Fig. 4.

Vibrational (high-resolution electron energy loss) spectra provide evidence of dihydrogen dissociation on palladium islands and migration of hydrogen atoms to silver. (A) Spectra taken after exposing dihydrogen to 4.00 ML of palladium on Ag(111) at 150 K are characteristic of palladium, showing peaks at 1,080 and 700 cm⁻¹, corresponding to vibrations of hydrogen atoms perpendicular and parallel to the surface, respectively. (B) Analogous data for palladium islands (θ_Pd = 0.5 ML) on Ag(111) show spectra at dihydrogen exposure of 10 and 50 L; at the higher exposure a new peak at 830 cm⁻¹ is observed and is assigned to hydrogen atoms bound to silver based on comparison to literature data for hydrogen atoms on Ag(111) at 850 cm⁻¹ and theoretical calculations. A quantitative interpretation of the intensities is not possible because such changes may be due to a change in sample reflectivity upon hydrogen atom adsorption and/or to the possible presence of subsurface hydrogen atoms (57). All vibrational spectra were collected at 100 K with a typical full width at half maximum of 48 cm⁻¹ for the elastic peak.