Desorption of nitrogen from amorphous carbon under electron and swift heavy ion bombardment

Desorption of nitrogen adsorbates from amorphous carbon (a-C) surfaces due to swift ion (Z=6–73, q=6–54, energy 6–13 MeV/u) and electron impact (5 keV) was studied by secondary electron emission (SEE) measurements. The fluence dependence of ion induced electron emission obeys an exponential decay law as the coverage rate is in the sub-mono-layer range. The desorption yield σ varies approximately as σ∼(dE/dx)3/2 or σ∼q3 as a function of the electronic energy loss dE/dx and the projectile charge q, respectively. We discuss the possible desorption mechanisms connected to electronic excitation. Furthermore, we present an ESCA investigation on the mechanism for the influence of nitrogen adsorbates on SEE yields. The mechanism for the SEE enhancement effect is connected to the surface termination: nitrogen adsorbates are believed to provide surface states which generate a downwards energy band bending, which on relative scale pushes up the Fermi level towards the conduction band minimum, reducing the work function.