High Energy Electron Impact Research Articles

Accurate Compton profiles for H2 and D2 including the effects of electron correlation and molecular vibration and rotation

Accurate isotropic Compton profiles J (q,R) for H2(X 1Σ+g) are calculated for 22 internuclear separations from the 126 configuration wavefunctions of Liu. These profiles, supplemented by the united atom and separated atom profiles, are then rigorously averaged over vibration–rotation wavefunctions computed by direct numerical solution of the radial Schrödinger equation using nearly exact potential energy curves including adiabatic and relativistic corrections. These averages are performed for a large number of vibration–rotation states of H2 and the ground vibration–rotation state of D2. It is shown that the effects of averaging Compton profiles over vibration–rotation states are significant and cannot be neglected. The isotope effect is shown to be smaller than the vibration–rotation effect. The peak of the calculated H2 Compton profile for the ground vibration–rotation state is found to be in excellent agreement with the very recent high energy electron impact measurements of Lee. A number of expansion techniques for vibration–rotation averaging, including a new, very simple, and reasonably accurate delta approximation, are presented.

Ejected-electron spectrum of Mg I and Mg II autoionising levels between 20 and 53 eV excited by low-energy electron impact on magnesium vapour

The ejected-electron spectrum of magnesium vapour has been observed at 75 degrees with respect to an incident electron beam of 62 to 400 eV incident energy. Ejected electrons corresponding to both Mg I autoionising levels and Auger transitions are reported. Comparison is made with photoabsorption data, ejected-electron spectra from ion impact and high-energy electron impact experiments and available theoretical calculations.

Dominant Auger Process in Electron-Impact Ionization of Mo Ions

Total ionization cross sections for the highly stripped Mo ions by high-energy electron impact are estimated, including both the direct ionization of outer-shell electrons and the excitation of inner-shell electrons followed by an Auger emission. For highly stripped ions, the excitation to discrete states predominates and enhances the Auger contribution. For the ${\mathrm{Mo}}^{24+}$ ions, the Auger ionization of the $L$-shell electrons dominates over the direct $M$-shell ionization.

High energy electron impact spectroscopy measurement on the Compton defect

Abstract Recent Compton scattering experiments with a 35 keV electron beam on helium, molecular hydrogen, and molecular deuterium, have shown that the energy loss spectra do not reduce to Compton profiles by means of the Binary Encounter theory. The observed Compton peak is shifted to smaller energy losses than those associated with a stationary unbound electron. This shift has been studied for momentum transfer from 1·5 to 12 a.u. for both He, and H2, and it is suggested that even at large momentum transfers electron correlations could be important.

Accurate determination of H2, He, and D2 Compton profiles by high energy electron impact spectroscopy

The effective nonrelativistic Compton profiles J (q,K) of H2 and He {defined in terms of the absolute generalized oscillator strength (GOS), df (K,E)/dE, as J (q,K) = (2K3/E)[df (K,E)/dE], where q is (E−K2)/2K, with K the momentum transfer and E the energy loss} have been studied as a function of scattering angle by means of high energy electron impact spectroscopy (HEEIS) with an incident energy of 25 keV over an angular range of 3°–10°. It was found that the binary encounter limit is not reached until the x-ray incoherent scattering factor reaches its large K asymptotic limit N, the number of target electrons. At 10° , the results of the Compton profiles of He and H2 were fitted by least squares to a polynomial of the form Σmn=1{an/[1+(q/ζn)2]n+2}, which leads to analytic expressions for the one electron momentum density and its integer powered moments. The fits proved to be radically different for He and H2. The experimental values for the moments of the momentum density function were found to be in excellent agreement with the best available theoretical results. In addition, the Compton profile of D2 at 8° was measured and no significant deviations from the Compton profile for H2 were observed. The new results reported here for H2 clear up a disagreement between theory and experiment and suggest that quoted uncertainties in the previous studies were unrealistically small owing to improper analysis of systematic experimental errors.

Determination of methane compton profiles by high energy electron impact spectroscopy

The Compton profile of methane is measured at 4.28 and 10 degrees using high energy electron impact spectroscopy with the 25 keV electrons. The 10 degree profile is found to agree with theoretical calculations through q = 5 au. Both experimental Compton profiles and the best available theoretical profile were fitted by least squares to a polynomial of form Σ M n=1 a n [1 + ( q/ξ n ) 2] − n−2 from which the electron momentum density and its integer powered moments were calculated. Agreement was found between the lower order moments, but not for the higher moments. The 4.28 degree experiments were used to define a valence shell Compton profile. Excellent agreement was found with the best theoretical results. Electron momentum density moments were also calculated for this data.

