Desorption
Desorption is the physical process where adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid.
The surface bond of a sorbant can be cleaved thermally, through chemical reactions or by radiation, all which may result in desorption of the species.
In the case of zeroth order, n = 0, the desorption will continue to increase with temperature until a sudden drop once all the molecules have been desorbed.
[4] In a typical thermal desorption experiment, one would often assume a constant heating of the sample, and so temperature will increase linearly with time.
For example, Redhead's peak maximum method[5] is one of the ways to determine the activation energy in desorption experiments.
However a drawback of this method, is that the rate constant in the Polanyi-Wigner equation and the activation energy are assumed to be independent of the surface coverage.
[5] Due to improvement in computational power, there are now several ways to perform thermal desorption analysis without assuming independence of the rate constant and activation energy.
[8] Another analysis technique involves simulating thermal desorption spectra and comparing to experimental data.
This technique relies on kinetic Monte Carlo simulations and requires an understanding of the lattice interactions of the adsorbed atoms.
These interactions are described from first principles by the Lattice Gas Hamiltonian, which varies depending on the arrangement of the atoms.
[9] In some cases, the adsorbed molecule is chemically bonded to the surface/material, providing a strong adhesion and limiting desorption.
As ionisation is required for this process, this suggests the atom cannot desorb at low excitation energies, which agrees with experimental data on electron simulated desorption.
In particular, in the beam vacuum systems the desorption of gases can strongly impact the accelerators performance by modifying the secondary electron yield of the surfaces.
This relaxation of the bonds together with a sufficient energy exchange from the incident light to the system will eventually lead to desorption.
[14] Generally, the phenomenon is more effective for weaker-bound physisorbed species, which have a smaller adsorption potential depth compared to that of the chemisorbed ones.
In fact, a shallower potential requires lower laser intensities to set a molecule free from the surface and make IR-photodesorption experiments feasible, because the measured desorption times are usually longer than the inverse of the other relaxation rates in the problem.
[15] The mode was discovered whilst investigating bromine absorbed on silicone using scanning tunnelling microscopy.
Instead, the optical phonons of the Silicon weaken the surface bond through vibrations and also provide the energy for electron to excite to the antibonding state.
Temperature programmed desorption (TPD) is one of the most widely used surface analysis techniques available for materials research science.
This physical process is designed to remove contaminants at relatively low temperatures, ranging from 90 to 560 °C, from the solid matrix.
[19] Thermal desorption systems operate at a lower design temperature, which is sufficiently high to achieve adequate volatilization of organic contaminants.
