Collisions of Electrons with Hydrogen Atoms. I. Ionization

Abstract

The cross section for ionization of the hydrogen atom on electron impact has been measured as a function of electron energy. A modulated beam of atoms and molecules in varying proportions, taken from a furnace, is crossed by a dc electron beam, and the positive ions formed are taken into a mass spectrometer. By using such modulation techniques, the ions formed by ionization of the beam are distinguished from the much larger number of ions formed by collisions of electrons with the residual gas in the vacuum chamber. From the study of the mass-spectrometer peak strengths as a function of temperature with constant gas flow, the ratio of the cross sections for ionization of the atom and the molecule is directly determined. The absolute atomic cross sections, determined by multiplying this ratio by the known molecular cross sections, are cross-checked by taking relative cross sections for the atom. Complete agreement with the first Born approximation is found only above about 250 ev. Deviations of experiment from theory at lower energies are as predicted qualitatively by theory.

Calculation of the total ionization cross section and spin asymmetry in electron-hydrogen scattering from threshold to 500 eV

“…The experimental situation is that the cross section is known to high accuracy (statistical errors of order 3%%uo) over the energy range from 14.6 eV to 4 keV through the pulsed crossedbearn measurements of Shah, Elliott, and Gilbody [1]. Their experiment supersedes the earlier studies of Fite and Brackmann [2] and Rothe et al [3]. The recent experiment of Shyn [4] yields results that are 2&o higher than those of Shah, Elliott, and Gilbody.…”

A polarized atomic hydrogen beam

“…In light of the uncertainty in the equivalence between optical and charged particle transparency for the two meshes, and with some reasonable assumptions about the efficiency curve provided by Mullard, we assign a conservative value of ± 0.025 to the uncertainty in 8. For the total ionization cross section, aI' we use the value of (0.762 ± 0.038) x 10-17 cm2 recently determined by Shah, Elliot, and Gilbody [43]. We note that this value is consistent with that obtained from the earlier absolute measurement of Fite and Brackman [44] at 20 eV and scaled downward to 15 eV in accordance with the relative measurements of McGowan and Clark [7]. With an ionization rate of 450 protons/s for an incident electron intensity of 10 nA, we obtain a value of (1.9±0.7) x 1010 atoms/cm3 for Peff, where we quote an overall uncertainty that includes the effect of the~100 meV resolution (full width at half maximum) of the electron beam 413 and the uncertainty in its energy centroid, as well as the specific uncertainties in 8 and aI and to a minor degree Re.…”

Radio Emission From Red-Giant Hot Jupiters

“…The cross-section 13 of hydrogen for electrons is ∼4 × 10 −17 cm 2 (Fite & Brackmann 1958), which implies a mean free path for ionized electrons of ∼2R J in the surrounding medium. As a result, nearly 2/3 of released energetic electrons will ionize a hydrogen atom within ∼2R J , and more than 99% will ionize a hydrogen atom within ∼10R J .…”