The cross sections of the electron impact ionization and dissociative ionization of CO, CH 4 and C 2 H 2 have been measured for electron energies from threshold to 600 eV. The present values agree with the previous measurements where a complete collection of the resulting ions has been demonstrated experimentally. However, for the processes where no complete collection has been demonstrated, the present results are usually higher if the ions are produced with higher kinetic energy. The present measurement demonstrates the complete collection of the ions experimentally and the results are thus believed to be reliable. The disagreement between the present results and the previous ones is explained by noting the kinetic energy distributions of the fragment ions.

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For reliable cross section data a focusing time-of-flight mass spectrometer has been presented which collects all the ions produced by the electron impact dissociative ionization of molecules. The focusing characteristic of the mass spectrometer generates the ions close to the axis and focuses the energetic ions back to the detector. By observing the deflection curves we can measure the cross sections conclusively. The complete collection allows one to extract the initial kinetic energy distribution of the ions from the time-of-flight profiles of the mass peaks. As a first example we measured the cross sections of electron impact dissociative ionization of CO2  for electron energies from threshold to 300 eV. The results agree very well with recent measurements of Straub et al. [J. Chem. Phys. 105, 4015 (1996)], although previous data are in very poor agreement with each other. We measured the initial translational energy distribution of the fragment ions from the dissociative ionization of CO2 . With respect to the kinetic energy distribution of the ions we analyzed the techniques which make use of the quadrupole mass spectrometer. We discuss why the results from the quadrupole mass spectrometer underestimate the cross sections for the fragment ions. Finally we suggest to recheck the data from earlier quadrupole mass spectrometer measurements.

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The cross sections of ionic products due to the electron impact ionization of N 2 and O 2 have been measured for the electron energies from threshold up to 600 eV. The contributions to the cross sections from the possible reaction channels after the multiple ionization of molecules are also determined. The experiment shows that for both N 2 and O 2 the singly charged fragment N + or O + originates mainly (∼70%) from the contribution of the dissociation of the singly charged parent ions, whereas the doubly charged fragments N 2+ or O 2+ are mainly (∼70%) from the dissociation channel of doubly ionized N 2 into N 2+ + N or O 2 into O 2+ + O, respectively. At high electron energies, 85% of singly ionized N 2 molecules stabilize into N + 2 , whereas about 70% of singly ionized O 2 molecules stabilize into O + 2 . More than 80% of doubly ionized N 2 and O 2 molecules dissociate through the equal charge separation channel. However, after triple ionization, most of the molecules dissociate into two charged fragments. The experiment also shows that the total cross sections for the different ionization stages decrease exponentially as the ionization stage increases.