Electron impact ionization of the hydroxyl radical

We employed the fast-neutral-beam technique in a measurement of absolute partial cross sections for the electron-impact ionization and dissociative ionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than 2.0×10^-16cm² at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of 1.85×10^-16cm² at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the D₂O molecule and found good agreement between our cross sections and the most recent measurements of Straub et al. [J. Chem. Phys. 108, 109 (1998)] as well as with recent calculations.