Surface Ionization. I. Desorption of U+ Ions from W and Re Surfaces

The mean residence times (τ) of U⁺ ions on polycrystalline W and Re surfaces have been measured as functions of the surface temperature. The U⁺ desorption parameters that were determined from the relationship 1/τ=ω₀ exp[—E_i/kT] are $$\begin{array}{ccc}\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\mathrm{Surface}& {\omega }_0({\sec }^{\text{−1}})& E_i(\mathrm{eV})\\ & & \\ \mathrm{Clean\; W}& {\text{(3.2±1.5)×10}}^{15}& \text{7.50±0.09}\\ \mathrm{Contaminated\; W}& {\text{(1.9±1.5)×10}}^{13}& \text{6.29±0.17}\\ \mathrm{Clean\; Re}& {\text{(0.7±0.3)×10}}^{13}& \text{6.70±0.09}\\ \mathrm{Slightly\; contaminated\; Re}& {\text{(1.2±0.7)×10}}^{13}& \text{6.84±0.14.}\end{array}$$ The effect of oxygen contamination on the W surfaces was dramatic. The Re results were not as clear cut. In the temperature range where uranium oxide ions were abundant, the simple desorption mechanism is complicated by surface reactions. The resulting log(τ⁻¹) vs T⁻¹ relationship is complex and nonlinear.