| AVS 54th International Symposium | |
| Vacuum Technology | Thursday Sessions |
| Session VT1-ThA |
| Session: | Adsorption/Desorption Phenomena on Vacuum Materials |
| Presenter: | P. Thissen, Max-Planck Institut for Iron Research, Germany |
| Authors: | P. Thissen, Max-Planck Institut for Iron Research, Germany O. Ozcan, Max-Planck Institut for Iron Research, Germany D. Diesing, Institut of Physical Chemistry Essen, Germany G. Grundmeier, Institut of Macromoleculare Chemistry Paderborn, Germany |
| Correspondent: | Click to Email |
Temperature-programmed desorption (TPD) techniques are important methods for the determination of kinetic and thermodynamic parameters of desorption processes or decomposition reactions. A sample is heated with a temperature program ß(t) = dT/dt (with the temperature T usually being a linear function of the time t) and the partial pressures of atoms and molecules evolving from the sample are measured, e.g. by mass spectrometry. 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 (TDS). A Monte Carlo model has been developed for describing the temperature-programmed desorption of adsorbates from single crystal surfaces. The developed Monte Carlo Program requires the input of frequency factors and unity bond order binding energies BE (for the top position) for every bond under examination. For the first time the required values are now calculated using a DFT code. The virtue of both methods (Monte Carlo and DFT) is combined in the present work. Our new model takes into account the effects of surface diffusion, the influence of surface-adsorbate (S-A) and adsorbate-adsorbate (A-A) interactions and the coverage dependence of the activation energy for desorption derived by precise calculations on an atomically defined level. The inclusion of localized (S-A) and (A-A) interactions has a pronounced effect on the shape of the predicted TPD spectrum. Only a single peak is observed in the absence of (S-A) and (A-A) interactions, whereas multiple peaks are found when these interactions are included. The inclusion of (S-A) and (A-A) interactions is also shown to produce a nonlinear decline in the activation energy for desorption as a function of increasing adsorbate coverage.