Bruce D. Kay

Evidence for the existence of structures in gas-phase homomolecular clusters of water

Abstract Presently, there is considerable theoretical and practical interest in small aqueous clusters which dictates the need for detailed information concerning their formation and properties. Extensive investigations of the formation of (H 2 O) N ( N ≤ 40) have been conducted employing a neutral free jet expansion with modulated molecular beam—electron impact ionization mass spectrometry. Cluster distributions are found to be generally dependent on a variety of factors including expansion characteristics of the source, attendant association and decomposition kinetics, and cluster ionization and dissociation processes in the detector. Detailed study of the influence of the various parameters has revealed the existence of certain reproducible features in the intensity distributions which can be divided into two general classes, one which is independent of ionization parameters but influenced by beam expansion conditions while the other is independent of the degree of beam expansion. Observed structures in the latter class are shown to arise subsequent to the ionization of certain neutral clusters leading to special structures corresponding to H + (H 2 O) N ( N = 21, 26, 28, 30); these are shown to be consistent with clathrate formation via an ion-induced mechanism. The existence of features at small cluster sizes involving well-defined structures similar to those occurring in bulk-ice crystals are explained on the basis of simple RRK arguments and internal energy considerations.

Studies of gas-phase clusters: the solvation of HNO3 in microscopic aqueous clusters

Abstract The formation and stability of aqueous clusters of nitric acid were studied utilizing a free-jet expansion technique, coupled with electron impact ionization mass spectrometry. Evidence for the onset of solvation at very small cluster sizes is presented.

The use of similarity profiles in studying cluster formation in molecular beams: Evidence for the role of pre-existing dimers

Abstract Studies of the influence of source temperatures and pressures on the distributions of water, ammonia and sulfur dioxide clusters are reported. The experiments reveal that nearly identical cluster distributions occur in cases where the pressure of dimer is maintained constant according to a simple equation involving stagnation temperature and pressure. In similar experiments covering the same range of pressures and temperatures, widely differing cluster distributions are obtained under conditions where the dimer concentration is not fixed. Our results suggest that large clusters proceed largely from pre-existing dimers, and that very few new ones are created early enough in the expansion to effect cluster growth. The gas-phase heat of dimerization of sulfur dioxide is determined to be 4.3 ± 0.3 kcal/mole.

Adsorption, Desorption, and Displacement Kinetics of H2O and CO2 on TiO2(110)

The adsorption, desorption, and displacement kinetics of H2O and CO2 on TiO2(110) are investigated using temperature programmed desorption (TPD) and molecular beam techniques. The TPD spectra for both H2O and CO2 have well-resolved peaks corresponding to desorption from bridge-bonded oxygen (Ob), Ti5c, and defect sites in order of increasing peak temperature. Analysis of the saturated surface spectrum for both species reveals that the corresponding adsorption energies on all sites are greater for H2O than for CO2. Sequential dosing of H2O and CO2 reveals that, independent of the dose order, H2O molecules will displace CO2 in order to occupy the highest energy binding sites available. Isothermal experiments show that the displacement of CO2 by H2O occurs between 75 and 80 K.

Desorption Kinetics of Ar, Kr, Xe, N2, O2, CO, Methane, Ethane, and Propane from Graphene and Amorphous Solid Water Surfaces

The desorption kinetics for Ar, Kr, Xe, N2, O2, CO, methane, ethane, and propane from graphene-covered Pt(111) and amorphous solid water (ASW) surfaces are investigated using temperature-programmed desorption (TPD). The TPD spectra for all of the adsorbates from graphene have well-resolved first, second, third, and multilayer desorption peaks. The alignment of the leading edges is consistent the zero-order desorption for all of the adsorbates. An Arrhenius analysis is used to obtain desorption energies and prefactors for desorption from graphene for all of the adsorbates. In contrast, the leading desorption edges for the adsorbates from ASW do not align (for coverages < 2 ML). The nonalignment of TPD leading edges suggests that there are multiple desorption binding sites on the ASW surface. Inversion analysis is used to obtain the coverage dependent desorption energies and prefactors for desorption from ASW for all of the adsorbates.

Desorption kinetics of methanol, ethanol, and water from graphene.

