Chemistry Reference
In-Depth Information
Chapter 7
Hydroxyl Group: Tunneling Dynamics
of Hydrogen Atom
Abstract I describe the production and characterization of isolated hydroxyl
species on a Cu(110) surface in this chapter. A hydroxyl group can be produced by
the STM-induced dissociation of a water molecule. A hydroxyl can be further
dissociated into atomic oxygen. It is found that a hydroxyl has an inclined
geometry against the surface normal and switches back and forth between the two
orientations via the H atom tunneling. The switching results in the characteristic
paired depression aligned along the [001] direction in the STM appearance of
hydroxyl. The tunneling switching can be observed directly for a deuterated
species (OD) within the time-resolution of STM, while it is smeared out for an OH
due to a significant increase of the tunneling rate. The switching is enhanced by the
vibrational excitation of the OH(OD) bending mode which is directly associated
with the switching reaction coordinate.
Keywords Tunneling dynamics of hydrogen atom
Hydroxyl groups on metal
surfaces
Single molecule dissociations
STM-IETS
7.1 Introduction
Quantum effects of H atom are of fundamental importance in diverse chemical,
physical, and biological processes [
1
-
5
]. Especially, tunneling in a two-state
system is a key interest because it is considered as one of the most ubiquitous
questions in physics and chemistry. For instance, the flipping of an isolated NH
3
molecule is the simplest model of H-atom tunneling, which was investigated by
Hund in the early days of quantum mechanics [
6
].
In surface science, quantum dynamics of H/proton has been investigated in
connection with the elementary processes of heterogeneous catalysis, energy
production and storage, namely fuel cells. The light mass of H atom results in the
unique behavior even at metal surfaces. For instance, Christmann et al. proposed
