Kinetic electron emission from clean polycrystalline gold induced by impact of slowC+,N+

Abstract

Ion-induced kinetic electron emission is commonly attributed to collisions of an energetic projectile with quasifree electrons, and to the promotion of atomic levels in binary collisions of the projectile with atomic particles in the solid. The contribution of the promotion processes to the electron emission has been estimated theoretically for all studied systems from molecular-orbital correlation diagrams. As quasifree electron collisional excitations have a sharp threshold at relatively high velocities of the projectiles, their contribution to the electron emission at lower impact velocities should be negligible. We will show, however, that the partial localization of the quasifree electrons due to the presence of the solid surface ''washes out'' this sharp threshold. This can lead to one-electron excitations at low impact velocities that may be more significant than excitations due to promotion. At the lowest impact velocities the electron emission yields conspicuously level off in some studied cases. Such behavior cannot be reconciled with any existing one-electron model ͑including the one proposed here͒, as they all predict a rapid decrease of the electron emission with decreasing impact velocity. In this paper we interpret the leveling-off of the yield in terms of a many-electron excitation mechanism, based on the assumption of spatial and temporal localizations of electronic excitation in the impact zone. The models discussed in this paper will be compared with experimental data on kinetic electron emission from polycrystalline gold bombarded by C ϩ , N ϩ , O ϩ , Ne ϩ , Ne 0 , Xe ϩ , and Au ϩ , with kinetic energies below ϳ15 keV, and perpendicular incidence on the surface.

Subsurface excitations in a metal

“…9,10,[17][18][19][20] For our measurements we can rule out the first two mechanisms as the incident energies employed are too low for close-collision-induced promotion and the projectiles are too simple ͑low Z͒ for multielectron effects to arise. Additionally, no Auger transitions analogous to those seen in previous systems exist for our projectiletarget combinations.…”

“…10,18,[22][23][24] Nonadiabaticity in this model is included via a time-varying ionsurface interaction that is characterized by the projectile velocity v, the potential V, and a distance-dependent parameter ␥ that accounts for the presence of the surface. 25 Applying this model to our system, we can obtain a probablity of kinetic electron excitation P KEE above the MOS barrier as…”

“…Therefore, the few experimental studies performed in this energy regime have been limited to measurements outside a target where emitted excitations ͑electrons͒ can be collected. 9,10 In particular, the work of Lakits et al used the detection of externally emitted electrons to provide precise statistical information on the yield of electrons emitted per incident ion. 9 Those data showed clear differences in the dependence of the yield on ion velocity, and distinct regimes for the yield were found that could be characterized by either potential emission ͑PE͒ or kinetic emission ͑KE͒.…”

Cryogenic micro-calorimeters for mass spectrometric identification of neutral molecules and molecular fragments

“…Studies of secondary electron emission from solid gold targets, however, have found only small secondary electron yields of Շ2 electrons per incident ion 66,67 and mean secondary electron energies of $10 eV (Ref. 68) for a broad range of projectile energies and projectile types.…”

Material Contrast of Scanning Electron and Ion Microscope Images of Metals

“…However, by our experiments, at energies from 1keV to 30 keV, γ for Ga + -ion impact decreases continuously with projectile energy and there is no cut off energy on the low energy side. Since the kinetic energy of the projectile for pKE must be several keV for the promotion of quasi-molecules 23 , it is reasonable to consider that the main process of electron emission for Ga + -ion impact is to be s-KE. For electron emission induced by ions with sub-threshold energies, several papers reported that calculated values of the threshold energies are higher than those of the experiment 29 and practical potential barriers across the surface are lower than the work function 30 .…”