R. Scott Smith

Adsorption, desorption, and clustering of H2O on Pt(111)

The adsorption, desorption, and clustering behavior of H2O on Pt111 has been investigated by specular He scattering. The data show that water adsorbed on a clean Pt111 surface undergoes a structural transition from a random distribution to clustered islands near 60 K. The initial helium scattering cross sections as a function of temperature are found to be insensitive to the incident H2O flux over a range of 0.005 monolayers (ML)/s-0.55 ML/s indicating that the clustering process is more complex than simple surface diffusion. The coarsening process of an initially random distribution of water deposited at 25 K is found to occur over a broad temperature range, 60<T<140 K, during thermal annealing. The desorption kinetics for submonolayer water are determined to be zero order for surface coverages greater than 0.05 ML and temperatures between 145 and 172 K. The zero-order desorption kinetics are consistent with a two-dimensional two-phase coexistence between a high-density H2O condensed phase (islands) and a low-density two-dimensional gaslike phase on the Pt surface.

Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. I. Surface limited desorption kinetics in amorphous solid water

The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programed desorption (TPD), and reflection-absorption infrared spectroscopy. The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness indicating that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage-dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, approximately 0.5 microm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.

The effect of the incident collision energy on the phase and crystallization kinetics of vapor deposited water films

Molecular beam techniques are used to grow water films on Pt(111) with incident collision energies from 5 to 205 kJ/mole. The effect of the incident collision energy on the phase of vapor deposited water films and their subsequent crystallization kinetics are studied using temperature programmed desorption and infrared spectroscopy. We find that for films deposited at substrate temperatures below 110 K, the incident kinetic energy (up to 205 kJ/mole) has no effect on the initial phase of the deposited film or its crystallization kinetics. Above 110 K, the substrate temperature does affect the phase and crystallization kinetics of the deposited films but this result is also independent of the incident collision energy. The presence of a crystalline ice template (underlayer) does affect the crystallization of amorphous solid water, but this effect is also independent of the incident beam energy. These results suggest that the crystallization of amorphous solid water requires cooperative motion of several water molecules.

Growth rate of crystalline ice and the diffusivity of supercooled water from 126 to 262 K

Significance Water is ubiquitous, but its physical properties are anomalous compared with most liquids. Because the anomalies become enhanced upon cooling, understanding the behavior of deeply supercooled water is critical. Unfortunately, experiments below ∼236 K at ambient pressure are difficult due to uncontrolled crystallization. Using a pulsed-laser–heating technique, we have determined the crystalline-ice growth rate and liquid-water diffusivity for temperatures between 180 and 262 K in ultrahigh-vacuum conditions. The fact that both of these quantities are smoothly varying rules out the hypothesis that water’s properties have a singularity at or near 228 K. However, the results are consistent with a previous prediction for the diffusivity that assumed no thermodynamic transitions occur in the supercooled region. Understanding deeply supercooled water is key to unraveling many of water’s anomalous properties. However, developing this understanding has proven difficult due to rapid and uncontrolled crystallization. Using a pulsed-laser–heating technique, we measure the growth rate of crystalline ice, G(T), for 180 K < T < 262 K, that is, deep within water’s “no man’s land” in ultrahigh-vacuum conditions. Isothermal measurements of G(T) are also made for 126 K ≤ T ≤ 151 K. The self-diffusion of supercooled liquid water, D(T), is obtained from G(T) using the Wilson–Frenkel model of crystal growth. For T > 237 K and P ∼ 10−8 Pa, G(T) and D(T) have super-Arrhenius (“fragile”) temperature dependences, but both cross over to Arrhenius (“strong”) behavior with a large activation energy in no man’s land. The fact that G(T) and D(T) are smoothly varying rules out the hypothesis that liquid water’s properties have a singularity at or near 228 K at ambient pressures. However, the results are consistent with a previous prediction for D(T) that assumed no thermodynamic transitions occur in no man’s land.

A unique vibrational signature of rotated water monolayers on Pt(111): Predicted and observed

Six H-bonds in the periodic di-interstitial structure that accounts for scanning tunneling microscope images of √37 and √39 wetting layers on Pt(111) are some 0.2 Å shorter than H-bonds are in ice Ih. According to a broadly obeyed correlation, this density functional theory result implies a stringent test of the di-interstitial motif, namely the presence of an OH-stretch band red-shifted from that of ice Ih by more than 1000 cm(-1). Infrared absorption spectra satisfy the test, in showing a feature centered at about 1965 cm(-1), which grows in as deposited water orders.

