TEMPERATURE PROGRAMMED DESORPTION

Some basic features of the experiment are:

• Prepare a clean sample surface.
• Adsorb a gas phase species of interest.
• Monitor the desorption of that adsorbate with a mass spectrometer while increasing the substrate temperature in a controlled fashion.

The preparation of the surface is accomplished by the standard set of "magic" incantations that are unique to every surface. The ideal sample would be perfectly clean and perfectly ordered. Those qualities are checked by AES and LEED respectively.

In the past, small molecules, such as CO or H₂O have been the main focus of TPD studies. Recently larger molecules have functioned as adsorbates. We are studying the desorption of large organometallics from silicon surfaces, e.g. CF₃AuCNCH₃ from Si(100).

Adsorption needs to occur at a sufficiently low substrate temperature (depends on the system) and usually liquid nitrogen cooling is necessary, as we do in our case.

The sample temperature is heated (by a variety of means) in a controlled fashion. By far the most common approach is to increase the sample temperature linearly in time at rates between 0.5 K/sec up to 20 K/sec. The mass spectrometer is tuned to monitor one or more mass fragments that can be correlated to the sample temperature. The desorption temperature, the shape of the desorption peak, and how all these change with initial surface coverage and heating rate are all analyzed to provide information about the binding character of the adsorbate/substrate system. Here is a typical TPD spectrum of our CF₃AuCNCH₃/Si(100) system.