Mean atomic velocities of uranium, titanium and copper during electron beam evaporation

Mean atomic velocities of uranium, titanium and copper during electron beam evaporation of the metal were measured by a microbalance technique as a function of evaporating metal surface temperature. In the measured temperature range, the mean velocity of uranium was up to 2.1 times the thermal mean velocity v_th, corresponding to the surface temperature of the evaporating metal. Mean velocities of titanium and copper increased to 1.6 and 1.3 times v_th, respectively. This meant that mean velocities of uranium and titanium exceeded the maximum flow speed of an ideal gas in adiabatic expansion, i.e. 1.4 times v_th, while the mean velocity of copper was almost equal to it. The electronic states of uranium and titanium were thermally excited to higher levels, then such excited energy should be converted to kinetic energy during adiabatic expansion. However, copper was rarely excited to higher levels because of the their fewer number. As a result, mean velocities of uranium and titanium were faster than the maximum flow speed of ideal gas.