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Journal ArticleDOI

Principles and procedures for determining absolute differential electron-molecule (atom) scattering cross sections

01 Sep 1989-Journal of Physics E: Scientific Instruments (IOP Publishing)-Vol. 22, Iss: 9, pp 730-738
TL;DR: In this article, the absolute elastic and inelastic differential cross sections (DCS) for electron impact on molecular (atomic) species are described and illustrated by examples, and a method of determining absolute differential electron-molecule scattering cross sections is generally applicable and provides reliable results.
Abstract: Procedures and calibration techniques for measuring the absolute elastic and inelastic differential cross sections (DCS) for electron impact on molecular (atomic) species are described and illustrated by examples. The elastic DCS for the molecule under study is first determined by calibration against helium using the relative flow technique. The second step involves the production of energy-loss spectra for the instrument response function, the unfolding of overlapping inelastic structures and the normalization of inelastic intensities to the elastic cross sections. It is concluded that this method of determining absolute differential electron-molecule (atom) scattering cross sections is generally applicable and provides reliable results.
Citations
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Journal ArticleDOI
TL;DR: Experimental collision cross-sections for electron-molecule scattering processes at low to intermediate energies (meV-100 eV ) are compiled and critically reviewed in this paper, where the authors discuss the effect of resonance effects in the context of the enhancement they produce in the various scattering cross-sectional.

285 citations

Journal ArticleDOI
TL;DR: In this article, the best available data for calculating a complete set of binary collision integral data for the computation of the mixture transport properties (viscosity, thermal conductivity, and ordinary and thermal diffusion) of 13-species weakly ionized air is presented.
Abstract: Ar eview of the best-available data for calculating a complete set of binary collision integral data for the computation of the mixture transport properties (viscosity, thermal conductivity, and ordinary and thermal diffusion) of 13-species weakly ionized air is presented. Although the fidelity of the data varies, all collision integrals presented herein, except for electron-neutral interactions, are estimated to be accurate to within 25% over the temperature range of interest (300‐15,000 K) for reentry and laboratory plasmas. In addition, most of the dominant atom‐atom and atom‐ion interactions for dissociated weakly ionized air were derived from ab initio methods that are estimated to be accurate to within 10%. The accuracy and valid temperature range for electron-neutral interactions vary because of scarcity of the required cross-sectional data.

266 citations

Journal ArticleDOI
TL;DR: A review of Born approximation and phenomenological scaling approaches that provide accurate excitation cross sections over a range of electron impact energies can be found in this article, where the methods are illustrated for a variety of atomic and molecular systems.
Abstract: Electron-atom and electron-molecule collisional cross sections are needed in the modeling and understanding of phenomena ranging from planetary atmosphere science to industrial applications of plasmas. This article reviews the Born approximation and phenomenological scaling approaches that provide accurate excitation cross sections over a range of electron impact energies. The methods are illustrated for a variety of atomic and molecular systems.

114 citations

Journal ArticleDOI
TL;DR: In this article, absolute angle-differential elastic and vibrational excitation cross sections for electron collisions with tetrahydrofuran were measured in the energy range 0.1-20 eV, extending existing measurements to lower energies.
Abstract: Absolute angle-differential elastic and vibrational excitation cross sections for electron collisions with tetrahydrofuran were measured in the energy range 0.1–20 eV, extending existing measurements to lower energies. The elastic cross sections were measured as a function of scattering angle from 10° to 180° at energies of 2 eV, 6 eV, 10 eV and 20 eV, and as a function of electron energy at 45°, 90°, 135° and 180°. The agreement with previous measurements and with the published theoretical work was generally satisfactory. The Ramsauer–Townsend minimum was observed at low energies, down to 0.24 eV at 180°. Three additional minima were observed at 1.13, 4.74 and 15.3 eV in the 180° elastic cross section. Vibrational excitation cross sections are reported as a function of electron energy from the threshold to 16 eV. They reveal threshold peaks and broad bands at 6.2 and 10.8 eV, attributed to shape resonances, in agreement with theoretical predictions, although the calculated energies are generally somewhat higher. A broad enhancement of the CH2 scissoring vibration is observed around 2.6 eV, implying a low-lying (shape) resonance similar to that observed earlier in cyclopropane.

