Electron impact ionization of CDx (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 CDx (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 x 10(-16) cm(2) 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.