Electron impact ionization of CD_x (x = 1-4)

We report measurements of absolute cross sections for the electron-impact ionization and dissociative ionization of the deuterated methane molecule CD₄ and the CD_x (x = 1-3) free radicals from threshold to 200 eV using the fast-neutral beam technique. We used the deuterated rather than the protonated target species to facilitate a better separation of the various product ions from a given parent molecule in our apparatus. Good agreement was found between our CD₄ results and the most recent measurements in CH₄. Our results for CD₃ and CD₂ agree well with earlier measurements carried out under different experimental conditions. The present results for the CD free radical are the first ionization cross section measurements for this target. A common feature of all four targets studied in this work was a dominant parent ionization cross section with essentially the same absolute value of 1.6-1.7 × 10^-16 cm² at 70 eV. A comparison of the experimentally determined total single ionization cross sections with calculated cross sections using a modified additivity rule showed very good agreement for all four targets.