Gregory A. Kimmel

Polarization- and Azimuth-Resolved Infrared Spectroscopy of Water on TiO2(110): Anisotropy and the Hydrogen-Bonding Network

We have investigated the structure and dynamics of thin water films adsorbed on TiO2(110) using infrared reflection-absorption spectroscopy (IRAS) and ab initio molecular dynamics. Infrared spectra were obtained for s- and p-polarized light with the plane of incidence parallel to the [001] and [11̅0] azimuths for water coverages ≤ 4 monolayers. The spectra indicate strong anisotropy in the water films. The vibrational densities of states predicted by the ab initio 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 monolayers on this surface. The results also demonstrate the capabilities of polarization- and azimuth-resolved IRAS for investigating the structure and dynamics of adsorbates on dielectric substrates.

Thermal and Nonthermal Physiochemical Processes in Nanoscale Films of Amorphous Solid Water

Amorphous solid water (ASW) is a disordered version of ice created by vapor deposition onto a cold substrate (typically less than 130 K). It has a higher free energy than the crystalline phase of ice, and when heated above its glass transition temperature, it transforms into a metastable supercooled liquid. This unusual form of water exists on earth only in laboratories, after preparation with highly specialized equipment. It is thus fair to ask why there is any interest in studying such an esoteric material. Much of the scientific interest results from the ability to use ASW as a model system for exploring the physical and reactive properties of liquid water and aqueous solutions. ASW is also thought to be the predominant form of water in the extremely cold temperatures of many interstellar and planetary environments. In addition, ASW is a convenient model system for studying the stability of amorphous and glassy materials as well as the properties of highly porous materials. A fundamental understanding of such properties is invaluable in a diverse range of applications, including cryobiology, food science, pharmaceuticals, astrophysics, and nuclear waste storage, among others. Over the past 15 years, we have used molecular beams and surface science techniques to probe the thermal and nonthermal properties of nanoscale films of ASW. In this Account, we present a survey of our research on the properties of ASW using this approach. We use molecular beams to precisely control the deposition conditions (flux, incident energy, and incident angle) and create compositionally tailored, nanoscale films of ASW at low temperatures. To study the transport properties (viscosity and diffusivity), we heat the amorphous films above their glass transition temperature, T(g), at which they transform into deeply supercooled liquids prior to crystallization. The advantage of this approach is that at temperatures near T(g), the viscosity is approximately 15 orders of magnitude larger than that of a normal liquid. As a result, the crystallization kinetics are dramatically slowed, increasing the time available for experiments. For example, near T(g), a water molecule moves less than the distance of a single molecule on a typical laboratory time scale (∼1000 s). For this reason, nanoscale films help to probe the behavior and reactions of supercooled liquids at these low temperatures. ASW films can also be used for investigating the nonthermal reactions relevant to radiolysis.

Adsorption Geometry of CO versus Coverage on TiO2(110) from s- and p-Polarized Infrared Spectroscopy.

The adsorption of CO on reduced, rutile TiO2(110) is investigated using IR reflection-absorption spectroscopy and temperature-programmed desorption. Experiments using s- and p-polarized IR light incident along the [001] and [11̅0] azimuths give detailed information on the adsorption geometry of the CO as a function of the CO coverage, θCO. The results indicate that for θCO ≤ 1 ML, CO adsorbs oriented perpendicular to the surface at Ti5c sites. For 1 < θCO ≤ 1.5 ML, the bonding geometry of the CO adsorbed at Ti5c sites is unchanged, whereas the additional CO molecules adsorb at Ob sites parallel to the surface and parallel to the [11̅0] azimuth. The results do not support previous suggestions that CO at Ti5c sites tilt ∼20° from normal at high coverages. The results demonstrate the utility of polarization-resolved infrared reflection-absorption spectroscopy for elucidating adsorption geometries on dielectric substrates.

Multiple Nonthermal Reaction Steps for the Photooxidation of CO to CO2 on Reduced TiO2(110).

