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 highresolution (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-a, 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. Copyright 2004 by the American Geophysical Union.