Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

cycles, another ten irradiation cycles were performed, however the oxygen physisorbed on the sample was
switched to 18 O2. For the eleventh irradiation cycle, the 16 O 18 O PSD signal reappears (Fig. 2 - upper right,
black line). For irradiation cycles 12 – 20, 16 O 18 O PSD signal decreases again. The results presented here
suggest that oxygen adsorbed on reduced TiO2(110) and then annealed to temperatures above 100 K
forms a distinct chemical species with interesting photochemical properties. At present, the chemical state
of the oxygen that reacts with physisorbed O2 and many of the details of that reaction are not known.
More research is needed to determine the detailed state of the reactive, chemisorbed oxygen. However,
two likely candidates are O2 adsorbed in a bridging oxygen vacancy and tetraoxygen. These results show
that the photochemistry of O2 on TiO2(110) is more complicated than previously appreciated.
Polarization- and azimuth-resolved infrared spectroscopy of water on TiO2(110): Anisotropy and the
hydrogen-bonding network
The structure of water at interfaces influences the physical, chemical and biological processes relevant to
a large number of natural and man-made systems. As a result, the structure of water at surfaces has been
extensively studied. Still, even the simplest and arguably most experimentally accessible issue – the
ground-state arrangement of the first water layer on a solid surface – has been hard to resolve. In
collaboration with Marcel Baer, Chris Mundy, Roger Rousseau, and Joost VandeVondele, we have
investigated the structure and dynamics of thin water films on TiO2(110) using polarization- and
azimuthal-resolved IRAS and density functional theory (DFT). 6 The IRAS spectra indicate strong
anisotropy in the water films. The vibrational densities of states predicted by the ab-initio molecular
dynamics (AIMD) simulations for 1 and 2 monolayer coverages agree well with the observations. The
results provide new insight into the structure of water on TiO2(110) and resolve a long-standing puzzle
regarding the hydrogen-bonding between molecules in the first and second monolayer on this surface.
More generally, the results also demonstrate the capabilities of polarization- and azimuthally-resolved
IRAS for investigating the structure and dynamics of adsorbates on dielectric substrates.
Figure 3 shows IRAS and AIMD results for 1 ML D2O films on TiO2(110). IRAS spectra for s-
and p-polarized light incident along the [001] and [ 11
0]
azimuths (Fig. 3d) indicate significant
Figure 3. Structure and dynamics for θ = 1 ML D2O on TiO2(110). a) top view and b) side view of the
calculated structure using ab-initio molecular dynamics. c) Calculated VDOS (solid lines) and d(ω)
parameters (dashed lines) versus frequency. The same colors are used for each direction for both the
VDOS and d parameters. d) IRAS spectra for s- and p-polarized light along both azimuths (solid lines)
and simulated spectra (dashed lines) using d-parameter theory.
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