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The reaction pathways of glycerol on TiO2(110) have been studied by molecular beam dosing and temperature-programmed desorption. The majority of adsorbed glycerol undergoes reactions to yield water...

Acetonitrile thin films were prepared on Pt(111) and on graphene on Pt(111) using molecular beam techniques. Temperature programed desorption (TPD) experiments of acetonitrile on Pt(111) displayed ...

The adsorption of cyclohexane, benzene, and alkyl-substituted benzene derivatives is studied on rutile TiO2(110) by a combination of molecular beam dosing, temperature-programmed desorption, and density functional theory (DFT). An inversion analysis is used to extract the coverage-dependent desorption energies from TiO2(110). The values of the suitable prefactors are derived from simple statistical mechanical models assuming different limits in the adsorbate mobility on the surface. The prefactor values determined using the vibrational frequencies from DFT calculations corroborate this analysis and show that the adsorbates are mobile in one or two dimensions on a corrugated TiO2(110) surface. The adsorption of benzene derivatives is found to be dominated by the dative Lewis acid–base interactions of the π system with the surface Ti ions. While the desorption energy generally increases with increasing the length and the number of substituents, the difference between the desorption energies decreases as the...

The interaction of carbon dioxide (CO2) with a graphene-covered Pt(111) surface was investigated using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). The TPD spectra show monolayer and multilayer desorption peaks; however, the multilayer peak is not well-separated from the monolayer peak. The TPD spectra for submonolayer and multilayer coverages align on separate common leading edges. This alignment is a signature of zero-order desorption kinetics. The RAIRS spectra for submonolayer coverages have a relatively sharp peak at ∼2350 cm–1, which is assigned to the ν3 asymmetric stretch. The peak is observed at the onset of CO2 adsorption, and the area of the peak increases linearly with coverage. This suggests that CO2 does not lie flat on the surface but instead has a component of its bond axis perpendicular to the graphene surface.

Crystalline acetonitrile has two polymorphs, a high-temperature (HT) phase that is stable between 217 K and its melting point at 229 K and a low-temperature (LT) phase that is stable below 217 K. Solid acetonitrile films can be prepared by vapor deposition in an ultrahigh vacuum chamber. To prevent sublimation of the film, temperatures are often kept below 150 K. While the LT phase is thermodynamically favored at these low temperatures, such preparation usually results in the formation of the metastable HT polymorph. In this work we use reflection adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) experiments to investigate the effects of the deposition temperature and underlying substrate on the morphology of acetonitrile films prepared with molecular beam deposition. We obtained the elusive LT phase when dosing at 120 K on a graphene substrate and on a crystalline decane layer. Dosing acetonitrile on other surfaces produced the HT phase, as did annealing of amorphous films. We used TPD experiments to determine the Gibbs energy difference between the HT and the LT phases. Our ΔG values agree with extrapolation of equilibrium calorimetry data. We also observed that acetonitrile films were amorphous when dosed at temperatures ≤ 60 K and porous for temperatures ≤ 50 K.

The desorption of benzene, 1,3,5-trifluorobenzene (TFB), and hexafluorobenzene (HFB) from a graphene covered Pt(111) substrate was investigated using temperature-programmed desorption (TPD). All three species have well-resolved monolayer and second-layer desorption peaks. The desorption spectra for submonolayer coverages of benzene and HFB are consistent with first-order desorption kinetics. In contrast, the submonolayer TPD spectra for TFB align on a common leading-edge, which is indicative of zero-order desorption kinetics. The desorption behavior of the three molecules can be correlated with the strength of the quadrupole moments. Calculations (second-order Møller-Plesset perturbation and density functional theory) show that the potential minimum for coplanar TFB dimers is more than a factor of 2 greater than that for either benzene or HFB dimers. The calculations support the interpretation that benzene and HFB are less likely to form the two-dimensional islands that are needed for submonolayer zero-order desorption kinetics.

The adsorption of the C2 hydrocarbons, including ethane, ethene, and ethyne, are studied on magnetite Fe3O4(001) by a combination of molecular beam dosing, temperature programmed desorption, and X-ray photoelectron spectroscopy. The ethane desorption profile has a single temperature invariant peak at 100 K, while ethene and ethyne exhibit additional peaks at ∼120 -and ∼135 K. An inversion analysis is used to extract coverage-dependent desorption energies as well as coverage-averaged prefactors for each molecule. Ethene and ethyne exhibit moderate coverage-dependent desorption energies decreasing from ∼80 to ∼30 kJ/mol at saturation, while ethane shows a relatively coverage invariant desorption energy ∼28 kJ/mol. The desorption energies of the C2 hydrocarbons increase in the order of increasing bond order. This is likely due to the enhanced interaction of the ethene and ethyne π system to the coordinatively unsaturated octahedral Fe sites (Feoct) on the oxide surface. The saturation coverages of each C2 hydrocarbon at 82 K were also determined. These match well with the 2-dimensional area determined from their liquid densities.

