Chemical Synthesis Induced by Dissociative Electron Attachment

Abstract

We have investigated reactions in condensed CF2Cl2 induced by electrons of subexcitation energies under ultrahigh vacuum conditions. The yields of the CF2Cl2 radiolysis products (C2F4Cl2, C2F3Cl3, and C2F2Cl4) were determined as functions of electron energy (Ei) (∼1 to ∼4.5 eV) and electron dose (fluence) (1.5 × 1016 e/cm2 to 2.2 × 1016 e/cm2) by postirradiation temperature-programmed desorption. In general, we ascribe the formation of these radiolysis products to reactions of •CFCl2 and •CF2Cl radicals, which are generated by dissociative electron attachment (DEA) to CF2Cl2. Consistent with condensed-phase experiments, which report the electron stimulated desorption of F− ions via transient negative ion formation at incident electron energies near 4 eV, the yield of products C2F2Cl4 and C2F3Cl3 derived from reactions of •CFCl2 show maxima near ∼3 eV. The production of C2F3Cl3 and C2F4Cl2 at electron energies as low as 2 eV indicates that •CF2Cl is generated; however, the absence of a clear resonance maximum in the C2F4Cl2 yield function indicates that the production of •CF2Cl radicals via DEA is not the sole or dominant pathway for the formation of C2F4Cl2. Indeed, the variation of radiolysis product yield with electron dose suggests that the C2F4Cl2 signal may additionally be generated by reactions of CF2Cl2 with the difluorocarbene radical (:CF2), which we propose is also formed by DEA to CF2Cl2. Although numerous studies have revealed chemical damage induced by DEA, our results represent one of the first studies to unambiguously demonstrate (via a clear signature of resonance) chemical synthesis induced by this process.