Dissociative chemisorption of hydrogen on Ir(111): Evidence for terminal site adsorption

Dissociative chemisorption of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ on the Ir(111) surface at liquid nitrogen temperature has been studied with high-resolution electron energy loss spectroscopy and thermal desorption mass spectrometry. Quantum delocalized motion of the H adatoms parallel to the surface is responsible for the absence of a significant isotopic frequency shift in the vibrational spectra collected for low surface coverages of H and D. Vibrational coupling between this delocalized motion and iridium phonon modes is suggested by the broad linewidth of these low frequency loss features. As the surface coverage is increased, hydrogen adatoms shift into the terminal adsorption site from the delocalized overlayer that is present at low coverage. The high vibrational frequency of 2030 ${\mathrm{cm}}^{\mathrm{-}1}$ (1460 ${\mathrm{cm}}^{\mathrm{-}1}$) observed for high surface coverages of H (D) adatoms enables unambiguous assignment of this mode to the perpendicular motion of the adatoms adsorbed at the terminal site. This is evidence of terminal site hydrogen adsorption on unmodified single-crystalline transition metal surfaces under ultrahigh vacuum conditions.