The photooxidation of CO on reduced, rutile TiO2(110) is studied on a millisecond time scale. For CO coadsorbed with a saturation coverage of chemisorbed O2 (θsat), the CO2 photon-stimulated desorption (PSD) signal is initially zero, increases to a maximum after several tens of milliseconds, and then decreases at longer times. The initial CO2 PSD signal increases ∼5 times more quickly for an oxygen coverage of 0.5θsat. The initial rate of increase of the CO2 PSD signal is proportional to the flux of UV photons. The results show that two or more nonthermal reaction steps are required to photooxidize CO adsorbed on TiO2(110). The intermediate species involved in the reactions is stable for at least 100 s at 30 K. Previous models had suggested that CO photooxidation required only one nonthermal reaction. The likely initial and final charge states of the system suggest that an electron-mediated reaction and a hole-mediated reaction are needed for the complete photooxidation reaction.

A variable-energy electron microbeam: a unique modality for targeted low-LET radiation.

Abstract Sowa, M. B., Murphy, M. K., Miller, J. H., McDonald, J. C., Strom, D. J. and Kimmel, G. A. A Variable-Energy Electron Microbeam: A Unique Modality for Targeted Low-LET Radiation. Radiat. Res. 164, 695–700 (2005). We have designed and constructed a low-cost, variable-energy low-LET electron microbeam that uses energetic electrons to mimic radiation damage produced by γ and X rays. The microbeam can access lower regions of the LET spectrum, similar to conventional X-ray or 60Co γ-ray sources. The device has two operating modes, as a conventional microbeam targeting single cells or subpopulations of cells or as a pseudo broad-beam source allowing for direct comparison with conventional sources. By varying the incident electron energy, the target cells can be selectively exposed to different parts of the energetic electron tracks, including the track ends.

Anticorrelation between surface and subsurface point defects and the impact on the redox chemistry of TiO2(110)

By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary-ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO2 . STM results show that surface O vacancies (VO ) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on VO formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O2 was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (VO ) are largely responsible for mediation of the redox chemistry at the reduced TiO2 (110).

Electron-stimulated reactions and O2 production in methanol-covered amorphous solid water films

The low-energy, electron-stimulated desorption (ESD) of molecular products from amorphous solid water (ASW) films capped with methanol is investigated versus methanol coverage (0-4x10(15) cm(-2)) at 50 K using 100 eV incident electrons. The major ESD products from a monolayer (ML) of methanol on ASW are quite similar to the ESD products from bulk methanol film: H(2), CH(4), H(2)O, C(2)H(6), CO, CH(2)O, and CH(3)OH. For 40 ML ASW films, the molecular oxygen, hydrogen, and water ESD yields from the ASW are suppressed with increasing methanol coverage, while the CH(3)OH ESD yield increases proportionally to the methanol coverage. The suppression of the water ESD products by methanol is consistent with the nonthermal reactions occurring preferentially at or near the ASW/vacuum interface and not in the interior of the film. The water and molecular hydrogen ESD yields from the water layer decrease exponentially with the methanol cap coverage with 1/e constants of approximately 6 x 10(14) and 1.6 x 10(15) cm(-2), respectively. In contrast, the O(2) ESD from the water layer is very efficiently quenched by small amounts of methanol (1/e approximately 6.5 x 10(13) cm(-2)). The rapid suppression of O(2) production by small amounts of methanol is due to reactions between CH(3)OH and the precursors for the O(2)-mainly OH radicals. A kinetic model for the O(2) ESD, which semiquantitatively accounts for the observations, is presented.

Complete Wetting of Pt(111) by Nanoscale Liquid Water Films

The melting and wetting of nanoscale crystalline ice films on Pt(111) that are transiently heated above the melting point in ultrahigh vacuum (UHV) using nanosecond laser pulses are studied with infrared reflection absorption spectroscopy and Kr temperature-programmed desorption. The as-grown crystalline ice films consist of nanoscale ice crystallites embedded in a hydrophobic water monolayer. Upon heating, these crystallites melt to form nanoscale droplets of liquid water. Rapid cooling after each pulse quenches the films, allowing them to be interrogated with UHV surface science techniques. With each successive heat pulse, these liquid drops spread across the surface until it is entirely covered with a multilayer water film. These results, which show that nanoscale water films completely wet Pt(111), are in contrast to molecular dynamics simulations predicting partial wetting of water drops on a hydrophobic water monolayer. The results provide valuable insights into the wetting characteristics of nanoscale water films on a clean, well-characterized, single-crystal surface.