Electron-impact ionization of the chlorine molecule

Relative partial ionization cross sections for the formation of Cl-2(+), Cl + and Cl2+ from molecular chlorine have been recorded as a function of the ionizing electron energy. In these measurements particular attention has be en paid to the efficient collection of fragment ions with high translationa l energies and the minimization of any mass-dependent discrimination effect s. The cross sections show that at electron energies above the double ioniz ation threshold the yield of fragment ions can be comparable with the ion y ield of nondissociative ionization. Further analysis shows that at electron energies above 50 eV the yield of fragment ions from multiple ionization i s comparable with the yield of fragment ions from single ionization: dissoc iative multiple ionization contributes 14% of the ion yield at 50 eV electr on energy and 26% at 100 eV. The decay of Cl-2(2+) by heterolytic cleavage to form Cl2+ is a result of approximately 5% of the dissociative double ion ization events. This heterolytic process has a threshold of 41.8 +/- 1.5 eV . Electron-impact induced triple ionization to form long-lived Cl-2(3+) ion s has been detected for the first time. This nondissociative triple ionizat ion process makes up approximately 2% of the triple ionization events and t riple ionization is responsible for approximately 2% of the ion yield above 100 eV. The threshold for dissociative triple ionization is determined to be 65.3 +/- 1.5 eV, a value in good agreement with a trication precursor st ate energy derived from the kinetic energy release for the fragmentation of Cl-2(3+) to Cl2+ and Cl+, which provides the first experimental estimate o f the triple ionization energy of molecular chlorine. (C) 2000 American Ins titute of Physics. [S0021-9606(00)01224-1].