Measurement of the Compton profiles of CH 4 and C 2H 4 by high energy electron impact spectroscopy

We present a new experimental determination of the Compton profiles of CH 4 and C 2H 4 molecules using high energy electron impact. The observable q range has been extended up to 10 au (6 au for C 2H 4). Good agreement is found with an earlier X-ray scattering experiment and with new theoretical calculations.

Theoretical studies of excited states of H 3O and of core-excited NH 3: Comparisons with equivalent core analysis of NH 3 core-excitation spectra and implications for the radiation chemistry of aqueous systems

The H 3O radical has been studied within the ab initio LCAO SCF MO model. A flexible basis set including diffuse basis functions at both O and H has been used in order to represent the excited states adequately. Calculated excitation energies are 1.87, 2.87–3.16, and 3.36–3.47 eV; the calculated ionization energy is 4.75 eV. These represent well the experimental values (good to ±0.3 eV) of 1.6, 2.9, and 3.5 eV for excitation, and 5.0 eV for ionization, deduced by equivalent core analysis of high energy electron impact energy loss studies of NH 3. Similar explicit calculations on the N-1s core-excited states of NH 3 have also been made to examine directly the equivalent core concept. “Excited states” (relative to the lowest bound core-excited state) at 1.72, 2.85–3.09, and 3.32 eV, and the “ionization energy” of 4.68 eV, agree well with experiment and support the equivalent core concept. The possible significance of these H 3O results in the radiation chemistry of aqueous media is discussed in view of the fact that the maximum in the absorption spectrum of the hydrated electron lies near that of H 3O.

Inner-shell excitations and ionizations of atomic ions by high-energy electron impact

Cross sections for the inner-shell ionization of argon ions by high-energy electron impact are estimated. An improved form of the single-particle model with varying degrees of ionization is used in the calculation of the excitation amplitudes. Partial sums over the allowed final states are carried out using the semiclassical projection operator. This operator depends on a spectral cutoff parameter ${e}_{b}$, and a method for its determination is examined for the case of hydrogenic transitions. The ionization cross section incorporates both the direct excitation to the continuum and the Auger emission following the inner-shell excitations. For electron energies in the KeV range and for atomic targets with the core charges ${Z}_{C}$ less than 20, the direct ionization dominates, with the excitation Auger-emission mode making a small but appreciable contribution. The ionization cross sections for the neutral neon, and oxygen targets are also estimated.

The absolute generalized oscillator strength for the Lyman–Birge–Hopfield band of N2 as determined by high energy electron impact spectroscopy

The absolute generalized oscillator strength for the dipole forbidden quadrupole allowed Lyman–Birge–Hopfield band [a 1Πg ← X 1Σ+g] in molecular nitrogen at an energy loss of 9.35 eV is observed by electron impact spectroscopy using 25 keV electrons over the momentum transfer range 0.04?K2?10 a.u. The results agree in the zero angle (zero momentum transfer) limit with the previous observations of Skerbele and Lassettre, but are in disagreement with previous theoretical and experimental results for K2≳0.5.

Observation of a new electronic transition in N2 at 31.4 eV by means of high energy electron impact spectroscopy

A new transition in molecular nitrogen at an energy loss of 31.4±0.5 eV has been observed by high energy electron impact spectroscopy at momentum transfers larger than 2 a.u. The absolute generalized oscillator strength was measured over the momentum transfer range 0.5?K2?5.08 a.u. using 25 and 33 keV electrons. The observed angular behavior of this line suggests that it may be due to a dipole forbidden quadrupole allowed process. The term symbol 1Πg with the electronic configuration (2sσg)−1 (2pπg)1 is tentatively assigned to this transition and it is suggested that this state may decay into the D 2Πg state of N+2 by Auger cascade.

Maxima and minima in the angular dependence of the [np)(n+1)s] transition in the rare gases observed by high-energy electron impact spectroscopy

Maxima and minima in the angular dependence of the [<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>n</mml:mi><mml:mi>p</mml:mi><mml:mo>)</mml:mo><mml:mo>(</mml:mo><mml:mi>n</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn><mml:mn /><mml:mo>)</mml:mo><mml:mi>s</mml:mi></mml:math>] transition in the rare gases observed by high-energy electron impact spectroscopy

Energy Dependence of theKαSatellite Intensity from High-Energy Electron-Impact Ionization

The relative x-ray yields corresponding to $\mathrm{KL}$ and $K$ ionized atoms have been measured for elements ranging from $\mathrm{V} (Z=23)$ to $\mathrm{Ni} (Z=28)$ at incident electron energies of 20 keV and 2 MeV. The intensity ratios of $K\ensuremath{\alpha}$ satellite to diagram lines at 2 MeV are found to be lower by a factor \ensuremath{\sim} 0.6 than those measured at 20 keV. These results cannot be understood solely in terms of electron shakeoff. It is suggested that the observed energy dependence is due to a competing multiple Coulomb-ionization mechanism.

