Electron impact ionization of CH3D and CD4 near threshold: Isotope and temperature effects

Electron impact ionization near threshold was studied for the two isotopomers CH3D and CD4 using a high-resolution electron impact mass spectrometer. The appearance energies (AEs) of the parent and two fragment cations were determined for both molecules using a fitting procedure based on a modified Wannier type threshold power law. The present results are compared with the ionization thresholds for CH4 studied previously with the same experimental setup. The deuterated molecules have always higher AEs than undeuterated CH4. Moreover, for both molecules a downward shift in the AE was observed when the temperature was increased from 293 K to 693 K. The temperature effect measured on the AEs increases from CH4 to CH3D to CD4.