Chemistry Reference
In-Depth Information
Fig. 7.1 Diffusion rate of H
and D on Cu(001) as a
function of temperature
(Arrhenius plot) between 80
and 9 K measured by atom
tracking technique. The
hopping rates of H and D are
represented by open circle
and cross, respectively.
Reprinted with permission
from Ref. [
13
,
14
]. Copyright
2000, American Physical
Society
the quantum delocalization of H [
7
] and ''the H-band model'', where H is
described in terms of the two-dimensional atomic energy bands. The first theo-
retical study on the quantum motion of chemisorbed hydrogen was reported by
Puska et al. in 1983 [
8
]. The H-band model was experimentally examined by Mate
and Somorjai in 1986 [
9
], in which they showed the evidence of the quantum
delocalization in the study of H atom on Rh(111) using high resolution electron
energy
loss
spectroscopy (HREELS). Quantum-delocalization of H atom
on
transition-metal surfaces was summarized by Nishijima et al. [
10
].
Diffusion is a common process of H atom on metal surfaces. The diffusion
behavior was first investigated by means of field emission microscopy in 1957
[
11
,
12
]. More recently, the unique dynamics of H was directly observed on a
metal surface using variable temperature STM. In 2000 Lauhon and Ho showed
the clear evidence of quantum diffusion of H atom on Cu(100) [
13
,
14
]. They
investigated the temperature dependence of the diffusion rate using the atom
tracking technique of STM. They showed an explicit deviation from Arrnenius
equation and the rate becomes plateau below 60 K (Fig.
7.1
). The diffusion was
described as incoherent tunneling in the presence of lattice and electronic exci-
tations (electron and phonon scattering cause decoherence of the H-atom wave
function and particle localization). Interestingly, it was found that tunneling plays
a crucial role even in heavy atoms and molecule [
15
-
17
], where the tunneling of
Cu and Co atoms and CO molecule was characterized on metal surfaces by low-
temperature STM. These results suggested that the tunneling is feasible even for
heavy particles within a reduced potential in the atomic-scale distances.
?A3B2 twb=.25w?>Hydroxyls on surfaces have been discussed in conjunction
with the structure of a first wetting layer [
18
,
19
] and H/proton transfer processes
[
20
]. In general, a spontaneous dissociation of water molecules on transition metal
surfaces is not feasible at low temperature. However, the situation changes
considerably as following cases; (i) a surface is heated and thermally activated
processes takes place. (ii) Pre-adsorbed oxygen atoms exist on the surface.
For instance, the water dissociation was observed on the oxygen-covered Pt(111)
