Electron Impact Ionization of D_2O and OD

Despite the importance of H_2O and the OH radical in many areas of application, the collisional data base for these species is scarce, in particular for the OH radical. Electron impact ionization cross section data available in the literature are especially controversial with a factor of 4 discprepancy between various measurements of the total H_2O ionization cross section (and no ionization cross section data are available for the OH radical). We report measurements of absolute partial and total ionization cross sections for the D_2O molecule and the OD free radical from threshold to 200 eV using the fast-beam technique. The deuterated target species were used for experimental convenience. Our data are compared with other available experimental data and with the results of recent cross section calculations. This work was supported by the Planetary Atmospheres Program of NASA.