Kinetic energy distributions and line profile measurements of dissociation products of water upon electron impact

Abstract

The Doppler line profiles of H Ly-α (1216 Å) and O I (1302 Å and 1152 Å) resulting from electron impact dissociative excitation of H₂O have been measured with a high-resolution (λ/Δλ = 50,000) ultraviolet spectrometer. The line profiles are used to calculate the kinetic energy distribution of the hydrogen atoms produced in dissociative excitation and ionization of H₂O at electron impact energies 25, 35, and 100 eV. Three distinct populations of H(2p) atoms were found. The kinetic energy of hydrogen atoms is found to have contributions from a low-energy component, with a mean energy of ~0.2 eV at all the three electron impact energies. In addition, a medium-energy component appears with a mean energy of ~2.0 eV for 35 eV electrons, and a high-energy component, ~7 eV, for 100 eV electrons. The measurement of O I (1302 Å) and O I (1152 Å) line profiles indicate that the kinetic energy of excited O I atoms is very small (~1 eV or less) at all electron impact energies. Most of the energy released in dissociation is found in the translational energy of the hydrogen atoms. The excitation functions of H Ly-α, O I (1302 Å) feature of oxygen, and A(0) - X(0) molecular band of hydroxyl near 3050 Å from threshold to 600 V were also measured. The spectrum (1.0 Å FWHM) of the rotational structure of OH (A - X) from electron impact dissociation indicates a high degree of rotational excitation, which is almost identical to the rotational structure from dissociative recombination of H₂O⁺. The results presented in this paper have important applications to planetary bodies, like comets, icy satellites of outer planets, Saturn's magnetosphere, and rings, all of which have H₂O and its daughter products in large amount.