Breaking through the glass ceiling: The correlation between the self-diffusivity in and krypton permeation through deeply supercooled liquid nanoscale methanol films

Molecular beam techniques, temperature-programmed desorption (TPD), and reflection absorption infrared spectroscopy (RAIRS) are used to explore the relationship between krypton permeation through and the self-diffusivity of supercooled liquid methanol at temperatures (100-115 K) near the glass transition temperature, T(g) (103 K). Layered films, consisting of CH(3)OH and CD(3)OH, are deposited on top of a monolayer of Kr on a graphene covered Pt(111) substrate at 25 K. Concurrent Kr TPD and RAIRS spectra are acquired during the heating of the composite film to temperatures above T(g). The CO vibrational stretch is sensitive to the local molecular environment and is used to determine the supercooled liquid diffusivity from the intermixing of the isotopic layers. We find that the Kr permeation and the diffusivity of the supercooled liquid are directly and quantitatively correlated. These results validate the rare-gas permeation technique as a tool for probing the diffusivity of supercooled liquids.

Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. II. Diffusion limited kinetics in amorphous solid water.

The adsorption, desorption, and diffusion kinetics of N2 on thick (up to approximately 9 microm) porous films of amorphous solid water (ASW) films were studied using molecular beam techniques and temperature programmed desorption. Porous ASW films were grown on Pt(111) at low temperature (<30 K) from a collimated H2O beam at glancing incident angles. In thin films (<1 microm), the desorption kinetics are well described by a model that assumes rapid and uniform N2 distribution throughout the film. In thicker films (>1 microm), N2 adsorption at 27 K results in a nonuniform distribution, where most of N2 is trapped in the outer region of the film. Redistribution of N2 can be induced by thermal annealing. The apparent activation energy for this process is approximately 7 kJ/mol, which is approximately half of the desorption activation energy at the corresponding coverage. Preadsorption of Kr preferentially adsorbs onto the highest energy binding sites, thereby preventing N2 from trapping in the outer region of the film which facilitates N2 transport deeper into the porous film. Despite the onset of limited diffusion, the adsorption kinetics are efficient, precursor mediated, and independent of film thickness. An adsorption mechanism is proposed, in which a high-coverage N2 front propagates into a pore by the rapid transport of physisorbed second layer N2 species on top of the first surface bound layer.

Measuring diffusivity in supercooled liquid nanoscale films using inert gas permeation. II. Diffusion of Ar, Kr, Xe, and CH4 through methanol.

We present an experimental technique to measure the diffusivity of supercooled liquids at temperatures near their T(g). The approach uses the permeation of inert gases through supercooled liquid overlayers as a measure of the diffusivity of the supercooled liquid itself. The desorption spectra of the probe gas are used to extract the low temperature supercooled liquid diffusivities. In the preceding companion paper, we derived equations using ideal model simulations from which the diffusivity could be extracted using the desorption peak times for isothermal or peak temperatures for temperature programmed desorption experiments. Here, we discuss the experimental conditions for which these equations are valid and demonstrate their utility using amorphous methanol with Ar, Kr, Xe, and CH(4) as probe gases. The approach offers a new method by which the diffusivities of supercooled liquids can be measured in the experimentally challenging temperature regime near the glass transition temperature.

Measuring diffusivity in supercooled liquid nanoscale films using inert gas permeation. I. Kinetic model and scaling methods.

We describe in detail a diffusion model used to simulate inert gas transport through supercooled liquid overlayers. In recent work, the transport of the inert gas has been shown to be an effective probe of the diffusivity of supercooled liquid methanol in the experimentally challenging regime near the glass transition temperature. The model simulations accurately and quantitatively describe the inert gas permeation desorption spectra. The simulation results are used to validate universal scaling relationships between the diffusivity, overlayer thickness, and the temperature ramp rate for isothermal and temperature programmed desorption. From these scaling relationships we derive simple equations from which the diffusivity can be obtained using the peak desorption time or temperature for an isothermal or set of TPD experiments, respectively, without numerical simulation. The results presented here demonstrate that the permeation of gases through amorphous overlayers has the potential to be a powerful technique to obtain diffusivity data in deeply supercooled liquids.

Formation of supercooled liquid solutions from nanoscale amorphous solid films of methanol and ethanol

Molecular beam techniques are used to create layered nanoscale composite films of amorphous methanol and ethanol at 20 K. The films are then heated, and temperature programed desorption and infrared spectroscopy are used to observe the mixing, desorption, and crystallization behavior from the initially unmixed amorphous layers. We find that the initially unmixed amorphous layers completely intermix to form a deeply supercooled liquid solution after heating above T(g). Modeling of the desorption kinetics shows that the supercooled liquid films behave as ideal solutions. The desorption rates from the supercooled and crystalline phases are then used to derive the binary solid-liquid phase diagram. Deviations from ideal solution desorption behavior are observed when the metastable supercooled solution remains for longer times in regions of the phase diagram when thermodynamically favored crystallization occurs. In those cases, the finite lifetime of the metastable solutions results in the precipitation of crystalline solids. Finally, in very thick films at temperatures and compositions where a stable liquid should exist, we unexpectedly observe deviations from ideal solution behavior. Visual inspection of the sample indicates that these apparent departures from ideality arise from dewetting of the liquid film from the substrate. We conclude that compositionally tailored nanoscale amorphous films provide a useful means for preparing and examining deeply supercooled solutions in metastable regions of the phase diagram.