Thermal desorption spectroscopy

Thermal desorption spectroscopy (TDS), also known as temperature programmed desorption (TPD) is the method of observing desorbed molecules from a surface when the surface temperature is increased. Many researchers prefer the name TPD because it is not a spectroscopic method.

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When molecules comes in contact with a surface, they adsorb onto it, minimizing their energy by forming a chemical bond with the surface. The binding energy varies with the combination of the adsorbate and surface. If the surface is heated, at one point, the energy transferred to the adsorbed species will cause it to desorb. The temperature at which this happens is known as the desorption temperature. Thus TDS shows information on the binding energy.

Since TDS observes the mass of desorped molecules, it shows what molecules are adsorbed on the surface. Moreover, TDS recognizes the different adsorption conditions of the same molecule from the differences between the desorption temperatures of molecules desorping different sites at the surface, e.g. terrasses vs steps. TDS also obtains the amounts of adsorbed molecules on the surface from the intensity of the peaks of the TDS spectrum, and the total amount of adsorped species is shown by the integral of the spectrum.

To measure TDS, one needs a mass spectrometer, such as a quadrupole mass spectrometer or a time-of-flight (TOF) mass spectrometer, under ultrahigh vacuum (UHV) conditions. The amount of adsorbed molecules is counted by increasing the temperature at a heating rate of typically 2K/s-10 K/s. Several masses are simultaneously counted by the mass spectrometer, and the intensity of each mass as a function of temperature is obtained as a TDS spectrum.

The heating procedure is often controlled by a computer or a Eurotherm using a PID algorithm.

Quantitative Intrepetation of TDS data

This is usually done using the "Redhead" method^[1], assuming the exponential prefactor and the desorption energy to be independent of the surface coverage.

1. ^ Redhead, P. A. "Thermal desorption of gases" Vacuum 12 (1962) p.203-211