Here, the crystallization kinetics of nanoscale amorphous solid water (ASW) films are investigated using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). TPD measurements are used to probe surface crystallization and RAIRS measurements are used to probe bulk crystallization. Isothermal TPD results show that surface crystallization is independent of the film thickness (from 100 to 1000 ML). Conversely, the RAIRS measurements show that the bulk crystallization time increases linearly with increasing film thickness. These results suggest that nucleation and crystallization begin at the ASW/vacuum interface and then the crystallization growth front propagates linearly into the bulk. This mechanism was confirmed by selective placement of an isotopic layer (5% D2O in H2O) at various positions in an ASW (H2O) film. In this case, the closer the isotopic layer was to the vacuum interface, the earlier the isotopic layer crystallized. These experiments provide direct evidence to confirm that ASW crystallization in vacuum proceeds by a “top-down” crystallization mechanism.

Temperature programmed desorption and molecular beam scattering were used to study the adsorption and desorption of small hydrocarbons (n-alkanes, 1-alkenes and 1-alkynes of C1–C4) on rutile TiO2(110). We show that the sticking coefficients for all the hydrocarbons are close to unity (> 0.95) at an adsorption temperature of 60 K. The desorption energies for hydrocarbons of the same chain length increase from n-alkanes to 1-alkenes and to 1-alkynes. This trend is likely a consequence of additional dative bonding of the alkene and alkyne π system to the coordinatively unsaturated Ti5c sites. Similar to previous studies on the adsorption of n-alkanes on metal and metal oxide surfaces, we find that the desorption energies within each group (n-alkanes vs. 1-alkenes vs. 1-alkynes) from Ti5c sites increase linearly with the chain length. The absolute saturation coverages of each hydrocarbon on Ti5c sites were also determined. The saturation coverage of CH4, is found to be ~ 2/3 monolayer (ML). The saturation coverages of C2–C4 hydrocarbons are found nearly independent of the chain length with values of ~ 1/2 ML for n-alkanes and 1-alkenes and 2/3 ML for 1-alkynes. This result is surprising considering their similar sizes.

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.

Using scanning probe microscopy and temperature programed desorption we examined the interaction between water and two common clathrate-forming gases, methane and isobutane, at low temperature and low pressure. Water co-deposited with up to 10–1 mbar methane or 10–5 mbar isobutane at 140 K onto a Pt(111) substrate yielded pure crystalline ice, i.e., the exposure to up to ~ 107 gas molecules for each deposited water molecule did not have any detectable effect on the growing films. Exposing metastable, less than 2 molecular layers thick, water films to 10–5 mbar methane does not alter their morphology, suggesting that the presence of the Pt(111) surface is not a strong driver for hydrate formation. This weak water–gas interaction at low pressures is supported by our thermal desorption measurements from amorphous solid water and crystalline ice where 1 ML of methane desorbs near ~ 43 K and isobutane desorbs near ~ 100 K. As a result, similar desorption temperatures were observed for desorption from amorphous solid water.

The interaction of methanol with iron oxide surfaces is of interest due to its potential in hydrogen storage and from a fundamental perspective as a chemical probe of reactivity. We present here a study examining the adsorption and reaction of methanol on magnetite Fe3O4(001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The methanol desorption profile from Fe3O4(001) is complex, exhibiting peaks at 140 K, 173 K, 230 K, and 268 K, corresponding to the desorption of intact methanol, as well as peaks at 341 K and 495 K due to the reaction of methoxy intermediates. The saturation of a monolayer of methanol corresponds to ∼5 molecules/unit cell (u.c.), which is slightly higher than the number of surface octahedral iron atoms of 4/u.c. We probe the kinetics and thermodynamics of the desorption of molecular methanol using inversion analysis. The deconvolution of the complex desorption profile into individual peaks allows for calculations of both the desorption energy and the prefactor of each feature. The initial 0.7 methanol/u.c. reacts to form methoxy and hydroxy intermediates at 180 K, which remain on the surface above room temperature after intact methanol has desorbed. The methoxy species react via one of two channels, a recombination reaction with surface hydroxyls to form additional methanol at ∼350 K and a disproportionation reaction to form methanol and formaldehyde at ∼500 K. Only 20% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway.

