Temperature Programmed Desorption o Thermal Desorption Spectroscopy is a standard surface science technique which has conventionally been used to provide information on the binding energies of atomic and molecular species adsorbed on a solid surface. The process of thermal desorption, on which this technique is based, is the mechanism with which an adsorbate leaves the substrate upon annealing and enters the gas phase. Desorption takes place if a molecule has enough energy to overcome the activation barrier for desorption.
The process of thermal desorption can be formally described in terms of an Arrhenius equation, known as the Polanyi-Wigner equation:
R = -dΘ/dt = νn Θn exp(-Ed/kBT)
which basically states that the desorption rate (R) of a given species (R) is related to the actual surface coverage (Θ) and to the kinetic order of the process (n) by a power law, and depends on the desorption energy (Ed) through a Boltzmann factor.
In a typical thermal desorption experiment, the sample is first exposed to a flux of molecules at a given temperature, until the desired initial surface coverage is reached. The sample is subsequently placed in front of a quadrupole mass spectrometer and its temperature is increased at a constant rate, in such a way to induce the thermal desorption of the adsorbed species (either atoms or molecules). During the experiment, the partial gas pressures are measured, in such a way to monitor the desorption rate of each atomic/molecular species as a function of the temperature.
A basic assumption in TPD, in fact, is that the desorption rate of an adsorbate is proportional to the corresponding measured partial pressure. This conditionis rigorously met only if a high enough pumping speed is attainable in the UHV chamber.
Another important requirement in TPD is to keep a constant heating rate during the experiment, a condition that can be realised by an electronically controlled ramping of the temperature.
Provided these requirements are satisfied, the measured increase in partial pressure as a function of time can be fitted to the model equation above in order to obtain the relevant parameters, in particular the desorption energy. The analysis of the desorption curves (in particular the position of the desorption maxima and the shape of the spectra) also provides information on the desorption kinetics and on the adsorption mechanisms of the different species on the substrate.
|Thermal Desorption Spectroscopy|