Kinetic-Energy Distribution of Negative Ions Formed by Dissociative Attachment and the Measurement of the Electron Affinity of Oxygen

The kinetic-energy distribution of ions produced by a dissociative ionization process is derived, taking into account the effect of thermal motion of the target molecule. In the case of dissociative attachment of monoenergetic electrons to a diatomic molecule, the width at half-maximum of the negative-ion energy distribution is given by ${\left(11\mathrm{}\beta \mathrm{kT}{E}_0\right)}^{\frac{1}{2}}$, where $\beta$ is the ratio of the mass of the ion to that of the parent molecule, $T$ is the target-gas temperature, and $E_0$ is the most probable ion energy. Using a crossed-field velocity filter, ${\mathrm{O}}^{-}$ ion energy distributions arising from the attachment of essentially monoenergetic electrons to ${\mathrm{O}}_2$ are studied as a function of electron energy at two gas temperatures. The measured widths of the distributions are consistent with the above relationship. Measurements of $E_0$ as a function of the electron energy allow a determination of the electron affinity $A$ of atomic oxygen. The result, $A=1.5\mathrm{}\pm 0.1$ eV, is in excellent agreement with photodetachment-threshold determinations.