We have examined the adsorbate–substrate interaction kinetics of CO2 and H2O on a natural forsterite crystal surface, Mg2SiO4(011), with 10–15% Fe2+ substituted for Mg2+. We used temperature-programmed desorption and molecular beam techniques to determine the adsorption, desorption, and displacement kinetics for H2O and CO2. Neither CO2 nor H2O has distinct submonolayer desorption peaks, but instead both have a broad continuous desorption feature that evolves smoothly into multilayer desorption. Inversion of the monolayer coverage spectra for both molecules reveals that the corresponding binding energies for H2O are greater than those for CO2 on all sites. The relative strength of these interactions is the dominant factor in the competitive adsorption and displacement kinetics. In experiments in which the two adsorbates are codosed, H2O preferentially binds to the highest-energy binding sites available and displaces CO2. The onset of significant CO2 displacement by H2O occurs between 65 and 75 K.

The adsorption of 1,3-propylene glycol (1,3-PG) on partially reduced TiO2(110) and its conversion to products have been studied by a combination of molecular beam dosing and temperature-programmed desorption (TPD). When the Ti surface sites are saturated by 1,3-PG, ∼80% of the molecules undergo further reactions to yield products that are liberated during the TPD ramp. In contrast to ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG) that yield only alkenes and water at very low coverages ( 0.1 ML), propanal (CH3CH2CHO) and two additional products, 1-propanol (CH3CH2CH2OH) and acrolein (CH2═CHCHO), are observed. The desorption of 1-propanol is found to be coupled with the desorption of acrolein, suggesting that these p...

We have studied the reactions of 1,2-propylene glycol (1,2-PG), DOCH(CH3)CH2OD, on partially reduced, hydroxylated, and oxidized TiO2(110) surfaces using temperature-programmed desorption. On reduced TiO2(110), propylene, propanal, and acetone are identified as primary carbon-containing products. While the propylene formation channel dominates at low 1,2-PG coverages, all of the above-mentioned products are observed at high coverages. The carbon-containing products are accompanied by the formation of D2O and D2. The observation of only deuterated products shows that the source of hydrogen (D) is from the 1,2-PG hydroxyls. The role of bridging oxygen vacancy (VO) sites was further investigated by titrating them via hydroxylation and oxidation. The results show that hydroxylation does not change the reactivity because the VO sites are regenerated at 500 K, which is a temperature lower than the 1,2-PG product formation temperature. In contrast, surface oxidation causes significant changes in the product dist...

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).

The absolute coverages of water and small aliphatic alcohols (C1–C4) were determined on TiO2(110) using a combination of temperature-programmed desorption and liquid nitrogen cooled quartz crystal microbalance measurements. The absolute saturation coverages of water on Ti4+ and bridging oxygen, Ob, sites are found to be equal to 1 monolayer with respect to the number of Ti4+ and/or Ob sites (1 ML ≡ 5.2 × 1014 cm–2) in good agreement with prior studies. The saturation coverages of primary alcohols on Ti4+ sites are found to be approximately constant and equal to 0.77 ML. This indicates that the increasing length of the alkyl chains does not contribute to added steric hindrance. Additional steric hindrance is observed with increased branching as shown for secondary alcohols and tertiary t-butanol where the saturation coverage decreases to 0.62 and 0.44 ML, respectively. On Ob rows, a monotonic decrease of the alcohol coverages is observed for both increasing length of the alkyl chains and the chain number.

The interaction of Fe0 with chlorinated hydrocarbons in an aqueous environment is important for the utilization of Fe0 nanoparticles in groundwater remediation technologies. This article builds upo...

The interaction of Fe0 atoms with C2Cl6 and C2Cl4 multilayers over FeO(111) has been investigated using the “atom dropping” preparation technique and a combination of temperature-programmed desorption and X-ray photoelectron spectroscopy. The reactivity and reaction products are strongly dependent on the Fe0 coverage. On C2Cl6 multilayers, submonolayer Fe0 doses lead to high reactivity and primarily FeCl3 and C4Cl6, whereas multilayer Fe0 doses lead to the production of FeCl2 and C2Cl4 with much lower Fe0 reactivity. The data are consistent with a model where Fe atoms form intermediate species at low coverage, which consist of an Fe atom inserted into a C−Cl bond. When two Fe atoms react with C2Cl6, a different intermediate species is formed that produces the alternative reaction pathway and the formation of C2Cl4. Similar atom dropping experiments demonstrate that C2Cl4 is also reactive toward Fe0 atoms at low Fe0 dose, leading to the production of one FeCl2 molecule per C2Cl4 molecule reacted. At higher...

