Dissociative Attachment of Electrons to Hot Oxygen

Crossed beam experiments on dissociative attachment to form ${\mathrm{O}}^{-}$ ions in collisions of electrons with oxygen molecules are described. In these experiments the temperature of the oxygen beam source was varied and very strong changes in the cross section as a function of temperature were displayed indicating marked enhancement of the cross section with increasing internal energy in the ${\mathrm{O}}_2$ prior to the collision with the electron. A separate thermal equilibrium experiment verified the accepted affinity of ground state ${\mathrm{O}}^{-}$, and experiments using free jet expansion of the heated ${\mathrm{O}}_2$ indicated that the form of internal energy responsible for the enhancement of the cross section is vibrational excitation, in accord with the theory given by T.F. O'Malley. Absolute determinations of the cross sections at different temperatures were made and found to be in accord with O'Malley's predictions.