[Zurück]

F. Aumayr, Hp. Winter:
"Potential Electron Emission from Metal and Insulator Surfaces";
in: "Slow Heavy-Particle Induced Electron Emission from Solid Surfaces", Hp. Winter, J. Burgdörfer (Hrg.); herausgegeben von: Springer Tracts in Modern Physics, Vol. 225; Springer-Verlag, 2007, ISBN: 978-3-540-70788-2, S. 79 - 112.

Impact of slow ions (impact velocity < 1 a.u. = 25 keV/amu) on solid surfaces
is of genuine interest in plasma- and surface physics, and related applications.
Nature and intensity of the resulting inelastic processes depend both on the
kinetic and the potential (= internal) ion energy carried toward the surface.
For most practical applications the kinetic projectile energy is of foremost relevance
as, e.g., in ion-induced kinetic electron emission (KE [1-4]), ion-surface
scattering and kinetic sputtering [5, 6]. However, ion-induced processes can
also depend on the internal (potential) energy of the projectile, especially if
the latter exceeds the kinetic projectile energy, resulting in additional electron
emission or sputtering (potential electron emission PE [4, 7-10], potential
sputtering [11-13]. Considerable potential energy will be stored in a multiply
charged ion (MCI) Zq+ during its production where q electrons are removed
from an originally neutral atom. The same potential energy will become again
available if the MCI encounters a solid surface.
PE due to impact of slow singly, doubly and multiply charged ions on
atomically clean metal surfaces has been thoroughly studied by Hagstrum [7,
8, 14, 15] who measured the yields and energy distributions of ion-induced
slow electrons. From these studies he concluded that PE arises from relatively
fast electronic transitions (rates >= 1014 sÅ|1) from the surface into empty
projectile states, which require no minimum impact velocity and already start
before the ion has entered the surface selvedge. PE yields increase strongly
with the projectile´s potential energy, i.e. their charge state. At higher impact
velocity also kinetic electron emission (KE [1-4]) will produce slow electrons
(see also Sect. 3.2 in the contribution of C. Lemell and J. Burgd¨orfer and
the contribution by Helmut Winter et al. to this book), the fraction of which
cannot simply be distinguished from the one due to PE. Based on theoretical
studies on "radiationless" electronic transitions between a metal surface and a
slow ion or excited atom by Massey [16-20], Hagstrum identified four types of
one- and two-electron transitions (a-d) as being relevant for PE (see Fig. 3.1).