The interaction of Fe0 atoms with D2O layers on FeO(111) has been investigated using the “atom-dropping” preparation technique and a combination of temperature-programmed desorption, X-ray photoele...

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.

The adsorption of water on FeO(111) is investigated using temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Well-ordered 2 ML thick FeO(111) films are grown epitaxially on a Pt(111) substrate. Water adsorbs molecularly on FeO(111) and desorbs with a well resolved monolayer peak. IRAS measurements as a function of coverage are performed for water deposited at 30 and 135 K. For all coverages (0.2 ML and greater), the adsorbed water exhibits significant hydrogen bonding. Differences in IRAS spectra for water adsorbed at 30 and 135 K are subtle but suggest that water adsorbed at 135 K is well ordered. Monolayer nitrogen TPD spectra from water covered FeO(111) surfaces are used to investigate the clustering of the water as a function of deposition or annealing temperature. Temperature dependent water overlayer structures result from differences in water diffusion rates on bare FeO(111) and on water adsorbed on FeO(111). Features in the nitrogen TPD spectra allow the monolayer wetting and 2-dimensional (2D) ordering of water on FeO(111) to be followed. Voids in a partially disordered first water layer exist for water deposited below 120 K and ordered 2D islands are found when depositing water above 120 K.

Well-ordered FeO(111) thin films are epitaxially grown on a Pt(111) substrate. A series of molecules including CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 are used as probes to test the chemical reactivity of the FeO(111) surface. The temperature-programmed desorption spectra show no evidence of dissociative adsorption or chemical reaction between the substrate and the adsorbates. The desorption kinetics studies reveal that all the molecules are physisorbed and have desorption kinetics with an order between 0 and 1. Kinetic analysis is conducted, assuming both zero- and first-order desorption, and shows that an uncertainty in the desorption order introduces an error in determination of the terrace site desorption energies (θ = 0.5 ML) of only ∼2%. The desorption energies for the series of molecules increase with the number of chlorine atoms in the molecule. The increase of desorption energies is not linear with the molecular polarizability, and the deviations from linearity are attributed to the permanent dipole ...

The adsorption dynamics and desorption kinetics of Ar and CH4 on MgO(100) are studied using a combination of molecular beam scattering and temperature-programmed desorption (TPD). Both Ar and CH4 exhibit an initial trapping probability that decreases dramatically with increasing kinetic energy and is independent of incident angle indicating a barrierless process obeying total energy scaling. The trapping probability for both systems increases roughly linearly with increasing adsorbate coverage in the first layer. Analysis of the TPD spectra yields desorption energies of 8.5 and 13 kJ/mol for Ar and CH4, respectively.

We have examined the adsorption of the weakly bound species N2, O2, CO, and Kr on the (√37×√37)R25.3° water monolayer on Pt(111) using a combination of molecular beam dosing, infrared reflection absorption spectroscopy, and temperature programmed desorption. In contrast to multilayer crystalline ice, the adsorbate-free water monolayer is characterized by a lack of dangling OH bonds protruding into the vacuum (H-up). Instead, the non-hydrogen-bonded OH groups are oriented downward (H-down) to maximize their interaction with the underlying Pt(111) substrate. Adsorption of Kr and O2 have little effect on the structure and vibrational spectrum of the "√37" water monolayer while adsorption of both N2, and CO are effective in "flipping" H-down water molecules into an H-up configuration. This "flipping" occurs readily upon adsorption at temperatures as low as 20 K and the water monolayer transforms back to the H-down, "√37" structure upon adsorbate desorption above 35 K, indicating small energy differences and barriers between the H-down and H-up configurations. The results suggest that converting water in the first layer from H-down to H-up is mediated by the electrostatic interactions between the water and the adsorbates.

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.

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.

We compare the adsorption, desorption, trapping, and release of Ar, N 2 , O 2 , CO, and CH 4 by dense (nonporous) and highly porous amorphous solid water (ASW) films. Molecular beam deposition techniques are used to control the porosity of the vapor-deposited ASW thin films. Experiments where the gas species is deposited on top of and underneath dense and porous ASW are conducted. For the film thickness used in this study, the porous films are found to adsorb between 20 and 50 times more gas than the dense films. The desorption temperature of the adsorbed gas is also dependent on the porosity of the ASW film. Differences between desorption from porous and dense ASW films are attributed to differences in their ability to trap weakly physisorbed gases. The results are largely independent of the gas studied, confirming that the adsorption and trapping of gases are dominated by the ASW porosity. These findings show that laboratory studies must account for the growth conditions and their effects on ASW morphology in order to accurately predict the properties of astrophysical ices.

