Department of Physics and Astronomy
Condensed Matter and Materials Physics
Alex Shluger's group
UCL Logo

Photo-induced desorption of atoms and ions from surfaces of ionic materials (pdf presentation)

Introduction

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.

Desorption of alkali halides

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

Desorption products

Laser excitation: Experimental setup

Atoms emitted from the surface are resonantly ionized and then detected in MS

Typical velocity profiles

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

Theoretical modelling can predict the mechanism: Embedded cluster method

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 - Telephone: +44 (0)20 7679 2000 - Copyright © 1999-2005 UCL


Search by Google