F J de Heer

Total cross sections for electron scattering by Ne, Ar, Kr and Xe

A set of total cross sections for scattering of electrons by Ne, Ar, Kr and Xe has been evaluated over the energy range of about 20 to 3000 eV by means of the analysis of experiments and theories on total cross sections for elastic scattering, ionisation and excitation, and on differential cross sections for elastic and inelastic scattering. The total cross sections for scattering of electrons are evaluated by adding those for ionisation, excitation and elastic scattering and they are accurate to about 5-10%. They appear to be in very good agreement with the recent experimental results on total electron scattering of Wagenaar et al. (1978).

Absolute differential cross sections for elastic scattering of electrons by helium, neon, argon and molecular nitrogen

An electron spectrometer has been constructed for the study of elastic and inelastic electron scattering processes. Up to now the apparatus has been used to measure differential cross sections of electrons elastically scattered by He, Ne, Ar and N2. Direct absolute cross section measurements were performed on N2 at 500 eV impact energy and at scattering angles between 5 degrees and 9 degrees . Relative cross section measurements were done on He, Ne, Ar and N2 at impact energies between 100 and 3000 eV and scattering angles between 5 degrees and 55 degrees . The relative cross sections were put on an absolute scale by means of the apparatus calibration factor derived from the absolute measurements on N2. Analysis of the exponential behaviour of the differential cross section as a function of momentum transfer yielded apparent polarizabilities of the target.

Electron impact induced light emission from CF4, CF3H, CF3Cl, CF2Cl2 and CFCl3

Abstract Several aspects have been studied of the light emission following electron impact excitation of CF 4 , CF 3 H, CF 3 Cl, CF 2 Cl 2 and CFCl 3 . Dissociative excitation processes, leading to excited atomic or diatomic fragments, have been analysed using the Bethe theory. The continuous emissions, found in all compounds, have been assigned to transitions between ionic states of the parent molecules, where the lower level is populated above its dissociation limit.

Emission cross sections of balmer-α,β,γ radiation for electrons (0–2000 eV) on H2 and D2

Abstract Absolute emission cross sections (σem) of Balmer-α,β, γ radiation have been measured in the case of electrons (0–2000 eV) incident on H2 and D2. The H Balmer σems are larger than the corresponding D Balmer σems for every electron impact energy. It has been found that this isotope effect varies strongly as a function of the incident electron energy below 100 eV. The isotope effect will be interpreted using the H2 and D2 potential energy curves.

Total cross sections for electron scattering by He

A set of total cross sections for scattering of electrons by He has been evaluated over the energy range of zero to 3000 eV by means of the analysis of experiments and theories on total cross sections for elastic scattering, ionisation and excitation, and on differential cross sections for elastic and inelastic scattering. Between 0 and 19.8 eV, where no inelastic processes occur, the total cross sections for scattering are equal to those for elastic scattering. Above 19.8 eV the authors have evaluated total cross sections for scattering of electrons by adding those for ionisation, excitation and elastic scattering. The total cross sections thus obtained are probably accurate to about 5% over a large part of the energy range. They appear to be in good agreement with the experimental results of Blaauw et al. (1977).

Radiative lifetimes of the CO a′3Σ+, b3Σ+, c3Π, d3Δ and B1Σ+ states. Absolute emission cross sections for t

Abstract Radiative lifetimes have been measured for the CO a′ 3 Σ + (ν′=4–9), b 3 Σ + (ν′= 0), c 3 Π(ν′=0), d 3 Δ(ν′=1–16) and B 1 Σ + (ν′= 0) states. Our experimental values, arranged in the same order, are 7–10 μs, 56 ns, 16 ns, 3–7.5 μs, 34 ns. Some of these values disagree with the results of previous experiments. To our opinion this is due to an incomplete identification of the emission spectrum in regions where many bands may overlap, dependent on the applied spectral resolution. For the a′Σ + −a 3 Π and d 3 Δ−a 3 Π emissions effective cross sections for quenching by CO molecules are given. In connection with the identification of the spectrum, absolute emission cross sections for electrons incident on CO have been measured for the b 3 Σ + −a 3 Π and c 3 Π−a 3 Π transitions. For an electron energy, corresponding to the maximum of the excitation function we find cross sections of 5.94 (∓1.2) × 10 −18 cm 2 and 0.630 (∓ 0.13) × 10 −18 cm 2 , respectively.

