Associative ionization in U + O and U + O2 collisions

Collisions between thermal energy uranium atoms and oxygen atoms and molecules have been studied in four experiments. A triple crossed beam experiment has determined that the ratio of effective cross sections for the associative ionization processes U + O → UO⁺ + e and U + O₂ → UO₂ + e is 97±20. An experiment measuring the number of ions formed per unit length by uranium atoms passing through O₂ has determined that the absolute cross section for $\mathrm{U}+{\mathrm{O}}_2\to {\mathrm{UO}}_2^{+}\mathrm{+\; e}$ is 1.68±0.27 × 10⁻¹⁷ cm². A total attenuation experiment indicates that the total reaction cross section between U and O₂ is 1.78±0.45×10⁻¹⁵ cm², implying that over 99% of the reactions lead to the neutral products UO+O while less then 1% give associative ionization. A modulated beam mass spectrometry experiment, employing phase spectrometry, indicates that the activation energy for $\mathrm{U}+{\mathrm{O}}_2\to {\mathrm{UO}}_2^{+}\mathrm{+\; e}$ is extremely small if existent at all. Two Appendices treat the relations between effective cross sections as measured in various experiments to true cross sections as derived from theory and the generation of phase shifts in modulated beam experiments where a beam is detected by reactions with a Maxwellian gas.