ELECTRON-IMPACT IONIZATION OF SPATIALLY ORIENTED CH3CL AND CCL3H

The influence of molecular orientation on 200 eV electron impact ioniz ation has been probed with oriented target molecules in crossed partic le beams. Ionization of oriented CH3Cl to form the molecular ion, CH3C l+, is favoured for electron impact on the positive end of the dipole, the CH3 end. Formation of the CH3+ fragmentation ion shows no prefere nce for either end of the dipole which has been interpreted as favouri ng broadside collisions with the molecule. The ionization cross-sectio n and the fragmentation pattern or mass spectrum for CH3Cl therefore e xhibit an orientation dependence. Ionization of oriented CCl3H to form the fragmentation product CCl2H+ is favoured for electron impact at t he positive end, the H end, of the molecule. An electrostatic or Coulo mb model for electron impact ionization which reproduces the main expe rimental features is introduced.