The laser induced fluorescence technique has been used to determine the population ratio of three metastable states, 620, 4, 275 and 7, 005cm^-1, to the ground state of ²³⁸U which was evaporated into vacuum by electron beam heating. The population ratio of metastable states of 620, 4, 275 and 7, 005cm^-1 decreased by increasing the temperature of evaporation surface in the range of 2, 700-3, 000K, and the rate of decrease was larger for the metastable states with higher energies. Excitation temperatures of the metastable states, 4, 275 and 7, 005cm^-1 were 4, 500 and 3, 300K, respectively, at the surface temperature of melt, 2700K, and were higher than the surface temperature of melt. The excitation temperatures were cooled down rapidly to 2, 800 and 2, 200K, respectively, by increasing the surface temperature to 3, 000K.
The experimental results indicate that ²³⁸U atoms are excited not only by thermal excitation but also by electron impact excitation. Relaxation of metastable atoms is due to the collision of atoms of which density above the evaporation surface increases with the increase of the surface temperature.