The interaction of K with Si(100)(2 x 1)-H has been studied by using t
hermal desorption and electron energy loss spectroscopy. We have found
that above 360 K potassium adsorption induces concomitant H-2 desorpt
ion. This mechanism has been ascribed to a two step process consisting
of the reaction of an incoming K atom with surface Sill species to fo
rm a surface complex, followed by the decomposition of this complex to
give rise to the H-2 desorption. We have found that the isolated reac
tion intermediate, whose decomposition leads to the H-2 desorption aro
und 450 K, is formed by the deposition of a small amount of K at 90 K.
For the deposition of a large amount of K, the reaction intermediates
form two types of potassium hydrides upon annealing and give rise to
sharp H-2 desorption peaks at 360 and 380 K. Analysis of the K uptake
curves above 360 K in terms of the Kisliuk model shows that K adsorpti
on occurs via a precursor-mediated process, which is similar to that o
n clean Si(100)(2 x 1) but with a much low probability(< 1/10) for the
precursor to chemisorb. We suggest that the low probability for chemi
sorption is due to the activation barrier, estimated to be similar to
3 kcal/mol, for the precursor K atoms to react with surface SiH specie
s.