Laser fluorescence spectroscopy of sputtered uranium atoms

Abstract

Laser induced fluorescence (LIF) spectroscopy was used to study the sputtering of 99.8% 238U metal foil when bombarded by normally incident 500–3000 eV Ne +, Ar +, Kr + and O 2 +. A three-level atom model of the LIF processes is developed to interpret the observed fluorescent emission from the sputtered species. The model shows that close attention must be paid to the conditions under which the experiment is carried out as well as to details of the collision cascade theory of sputtering. Rigorous analysis shows that when properly applied, LIF can be used to investigate the predictions of sputtering theory as regards energy distributions of sputtered particles and for the determination of sputtering yields. The possibility that thermal emission may occur during sputtering can also be tested using the proposed model. It is shown that the velocity distribution (either the number density or flux density distribution, depending upon the experimental conditions) of the sputtered particles can be determined using the LIF technique and that this information can be used to obtain a description of the basis sputtering mechanisms. These matters are discussed using the U-atom fluorescence measurements as a basis. Both the v-parallel (the exciting laser beam is parallel to target surface) and the v-perpendicular (the laser beam is perpendicular to the target surface) number density velocity distributions were measured using the LIF Doppler shifted absorption frequencies of the sputtered ground state U(1) atoms. The results are compared with the collision cascade sputtering theory using the LIF model presented here. The relative sputtering yields for various incident ions on uranium were also measured for the first time using the LIF technique. A surprisingly high fraction of the sputtered uranium atoms were found to occupy the low lying metastable energy levels of U(I). The population of the sputtered metastable atoms were found approximately to obey a Boltzmann distribution with an effective temperature of (920 ± 100) K.