Band structure effects in ejection of Ni atoms in fine structure states.

The formation of excited atomic electronic states subsequent to keV ion bombardment of metals has been a research focus for nearly three decades in order to establish the role of inelastic energy transfer in electronic device fabrication and to further the basic understanding of ionsolid interactions. The consensus resulting from quantumstate specific kinetic energy distribution measurements of sputtered particles is that the final population of metastable excited states is dominated by nonradiative deexcitation events that depend largely on the magnitude of the energy gap between the ground and excited state [1]. More recently, experiments with ion-bombarded In [2] and Rh [3] metal, using multiphoton ionization (MPI) for detection of quantum-specific excitations, suggest that the character of the electronic state is at least as important as the magnitude of the energy gap in determining the nonradiative relaxation rate and hence the final population. In this Letter we report on a systematic study of the energy distributions and populations of Ni atoms ejected from an ion-bombarded Ni{001} crystal. This system possesses the essential attributes necessary to disentangle the influence of the magnitude of the excitation energy from the electronic state character on the final populations since there are two distinct electronic configurations 3d 8 4s 2 ha 3 F4,3,2j and 3d 9 4s 1 ha 3 D3,2,1 and a 1 D2j that have closely spaced and intertwined energy levels [4]. In contrast to previous studies [5] of metastable states of Ni, our results show for the first time that the peak position of the kinetic energy distribution depends solely on the electronic structure of the sputtered atom. Moreover, the populations exhibit a remarkable behavior in that the excited 3 D3,2 states are more heavily populated than the ground 3 F4 state, a result consistent with the D-like character