Electronic excitations of solids create bulk and surface defects and can induce surface decomposition and particle emission and lead to desorption of adsorbed species.
Techniques such as desorption induced by electronic transitions (DIET), laser ablation and photon-induced desorption have many technological applications.
Irradiation sources used in these techniques include ion and electron beams, x-rays and lasers. These induce a variety of defect formation processes that are difficult to distinguish or control.
One of the major challenges, therefore, is to understand the mechanisms of defect formation, diffusion and inter-conversion under the given irradiation conditions and to achieve control over the desorption products.
Different types of irradiation create emission of electrons and surface atoms.
The surface evolves with square sided holes as can be seen in this AFM image (right).
Parameters of irradiation
What do we create first
Atoms emitted from the surface are resonantly ionized and then detected in MS
Velocity profile of desorbing Br atoms depends on the laser energy and intensity of irradiation (K.M.Beck et al., Phys. Rev. B, 63, 125423, (2001))
Models: Hyper-thermal desorption - decay of the near-surface exciton, Near-thermal desorption – diffusion of H centres to the surface
Modelling a step on MgO using a quantum cluster (Mg15O15) embedded into classical region.
Desorption of MgO proceeds in the following way according to the quantum mechanical calculations:
Step 1:Selective excitation of O corners with 4.7 eV photons
Step 2:Exciton relaxation into the lowest triplet state
Step 3:Ionisation of the triplet exciton
Step 4:Excitation of the O- corner centre
Step 5:Spontaneous desorption of O atom
University College London - Gower Street - London - WC1E 6BT - +44 (0)20 7679 2000 - Copyright © 1999-2005 UCL