Thermal desorption spectroscopy

When experiments are performed using well-defined surfaces of single-crystalline samples in a continuously pumped ultra-high vacuum (UHV) chamber, then this experimental technique is often also referred to as thermal desorption spectroscopy or thermal desorption spectrometry (TDS).

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.

TPD also obtains the amounts of adsorbed molecules on the surface from the intensity of the peaks of the TPD spectrum, and the total amount of adsorbed species is shown by the integral of the spectrum.

The amount of adsorbed molecules is measured by increasing the temperature at a heating rate of typically 2 K/s to 10 K/s.

TDS spectrum 1 and 2 are typical examples of a TPD measurement.

Both are examples of NO desorbing from a single crystal in high vacuum.

The crystal was mounted on a titanium filament and heated with current.

Before 1990 analysis of a TPD spectrum was usually done using a so-called simplified method; the "Redhead" method,[3] assuming the exponential prefactor and the desorption energy to be independent of the surface coverage.

[5] These methods assume the exponential prefactor and the desorption energy to be dependent of the surface coverage.

Several available methods of analyzing TDS are described and compared in an article by A.M. de JONG and J.W.

[6] During parameter optimization/estimation, using the integral has been found to create a more well behaved objective function than the differential.

where This equation is difficult in practice while several variables are a function of the coverage and influence each other.

[8] The “complete analysis method” calculates the pre-exponential factor and the activation energy at several coverages.

First we assume the pre-exponential factor and the activation energy to be independent of the coverage.

(equation 1) where: We assume that the pump rate of the system is indefinitely large, thus no gasses will absorb during the desorption.

is indefinitely large so molecules do not re-adsorp during desorption process and we assume that

One can use data in an experiment, which are a function of the pressure like the intensity of a mass spectrometer, to determine the desorption rate.

Since we assumed the pre-exponential factor and the activation energy to be independent of the coverage.

for n=2 M. Ehasi and K. Christmann[9][10] described a simple method to determine the activation energy of the second order.

TDS Spectrum 1 A thermal desorption spectrum of NO absorbed on platinum-rhodium (100) single crystal. The x axis is temperature in kelvins , the unit of the y axis is arbitrary, in fact the intensity of a mass-spectrometer measurement.
TDS Spectrum 2 A thermal desorption spectrum of NO absorbed on platinum-rhodium (100) single crystal. The spectra of several NO coverages are combined in one spectrum. The x axis is temperature in kelvins, the unit of the y axis is arbitrary, in fact the intensity of a mass-spectrometer measurement.