Unusual adsorption kinetics of formic acid on Cu(1 0 0) studied by dc resistance and nonresonant infrared reflectance changes

The combination of in situ dc resistance measurements and simultaneous measurements of the nonresonant infrared reflectance provides a sensitive, noninvasive, real-time probe of adsorption kinetics on thin metal films. The two techniques are complementary, with the resistance sensitive to first-layer adsorption while the reflectance shows effects due to both first-layer and multilayer adsorption. For formic acid adsorption on Cu(1 0 0), the sticking coefficient for the first monolayer is at least 40 times smaller than for subsequent layers, and is therefore no larger than a few percent. An activation barrier to chemisorption is excluded by the weak and nonmonotonic dependence of the initial sticking coefficient on surface temperature. The rate-limiting step for first-layer adsorption is probably trapping into a weakly bound precursor state, with inefficient energy transfer to the surface leading to a low trapping rate. The additional vibrational modes available in the adsorbed layer lead to a greatly enhanced trapping rate for subsequent layers, resulting in rapid multilayer growth. The transition from slow first-layer growth to fast multilayer growth is not immediate, however, suggesting that the increased trapping becomes significant only once multilayer clusters reach a critical size.