Population inversion of metastable Ni atoms sputtered from Ni(100), Ni3Al(100), and NiAl(110)

We have measured the ratio of the population of Ni neutral atoms sputtered into the a³D₃ metastable state to that into the a³F₄ ground state using two-color ionization schemes. The ratio is 2.27±0.31 for Ni, 2.34±0.17 for Ni₃Al, and 2.61±0.27 for NiAl. Within experimental error, we observed no effect of the valence band electron structure on the population ratio. We suggest that sputtered atoms should experience resonant neutralization and nonradiative relaxation. Before neutralization, departing Ni ions from the surface have an electron configuration of 3d⁹ or 3d⁸4s¹. The 4s electron orbital of the departing ions captures a valence electron and formed nascent Ni atoms in the D (3d⁹4s¹) and F (3d⁸4s¹) states, respectively. The population inversion can be attributed to the character of ions and atoms of Ni. Because of the character of Ni ions, most of the ejected nascent Ni atoms are in the D state. The character of Ni atoms leads to a low relaxation rate of the D to F state. Many of the nascent Ni atoms in the D state can survive and so the population inversion was observed.