions obtained for protons from the 2 Sigma u+ state of H2+ are consistent with theoretical arguments and confirm the results of previous measurements. The first angular distribution of protons from the 2 Sigma g+ state of H2+ is also reported. Absolute cross sections for the production of protons from both the 2 Sigma g+ and 2 Sigma u+ state of H2+ at electron energies close to the respective thresholds are also presented. The ratio H+/H2+ is measured as a function of incident electron energy for ions originating from the 2 Sigma g+ state.

Defect production in thorium by low-temperature electron irradiation

Abstract The increase in the electrical resistivity of 99.98% pure thorium per unit electron flux was measured as a function of incident electron energy from 1.6 to 2.8 MeV. During irradiation, the temperature of foil samples was held below the temperature of the fist major recovery peak at 10°K. For unit displacement probability, the theoretical displacement cross-section best reproduces the experimental damage rates at an effective threshold displacement energy of 35 eV and a Frenkel resistivity of 15 × 10−4 ohm-cm per unit fractional concentration of Frenkel pairs.