. A pulsed molecular beam is directed at a very
thin single crystal (about 2000 Å thick). A
proportion of these molecules adsorb at the surface and cause the
liberation of adsorption heat within the surface region. The small
heat capacity of the crystal leads to a measurable rise in
temperature (the adsorption of 1 percent of a monolayer typically
leads to a temperature rise of 0.1 K).
The heat is conducted very efficiently to the back of the crystal and
very in-efficiently to the sides. This means that the cooling of the
crystal occurs mostly by the emission of thermal radiation. For this
reason the back-side of the crystal is made black with a carbon film
such that it will emit most of the radiation which is then measured
with an infrared detector. The detector measures a short infrared
pulse and the crystal is cooled down again before the next pulse of
molecules arrives.
Figure: A single-crystal adsorption calorimeter.