Kinetic electron emission from Al surfaces by slow ions

Abstract

We measured energy distributions of electrons emitted in the interaction of Na + with Al surfaces at incident ion energies in the range 150-4000 eV. The data allow to correlate emission intensities with spectral signatures of electron excitation processes. We find a remarkable contribution to electron emission from asymmetric collisions between incoming ions that have survived neutralization at the surface and target atoms, leading to Al-2p excitation via a vacancy transfer process. We observe that the total electron emission yields sharp increases by more than an order of magnitude at impact energies above the threshold for this process.

Kinetic electron emission from metals induced by impact of slow atomic particles

“…The model in (8)(9)(10)(11) describes the substrate as an electron gas and assumes that multiple electron excitations occur around the particle. Such a concept is compatible with an intuitive picture of a strong, local perturbation caused by the moving atomic particle in metal.…”

“…For example, in Na-Al systems (not shown in Fig. 1 and Table 1) Na and Al Auger electrons and Al bulk plasmons contribute significantly to the sub-threshold emission [8]. Similarly, an unidentified promotion mechanism in Xe-Au is apparently dominant in the KEE for 1/m < 40.The difficulty to interpret the 1/m dependence and to determine c e precisely in this case is indicated by the question mark in the table.…”

“…(c) The localized nature of deeper substrate-particle electronic levels may cause promotions of these levels during close collisions and creation of deep holes in some substrates. Subsequent decay of these holes by Auger processes creates high energy electrons with usually clearly defined energy spectra well above the maximum excitation energy (1), even for particles with m < m th [8].…”

Kinetic electron emission due to perpendicular impact of carbon ions on tungsten surfaces

“…The promotional processes, leading to these excitations, are expected to have sharp thresholds in the velocity of the primary particle. However, significant KEE has been observed [25] at primary particle velocities well below the threshold for the excitation of the shallowest deep level of the system (Na 3p level). This observation supports the conjecture that the gross features of KEE are not determined by promotion processes, though the detailed atomic-like features of KEE observed in some systems are undoubtedly due to deep-level excitations.…”

“…One of the possible contributions to this sub-threshold emission is the electronic excitation due to the collision-induced promotion of deeper atomic levels. However, as we already mentioned the subthreshold KEE cannot be fully accounted for by the orbital promotion models due to the following arguments which are based on already published experiments [5,22,25]: (i) the KEE spectra are mostly without atomic features that would be a clear signature of core level promotion processes. (ii) As the relative positions of deep levels vary considerably with the used projectiles and substrates, one would expect that KEE and its velocity dependence strongly varies with various projectile-substrate systems.…”