Electron Attachment Processes in Liquid Argon containing Oxygen or Nitrogen Impurity

The formation of negative ions of oxygen and nitrogen has been investigated by measurement of the attachment coefficients for electrons in dilute solutions of these impurity gases in liquid argon. Pulses of electrons were injected into the liquid by α-particle ionization, and the attachment coefficients were obtained by two methods involving changes of the magnitude of the pulses and of the average ionization current when the electrode spacing and impurity concentration were varied. The attachment cross section for oxygen is appreciably greater than that observed in oxygen gas for low energy electrons (similar 10 -17 cm 2 ), while for nitrogen the cross section is as small as 3 × 10 -20 cm 2 . The field variation of the cross sections suggests that the attachment is a three-body process in both cases. Uncertainty in the exact composition of the liquids, particularly in the case of argon-nitrogen mixtures, makes detailed interpretation of these results difficult.