Zusammenfassung
A new microcalorimeter for measuring heats of adsorption on clean
single-crystal surfaces is described, and its operational characteristics
are presented. The principle is similar to that pioneered by David
King's group: A pulse of gas from a molecular beam adsorbs on an
ultrathin single crystal's surface, causing a measurable transient
heat input and temperature rise. Our novel heat detector is a 9 mu
m thick pyroelectric polymer ribbon, which is mechanically driven
to make a gentle mechanical/thermal contact to the back of the single-crystal
sample during measurements. Advantages include use of thicker samples
(1 mu m), sample preparation at very high temperatures, and potential
measurements at cryogenic temperatures. A novel chopped molecular
beam of metal vapor and a method of correcting for absorbed radiation
from the hot effusion cell are also described. This system is applied
to study the heats of adsorption of metals on clean, well-defined
and single-crystalline surfaces as a detailed function of coverage,
from which metal/substrate adhesion energies can also be extracted.
We obtain pulse-to-pulse standard deviations of <2% in the heat of
adsorption for pulses containing <0.03 ML of Cu and Pb, and absolute
accuracy to within a few percent.
Nutzer