Spurious Dissociative Attachment Peaks from Inelastic Loss Reactions

Peaks in the electron energy dependence of negative ion production are usually the direct result of dissociative attachment reactions of the type e+XY→XY*−→X−+Y. Each such peak is related to a particular intermediate molecular negative ion state XY*− and provides an approximate estimate of the energy of that state in the Frank–Condon region. A dissociative attachment peak may, however, be accompanied by a smaller “partner” peak at a higher electron energy at which an inelastic energy loss collision leaves the electron with an energy at which subsequent dissociative attachment can occur. Thus the separation of the partner peak from the main dissociative attachment peak equals the most probable energy loss. A peak in the production of O− from O2 at 15 eV, observed previously, but not satisfactorily explained, has been studied experimentally and is shown to arise from the above described reaction sequence involving the loss of ∼ 8.4-eV energy by excitation of the B 3Σu− dissociation continuum. Satisfactory correlation of the observations with published energy loss spectra is demonstrated and the expected quadratic pressure dependence of the signal is confirmed. In addition a technique is described whereby the energy of the electrons directly responsible for the ion production is measured even though it differs from that of the primary electron beam. On the basis of the present observations it is concluded that in the region of 15 eV essentially all of the energy loss occurs through the 8.4-eV process, and the inelastic cross sections of Hake and Phelps are appropriately re-apportioned. Peaks in the production of O− from N2O in the region of 7–15-eV beam energy are shown to have a quadratic pressure dependence and are ascribed to the same general mechanism. Correlations with known energy losses are, however, less satisfactory than in the O2 case. Possible reasons for this are suggested. Peaks in O− production from CO at 16 and 18.3 eV arise from excitation of the A 3π and A 1π states. The relative magnitudes of these two excitation cross sections apparently changes dramatically with increasing electron energy. Two peaks in O− production from NO, at 14.2 and 15 eV, are similarly correlated with known energy losses and the first of these peaks ascribed to excitation of the c 2π and/or the D 2Σ+ state.