The influence of molecular orientation on 200 eV electron impact ionization has been probed with oriented target molecules in crossed particle beams. Ionization of oriented CH3Cl to form the molecular ion, CH3Cl+, is favoured for electron impact on the positive end of the dipole, the CH3end. Formation of the CH3+fragmentation ion shows no preference for either end of the dipole which has been interpreted as favouring broadside collisions with the molecule. The ionization cross-section and the fragmentation pattern or mass spectrum for CH3Cl therefore exhibit an orientation dependence. Ionization of oriented CCl3H to form the fragmentation product CCl2H+is favoured for electron impact at the positive end, the H end, of the molecule. An electrostatic or Coulomb model for electron impact ionization which reproduces the main experimental features is introduced. © 1995.
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