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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.
- What do we need to know in order to control desorption?
- What do we learn about the surface from studying desorption?
- Any possible applications?
- What is the surface we desorb?
- How to register desorbing species?
- How to control what is desorbed and from where?
- How to observe what happened to the surface?
- What is the mechanism of desorption?
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
- Type of irradiation source
- Energy, intensity
- Number of sources, delay between pulses
What do we create first
- Electron - hole pairs
- Excitons
- Point defects
- Defect aggregates
Desorption products
- Atoms, ions
- Electronic states - Br(2P3/2) & Br*(2P1/2)
- Kinetic energy
- Angular distribution
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.
- Code - GUESS (Gaussian98)
- QM cluster - up to 50 atoms
- Classical region - shell model
- Region I (relaxed) - up to 700 atoms
- B3LYP functional - ground states
- TD-DFT - excited states
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