Investigation on the quantum motion of H on transition metal surfaces

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H is the lightest element and thus the motion of H is often described by not classical but quantu mechanics, As for H on metal surfaces, the activation energy of for surface diffusion is almost tr same order as the vibrational energy of H.Therefore, H in the vibrational excited states can be expected to be quantum-mechanically delocalized and thus the vibrational level is described by atomic band model just like an electron in the periodical potential well (Bloch state). The theo ret calculation for H on Ni surfaces indicates that the vibrational excited levels have widths of a few. several tens meV.We have investigated the vibrational spectra of Hon Pd(110), Ni(111) and Rh(111) at 90 K by using high resolution electron energy loss spectroscopy (HREELS). For H on Pd(110) surface, three losses are observed at 87-89, 96-100 and 121-122 meV for low H coverage regions. The coverage dependences of the 87-89 and 96-100 meV losses are compar with the model calculation based on the quantum delocalization of H.The agreement between experimental and theoretical results is reasonably good, and hence it is concluded that H in the vibrational state is quantum-mechanically delocalized on Pd(110). The vibrational spectra for H Ni(111) has been investigated with the energy resolution of 1-2 meV.For the (2x2)-2H phase, asymmetric stretching modes for H in hcp and fcc sites are observed at 90 and 96 meV.The wi of these losses are narrow (about 2 meV). As the H coverage is decreased, weak but broad structure ranging from 60 to 110 meV is observed which may be related to the quantum motior H.For H on Rh(111), EELS spectra do not show any evidence of the quantum delocalization of H However, energy resolution is not so good (7-8 meV) and thus this system should be reinvestigated with much high energy resolution.