Adsorbate-induced roughening of nanosized palladium particles after coherent laser excitation

Femtosecond laser induced adsorption site changes of CO at nanoparticulate palladium aggregates have been investigated using laser light at lambda=400 nm and pulse lengths of 70 fs. The average sizes of the aggregates grown by evaporating palladium atoms onto an epitaxial Al2O3 film on NiAl(110) were varied between 100 and 6000 atoms per island. Amorphous aggregates grown at 120 K and crystalline aggregates grown at 300 K have been investigated. Beyond laser-induced migration of CO already reported for nanosecond experiments, adsorbate-induced surface roughening is apparent from Fourier-transform infrared reflection absorption spectroscopy. The laser-induced processes are correlated to the presence of higher local adsorbate densities and a substantial population of edge sites. A model is suggested attributing surface roughening to the formation of local hot spots after coherent electronic excitation via heating through highly vibrationally excited adsorbate states and energetic channelling along chains of elevated local adsorbate densities. The processes are most efficient for intermediate aggregate sizes. In contrast to nanosecond experiments, a few percent of CO molecules are desorbed during the femtosecond experiments from smaller aggregates.