The adsorption of small alkynes on Pd(100) at 80 K has been studied using r
eflection-absorption infrared spectroscopy (RAIRS). Ethyne is found to form
a physisorbed multilayer at 80 K, which converts to a chemisorbed monolaye
r when heated above 100 K. The ethyne is adsorbed with the C=C bond paralle
l to the surface, through a di-sigma+di-pi interaction, and the H atoms til
ted away from the surface. Propyne exhibits similar behaviour: at 80 K a ph
ysisorbed multilayer forms which converts to a chemisorbed monolayer on hea
ting to 100 K. Again adsorption is via the C=C bond with the H and CH3 grou
ps tilted away from surface parallel. A disordered multilayer forms above t
he chemisorbed layer. Small doses were found to chemisorb directly at 80 K.
But-2-yne chemisorbed directly via the C=C bond at 80 K, and further addit
ion of but-2-yne formed a disordered, physisorbed multilayer above the chem
isorbed layer. The geometry of the chemisorbed species is discussed; ethyne
is assigned conclusively to a configuration in which the HCCH plane is nor
mal to the surface, but the spectra obtained for propyne and but-2-yne are
consistent with either this or a configuration in which the HCCC/CCCC plane
is tilted with respect to the surface normal.