K-shell ionization cross sections following 0.6-4-keV e-+H2O collisions

Collisions between electrons and a H2O vapor target at projectile energies ranging from 600 eV to 4 keV have been investigated experimentally and theoretically, in order to determine cross sections for the production of a K-shell vacancy of the H2O molecule. The electrons originating from direct ionization and from autoionization were detected at angles in the range 30 degrees-130 degrees with respect to the incident beam direction. From the spectra, cross sections for the emission of a target electron following the production of a K-shell vacancy in oxygen are determined as a function of the projectile energy. The present experimental results are compared with semiempirical formula that have been extensively used previously for electron-atom collisions, and with model calculations using plane wave Born approximation. The ratio between K-shell and total ionization cross sections is found to be similar to that determined for an O atomic target, except at the highest projectile energies, for which it is equivalent to that determined for Ne. We show that this ratio is essentially governed, for sufficiently high collision energies, by the ratio between the outer- and inner-shell ionization potentials.