93 citations

Journal ArticleDOI
TL;DR: In this paper, an absolute differential cross section (DCSS) for vibrational excitation of O2, with improved sensitivity and resolution, is presented, and the 90 degrees DCSS is given as a function of energy up to 16 eV.
Abstract: New measurements of absolute differential cross sections (DCSS) for vibrational excitation of O2, with improved sensitivity and resolution, are presented. The 90 degrees DCSS are given as a function of energy up to 16 eV for elastic scattering and vibrational excitation of up to v=7. A weak continuous, probably non-resonant, background scattering is found in the v=1 cross section in addition to the well known sharp 2 Pi g resonances dominating vibrational excitation in the energy range up to 2.5 eV. The sharp 2 Pi g resonances interfere coherently with the background scattering both in the v=1 and in the elastic channels. New absolute values for the energy-integrated DCSS for 2 Pi g resonances are given. They exhibit oscillatory Franck-Condon factors in the v=4-7 exit channels. The broad resonance peaking at 9 eV, dominating vibrational DCSS at higher energies, causes excitation of high vibrational levels (up to the dissociation limit) of the X 3 Sigma g- ground state, but not of the a1 Delta g and b1 Sigma g+ electronically excited states, supporting its assignment as the 4 Sigma u- resonance, the selectivity being caused by spin selection rules. The excitation of high vs indicates a relatively narrow autodetachment width for this resonance. The angular dependence at 9 eV is, however, not a simple psigma wave as would be expected for a simple sigma u* shape resonance. Preliminary experiments with a free jet cooled sample, concerned with rotational broadening of the 2 Pi g resonance, are reported.

93 citations

References
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Book
01 Jun 1969
TL;DR: In this paper, Monte Carlo techniques are used to fit dependent and independent variables least squares fit to a polynomial least-squares fit to an arbitrary function fitting composite peaks direct application of the maximum likelihood.
Abstract: Uncertainties in measurements probability distributions error analysis estimates of means and errors Monte Carlo techniques dependent and independent variables least-squares fit to a polynomial least-squares fit to an arbitrary function fitting composite peaks direct application of the maximum likelihood. Appendices: numerical methods matrices graphs and tables histograms and graphs computer routines in Pascal.

12,737 citations

Journal ArticleDOI
TL;DR: Numerical methods matrices graphs and tables histograms and graphs computer routines in Pascal and Monte Carlo techniques dependent and independent variables least-squares fit to a polynomial least-square fit to an arbitrary function fitting composite peaks direct application of the maximum likelihood.
Abstract: Uncertainties in measurements probability distributions error analysis estimates of means and errors Monte Carlo techniques dependent and independent variables least-squares fit to a polynomial least-squares fit to an arbitrary function fitting composite peaks direct application of the maximum likelihood. Appendices: numerical methods matrices graphs and tables histograms and graphs computer routines in Pascal.

10,546 citations

Journal ArticleDOI
Gregory H. Wannier1
TL;DR: In this paper, the dependence of the yield on the energy just above a threshold is derived and the derivation is not rigorous because it circumvents some of the difficulties of the three-body problem by applying ergodicity, albeit in a weakened form.
Abstract: When an electron hits an atom or ion, it may knock off an electron This process is fundamental in almost all types of gas discharge The reaction is endothermic; hence there is a threshold value in the electron energy below which it does not occur In this paper, the dependence of the yield on the energy just above this threshold is derived The derivation is not rigorous because it circumvents some of the difficulties of the three-body problem by applying ergodicity, albeit in a weakened form The result is that, for atoms, the yield rises as the 1127th power of the energy excess For ions the exponent lies between this number and unity

835 citations

Journal ArticleDOI
TL;DR: In this paper, the Schrodinger equation for two electrons in a Coulomb field is studied in the critical region where both electrons have near-zero kinetic energies, and the main feature of this problem is that the mutual screening between the two electrons determines and is determined by the partition of the available energy between them.
Abstract: The Schr\"odinger equation for two electrons in a Coulomb field is studied in the critical region where both electrons have near-zero kinetic energies. The main feature of this problem is that the mutual screening between the two electrons determines and is determined by the partition of the available energy between them. This energy-dependent screening can be taken into account to yield a complex potential in the radial variable $R={({r}_{1}^{2}+{r}_{2}^{2})}^{\frac{1}{2}}$ of the six-dimensional configuration space of the two electrons. Solutions of this equation are obtained and are shown to correspond to the classical orbits given in an early paper by Wannier. A possible way is indicated of using these wave functions to establish the Wannier threshold law which, for ionization of neutral atoms, is $\ensuremath{\sigma}\ensuremath{\propto}{E}^{1.127}$. Finally, the interplay between the total energy and the Coulomb potential is discussed both for this problem and for the case of one electron in the field of a nucleus.

223 citations

Journal ArticleDOI
TL;DR: In this article, the angular distributions for elastically scattered electrons were measured in a crossed-beam geometry using a collimated, differentially pumped atomic-beam source which requires no effective-path-length correction.
Abstract: Absolute differential, integral, and momentum-transfer cross sections for electrons elastically scattered from helium are reported for the impact energy range of 5 to 200 eV. Angular distributions for elastically scattered electrons are measured in a crossed-beam geometry using a collimated, differentially pumped atomic-beam source which requires no effective-path-length correction. Below the first inelastic threshold the angular distributions were placed on an absolute scale by use of a phase-shift analysis. Above this threshold, the angular distributions from 10 to 140 deg were fitted using the phase-shift technique, and the resulting integral cross sections were normalized to a semiempirically derived integral elastic cross section. Depending on the impact energy, the data are estimated to be accurate to within 5 to 9%.

194 citations