Electron impact ionization cross sections of Cu and Au between 40 and 250 eV, and the velocity of evaporated atoms

The total ionization cross sections of Cu and Au were measured with a simple crossed-beam apparatus for bombarding electron energies between 40 and 250 eV. The maximum cross sections were σ (Cu)=(7.6 ± 1.1 × 10−16cm2 at 95 ± 10 eV and σ (Au) =(15.3 ± 2.1) × 10−16cm2 at 100 ± 10 eV. The ratio of the cross sections is σ (Au)/σ (Cu) =2.0 ± 0.2 between 50 and 180 eV. The shape of our cross section curve for Cu compares well with a previous measurement by Crawford. An evaporation model which predicts thermal equilibrium between a liquid and the atoms evaporating into a vacuum is discussed, and the average velocity of atoms evaporated from an evaporation boat is calculated.