Double ionization-a reply to the comments of Byron, Joachain and van der Wiel

Byron et al (J. Phys. B. Atom. Molec. Phys.; 6: 357 (1973)) threw doubt on previous work of the present author (Tweed, J. Phys. B. Atom. Molec. Phys.; 6: 256 to 69 and 270 to 85 (1973)) concerning double ionization of helium- like atoms by high energy electron impact. It is agreed that, on the basis of experimental evidence, the factor of three, previously given, must be spurious. The reasoning which leads to this factor is again considered. (UK)

Multi-Ionization of Krypton and Its Ions by High-Energy Electron Impact

Estimates of the total ionization and multi-ionization high-energy-electron collision cross sections have been obtained for krypton and its ground-state ions. The total cross sections were computed within the framework of the Bethe-Born high-energy-approximation theory using the semiclassical projection-operator technique of Hahn and Watson and the independent-particle model for atoms due to Green, Sellin, and Zachor. Both direct continuum ionization and inner-shell excitation followed by Auger ionizing transitions were taken into account. Multiple-ionization cross sections were then estimated using experimental information on electron emission following ejection of inner-shell electrons in neutral krypton and a simplified model of Auger cascading in ions. The results, applied to a calculation of the cumulative ionization produced by a relativistic electron ring beam interacting with a krypton pressure pulse, indicate that Auger processes have a significant influence in speeding up ionization rates in such a system.

On the double ionization of helium-like atoms by high-energy electron impact

The authors reply to the work of Tweed (see abstr. A20044/5 of 1973) which has thrown doubt on the previous work of Byron and Joachain (see abstr. A28934 of 1967) and Van der Wiel and Wiebes (1971).

High energy electron impact ionization for elemental analysis by energy dispersive X-ray spectrometry

A method has been demonstrated in which the emission of characteristic X-rays following impact ionization by electrons of several MeV energy is used for quantitative elemental analysis. Because of the unique properties of MeV electrons with respect to their X-ray production yield and range in solids, the method has comparable sensitivity over a wide range of elements and relatively less stringent sample preparation requirements. By using energy dispersive techniques to detect the X-rays, a simultaneous determination of the weight fraction of the elements in an unknown sample is possible from a procedure based entirely on empirically determined relative X-ray yields. The method was tested by analyzing three standard metal alloy samples obtained from the National Bureau of Standards. Elements reaging from iron (Z=26) to lead (Z=82) with concentrations from 0.06% to 88% were quantitatively determined. In general, good approximate agreement was found wit the reported assays even for the minor constituents.

Determination of optical sums from the small angle electron impact spectra of argon

A method for obtaining optical sums directly from high energy (25 keV) electron impact spectra in the forward direction is suggested. The electron energy loss spectrum of argon at an energy resolution of ±600 meV was obtained for energy losses from zero to 50 eV and 200 to 300 eV at an energy resolution of 5 eV averaged over an angular range of ±5 × 10 −4 radians. The data were corrected for finite detector size, gas and electron beam spatial distributions to obtain an estimate of zero angle scattering spectrum. A hydrogenic contribution was assumed for the 3p, 3s continuum for energy loss greater than 5- eV and for the 2p continuum for energy loss greater than 280 eV. Various moments of the spectrum were computed and related to optical sums by use of the measured experimental moments of the energy distribution of the incident beam and the scale factor needed to place the data on an absolute scale. The scale factor was obtained by use of the Thomas-Reiche-Kuhn sum rule. The results are in excellent agreement with previous estimates of the various sum rules and the present approach shows promise for providing useful data for a large number of atomic and molecular targets.

Ionization—Excitation Processes in Zinc, Cadmium, and Mercury

Cross sections for ionization—excitation by photon and high-energy electron impact have been calculated for several states of zinc, cadmium, and mercury. Such processes are of interest both theoretically, in that they are an extremely sensitive probe into electron correlation effects in atoms, as well as practically, in that they may play an important role in the population of many upper ion laser levels. A model for the ionization—excitation process has been developed which takes full account of correlation in the initial state but includes only the screening part of the final-state interaction.

Bound-state effects in atomic rearrangement collisions

Abstract The influence of target structure effects in atomic rearrangement collisions is investigated. This bound-state problem is studied for two simple reactions: excitation of the 23S state of helium from the ground state by high-energy electron impact and charge transfer of fast protons on helium.