Dissociative Electron Attachment to Molecules

Dissociative-electron-attachment cross sections (some unpublished) for 30 molecules are summarized, evaluated, and discussed within the framework of the resonance scattering theory. The dissociative-attachment peak cross section, σc(εmax), is found to be a strong function of the peak (resonance) energy, εmax, with a break in this dependence at the energy where electronic excitation of the neutral molecule begins to occur. Based on the experimental data, three groups of molecules have been distinguished: (i) those where εmax is less than the energy, εN, of known electronic excited states of the neutral molecule, and the negative-ion state is purely repulsive in the Franck-Condon region, (ii) those where εmax ≥ εN, and (iii) those with exceptionally small σc(εmax) for which a vertical onset for dissociative attachment occurs. For the molecules in group (i) σc(εmax) varies almost as (εmax)−1, while for group (ii) σc(εmax) is a much stronger decreasing function of the resonance energy εmax. For group (i) the effect of autoionization on σc(εmax) is small, and isotopic effects on σc(εmax) are contained within the square root of the inverse ratio of the reduced masses of the products. For group (ii) and especially group (iii) the effect of autoionization on σc(εmax) is large and so are the isotopic effects on σc(εmax).