# Optical Radiation during Sputtering of Lithium into a Noble Gas Using a Nanosecond Electron Beam
## 4 Discussion  
As can be seen from Figure 4, with an increase in the lithium layer temperature from 670 to 870 K, the lithium line intensity increases from 3 to 4 times, while the saturation vapor pressure of the lithium at these temperatures increases by \(\approx\)500 times. The radiation on the lines of the lithium atom cannot be explained by the thermal evaporation of the lithium. Consequently, the lithium vapor density, which is different from the density of the saturated steam, is generated in the gas excitation area. The sputtering mechanism here seems to have been somewhat different, unrelated to the displacement of the crystal lattice atoms and analogous to the electron sputtering caused by the MeV-ions. A major part of the energy of the heavy ions is transferred to the electrons along the ion track. A coupling between the electrons and phonons causes  
Figure 6: Oscillograms of radiation on lines of lithium 670.8 nm (1, in red), 610.4 nm (2, in blue), argon 696.5 nm (3, in black scatter, in Figure 6a), krypton 760.2 nm (4, in black scatter, in Figure 6b), and xenon 828.0 nm (5, in black scatter, in Figure 6c). The initial pressure at 295 K of argon and krypton is 40 kPa, xenon 10 kPa, and lithium layer temperature 870 K (**a**), 820 K (**b**), and 770 K (**c**). large local heating in cylindrical volume. The surface atoms may be removed through evaporation in a jet from the heated volume [6]. The lithium at temperatures 1.5-2 times the melting point (453 K) was in a liquid state. This facilitates lithium evaporation in the fast electron track. As far as we know, there is no developed theory of sputtering from liquid metal. The optical radiation arises from both at the 2p-1s transitions of the noble gas atoms and the transitions of the lithium atom in a time of about 20-30 ns (see Figure 6). The characteristic time of the plasma-chemical reactions in a gas is hundreds of nanoseconds; the luminescence of the lithium during the initial period cannot be explained by the processes in the plasma. In [1], upon the irradiation of a lithium layer at a residual gas pressure of 10 Pa with the products of the \({}^{6}\)Li(n,\(\alpha\))\({}^{3}\)H nuclear reaction, the radiation intensity of the lithium lines was at least \(10^{4}\) times lower than during irradiation in a gaseous medium. The emission of the excited atoms during lithium sputtering by the products of the nuclear reaction does not contribute much to the emission of the lithium atoms, as compared with the plasma-chemical processes in the noble gas medium using sputtered lithium. Apparently, in the case of bombardment caused by fast electrons, the radiation in the lithium lines during the initial period cannot also be explained by the sputtering of the excited lithium atoms under the influence of an electronic beam. The similarity of the time dependence of the intensities of the noble gas and lithium lines suggests that the optical radiation from the noble gas atoms and lithium atoms during this period is formed by the same processes. The radiation on the lines of the 2p-1s transition of the noble gas atoms in the initial period is associated with the excitation of atoms caused by secondary electrons [21]. The excitation of the levels of noble gas atoms arises in the processes with electrons, with energies above the electron excitation threshold (Im); the Im for argon is 11.55 eV and for xenon is 8.315 eV. When xenon is irradiated with argon ions at 70 MeV, about 12% of the radiation is due to direct excitation caused by secondary electrons [22]. Most of the optical radiation from the noble gases is due to the dissociative recombination of the molecular ions (R\({}_{2}^{+}\), R\({}_{3}^{+}\)) with the electrons in the plasma [23; 24]. In the sub-threshold region (energy of the electrons is less than Im), the electrons lose their energy in elastic collisions with noble gas atoms, as well as during the excitation of the lithium atom levels. The thermalization time of the electrons in noble gases is 5-10 ns [21].