Temperature dependencies in dissociative electron attachment to CCl4, CCl2F2, CHCl3 and CHBr3

The studies of dissociative electron attachment to CCl4, CCl2F2, CHCl3, and CHBr3 molecules have been performed in the electron energy range from 0 to 2 eV and in the gas temperature range from 300 to 550 K using a new crossed electron-molecular beams apparatus with a temperature regulated, effusive molecular beam source. In the case of dissociative electron attachment channels Cl-/CHCl3 and Cl-/CCl2F2 strong enhancements of the negative ion production with the gas temperature at low electron energies has been observed. Activation energies for dissociative electron attachment of 80 20 meV for CHCl3 and 130 +/- 20 meV for CF2Cl2 have been estimated. For the resonances at 0.27 eV in CHCl3 and 0.7 eV in CF2Cl2 no dependence of the cross-sections on gas temperature have been observed. In the case of the dissociative electron attachment to Cl-/CCl4 and Br-/CHBr3 the cross-sections do not depend on T in the whole measured electron energy range from 0 to 2 eV. At the elevated gas temperatures above 400 K, unusual temperature effects have been observed for CCl4, CHCl3 and CHBr3 molecules. These effects might be most likely attributed to the thermal decomposition of molecules on the surface of the molecular beam source. (C) 2002 Published by Elsevier Science B.V.