Chemi-ionization reactions in accelerated uranium–O2 crossed molecular beams

Abstract

Chemi-ionization and electron transfer reactions in accelerated uranium-atom–oxygen−molecule collisions have been studied by crossed molecular beam techniques: U+O2→UO+2+e, associative ionization (1); U+O2→UO++(O+e), rearrangement (reactive) ionization (2); U+O2→U++(O2+e), electron transfer (3). Cross sections for the three ionization channels were measured as a function of relative collision energy from 0.2 to 60 eV. The thresholds observed for channels (2) and (3), which are closed at thermal energies, are in good agreement with values calculated from thermochemical data. The cross section for associative ionization varied inversely with vr, the relative collision velocity, as predicted for weak autoionizing transitions. With the use of a rough absolute calibration and a 1/vr extrapolation to lower energies, the UO+2 cross section was found to be in agreement with the previously reported value measured with thermal beams. In the case of rearrangement ionization, the cross section rose rapidly at threshold, peaked near the threshold for the electron transfer process, and then declined with increasing collision energy. The electron transfer cross section increased with a power-law dependence of the excess energy above threshold throughout the range investigated. The maximum absolute cross section measured was 1.4×10−17 cm2. The dynamics of ionization processes (2) and (3) were elucidated from the energy dependences of the cross sections and from measurements of the angular and kinetic energy distributions of the product ions. The results support the conclusion that rearrangement ionization proceeds by a ’’spectator-stripping’’ mechanism. Both the UO+ from this reaction and the U+ from electron transfer are scattered predominantly in the forward direction.