Excitation cross sections for 3s, 3p and 3d sublevels of atomic hydrogen split from simple molecules by high-energy electron impact

Abstract Relative cross sections for the excitation of the 3s, 3p and 3d sublevels of atomic hydrogen were determined in the case of dissociative excitation of H 2 , HCl, H 2 O, NH 3 and CH 4 molecules by impact of 1000 and 2000 eV electrons. For this purpose the intensity as well as the polarization of the emitted Balmer-α radiation has been measured as a function of an applied homogeneous electric field. Using the theory of the Stark level mixing for H-atoms, it was derived from these data that within the experimental error only the 3s and 3d states of H were excited.

Dissociative excitation of some aliphatic hydrocarbons by electron impact

Abstract Cross sections and threshold energies are compared for radiation from fragments produced by electron impact on methane, ethylene, ethane and acetylene. Some previous measurements have been repeated. The emission cross sections for corresponding Balmer radiation are within 10% equal for these hydrocarbons. Also the thresholds for Balmer radiation lie close together. These results can be explained in a model where H fragments arise from Rydberg states excited by promotion of an inner valence electron to a non-bonding orbital. In this model a comparison between dissociative ionization yielding H+ and dissociative excitation yielding H+ is made. For radiation from molecular fragments it is shown that the CH(A2 Δ-X2Π) emission cross sections are particularly high in the case of acetylene. The electron impact data appear to be consistent with photoabsorption data.

Excitation and decay of the C2Σ+u state of N+u in the case of electron impact on n2

Abstract This paper deals with the mechanism for electron-impact excitation of the C2Σ+u and with the competition between the two observed C state decay modes: “2nd negative” radiation (C2Σ+u→X2Σ+g) and predissociation. The relevant data were obtained from a quantitative analysis of the 2nd negative Δu = −6 and Δu = −7 emission sequences; these were produced by the bombardment of a low-pressure N2 target with an electron beam whose energy could be varied up to 800 eV. The signal from interfering Lyman-Birge-Hopfield radiation was suppressed by means of a chopped-beam technique. The data are consistent with the recent proposal by Lorquet and Desouter that the C state can be excited through intensity borrowing by configuration interaction. For the C state vibrational levels υ> 3 in 14NS14NS+ the ratio of the probability for predissociation to that for 2nd negative emission is found to increase from 9.6-1.5+2.6 for υ = 3 to more than 60 for υ = 7. For 14N15N+ predissociation becomes about 6 times less probable, and for 15N15N+ about 10 times. These observations can possibly be explained by the accidental predissociation mechanism suggested by Lorquet and Desouter.

Subshell-selective electron capture in collisions of C4+, N5+, O6+ with H, H2 and He

By means of optical spectroscopy the authors have determined absolute subshell-selective single electron capture cross sections sigma nl for collisions of C4+, N5+ and O6+ with atomic and molecular hydrogen and helium at impact velocities between 0.1 and 0.5 au. The absolute sensitivity calibration of the monochromators is discussed briefly. In addition they have measured the total single electron capture cross section sigma t by means of charge state analysis, for atomic and molecular hydrogen as targets. In all cases it is found that one or two n shells are dominantly populated and that the total cross section is only weakly dependent on the impact velocity, in contrast to the subshell-selective cross sections sigma nl. The main features of the results obtained are qualitatively explained by means of simple diabatic potential diagrams and the classical overbarrier transition model. For several systems a detailed comparison can be made with recent theoretical calculations, showing satisfactory agreement.