The desorption kinetics of methanol, ethanol, and water from graphene covered Pt(111) are investigated. The temperature programmed desorption (TPD) spectra for both methanol and ethanol have well-resolved first, second, third, and multilayer layer desorption peaks. The alignment of the leading edges is consistent with zero-order desorption kinetics from all layers. In contrast, for water, the first and second layers are not resolved. At low water coverages (<1 monolayer (ML)) the initial desorption leading edges are aligned but then fall out of alignment at higher temperatures. For thicker water layers (10-100 ML), the desorption leading edges are in alignment throughout the desorption of the film. The coverage dependence of the desorption behavoir suggests that at low water coverages the nonalignment of the desorption leading edges is due to water dewetting from the graphene substrate. Kinetic simulations reveal that the experimental results are consistent with zero-order desorption. The simulations also show that fractional order desorption kinetics would be readily apparent in the experimental TPD spectra.

Molecular Beam Studies of Nanoscale Films of Amorphous Solid Water

What is Amorphous Solid Water? Amorphous solid water (ASW) is a solid phase of water that is metastable with respect to the crystalline phase [1,2]. It is metastable because it is “trapped” in a configuration that has a higher free energy than the equilibrium crystalline configuration [3]. Amorphous solids, also known as glasses, are often described as structurally arrested or “frozen” liquids. Amorphous solids are most often formed when a liquid is cooled fast enough that crystallization does not occur prior to the system reaching a temperature where the structural relaxation timescale is long compared to the laboratory timescale, i.e. 100 s. The temperature where this occurs is called the glass transition temperature, T g .

Isotope enrichment during the formation of water clusters in supersonic free jet expansions

Presently, investigation of the dynamics, energetics, and structure of microscopic molecular clusters constitutes an active area of research in chemical physics. Herein the RRKM theory of primary isotope effects is extended to qualitatively predict isotope enrichment in water cluster formation. The theoretical model is verified experimentally by neutral free jet expansion modulated molecular beam mass spectrometry of mixed (H2O)m(D2O)n clusters. The results further support the previously presented mechanism for neutral cluster growth in free jet expansions. The observed enrichment factors (∼30%) suggest that techniques involving clustering may find practical applications in the area of isotope separation.

The release of trapped gases from amorphous solid water films. II. “Bottom-up” induced desorption pathways

In this (Paper II) and the preceding companion paper (Paper I; R. May, R. Smith, and B. Kay, J. Chem. Phys. 138, 104501 (2013)), we investigate the mechanisms for the release of trapped gases from underneath amorphous solid water (ASW) films. In Paper I, we focused on the low coverage regime where the release mechanism is controlled by crystallization-induced cracks formed in the ASW overlayer. In that regime, the results were largely independent of the particular gas underlayer. Here in Paper II, we focus on the high coverage regime where new desorption pathways become accessible prior to ASW crystallization. In contrast to the results for the low coverage regime (Paper I), the release mechanism is a function of the multilayer thickness and composition, displaying dramatically different behavior between Ar, Kr, Xe, CH4, N2, O2, and CO. Two primary desorption pathways are observed. The first occurs between 100 and 150 K and manifests itself as sharp, extremely narrow desorption peaks. Temperature programmed desorption is utilized to show that these abrupt desorption bursts are due to pressure induced structural failure of the ASW overlayer. The second pathway occurs at low temperature (typically <100 K) where broad desorption peaks are observed. Desorption through this pathway is attributed to diffusion through pores formed during ASW deposition. The extent of desorption and the line shape of the low temperature desorption peak are dependent on the substrate on which the gas underlayer is deposited. Angle dependent ballistic deposition of ASW is used to vary the porosity of the overlayer and strongly supports the hypothesis that the low temperature desorption pathway is due to porosity that is templated into the ASW overlayer by the underlayer during deposition.

Molecular Hydrogen Formation from Proximal Glycol Pairs on TiO2(110)

Understanding hydrogen formation on TiO2 surfaces is of great importance, as it could provide fundamental insight into water splitting for hydrogen production using solar energy. In this work, hydrogen formation from glycols having different numbers of methyl end-groups has been studied using temperature-programmed desorption on reduced, hydroxylated, and oxidized rutile TiO2(110) surfaces. The results from OD-labeled glycols demonstrate that gas-phase molecular hydrogen originates exclusively from glycol hydroxyl groups. The yield is controlled by a combination of glycol coverage, steric hindrance, TiO2(110) order, and the amount of subsurface charge. Combined, these results show that proximal pairs of hydroxyl-aligned glycol molecules and subsurface charge are required to maximize the yield of this redox reaction. These findings highlight the importance of geometric and electronic effects in hydrogen formation from adsorbates on TiO2(110).