The ability to grow thin MgO(100) films of quality approaching that of vacuum-cleaved MgO(100) is demonstrated using low-energy electron diffraction and temperature-programmed desorption (TPD) of CO. Highly ordered MgO(100) surfaces are used to study the adsorption and desorption of CO. A linearly increasing sticking coefficient from 0.47 ± 0.03 to 0.90 is observed for relative CO coverages, θ, less than 0.8 monolayer (ML). For this coverage range, the total sticking coefficient is given by SMgO(1 − PCO) + SCOPCO, where SMgO (SCO) is the sticking on the bare (CO-covered) MgO and PCO is the probability of striking the CO-covered surface. In TPD, the desorption of CO is dominated at very low coverages by desorption from sites influenced by defects. At intermediate coverages (0.25−0.8 ML) the CO desorbs via first-order desorption. At 0.8 ML where the monolayer peak saturates, the desorption energy is 17 ± 2 kJ/mol and the preexponential factor is 1 × 1015±2 s-1. The desorption energy increases linearly as co...

The desorption kinetics of molecular beam deposited ultrathin films of CC${\mathrm{l}}_{4}$ and amorphous solid water (ASW) are studied. Overlayers of ASW impede CC${\mathrm{l}}_{4}$ desorption until the onset of crystallization, whereupon the CC${\mathrm{l}}_{4}$ desorbs abruptly. The abrupt desorption occurs through connected pathways that are formed in the water overlayer during the nucleation and growth of crystalline ice from ASW. The onset of the abrupt desorption corresponds to the threshold for dynamic percolation. As the crystallization proceeds, the number of connected pathways rapidly increases, giving rise to the episodic release of CC${\mathrm{l}}_{4}$.

Studies of the properties of crystalline ice and amorphous solid water are the focus of considerable diverse and interdisciplinary research. The reasons include understanding heterogeneous atmospheric processes, interstellar and cometary astrophysics, cryobiology, and the physics and chemistry of liquids. In this review we summarize our recent work using nanoscale ice films to characterize the kinetic behavior of crystalline ice and amorphous solid water. The adsorption, desorption, crystallization and diffusion kinetics of the nanoscale films are studies using molecular beam and programmed desorption techniques. The results of these experiments and their implications for the physical properties of nanoscale ice films are presented.

The amorphous to crystalline ice phase transition is studied by measuring the water desorption rate from nanoscale thin films of water vapor deposited on Au(111) and Ru(001) single crystal metallic substrates. The desorption kinetics are substrate dependent and suggest strongly that the film morphology is governed by the hydrophilicity of the substrate. The crystallization kinetics are independent of substrate but depend strongly on both temperature and film thickness and are consistent with a spatially random nucleation and isotropic growth model.

Adsorption and desorption of water on well‐ordered and sputter‐damaged single crystal MgO(100) surfaces were studied by a combination of molecular beam reflection and temperature programmed desorption techniques. Adsorption exhibits precursor‐mediated kinetics and desorption exhibits a strong dependence on substrate treatment, demonstrating the importance of surface defects.

The condensation of H2O on ice multilayers on Ru(001) was studied using both molecular beam and optical interference techniques as a function of surface temperature. From the beam reflection technique, the H2O sticking coefficient, S, was determined to be S = 0.99 ± 0.03 at temperatures between 85 and 150 K and was independent of incident angle (0−70°) and beam energy (1−40 kcal/mol). The condensation coefficient, α, was dependent on both the incident H2O flux and the desorption H2O flux at the various surface temperatures. The magnitude of α varied continuously from unity at T < 130 K to zero at higher temperatures. The optical interference experiments yielded condensation coefficients and sticking coefficients of α = S = 0.97 ± 0.10 at temperatures from 97 to 145 K where the H2O desorption flux was negligible with respect to the incident flux. The optical interference measurements monitored the ice film thickness versus H2O exposure time and were dependent on the refractive index, n, and the density, ρ,...

Desorption of carbon tetrachloride from beneath an amorphous solid water (ASW) overlayer is explored utilizing a combination of temperature programmed desorption and infrared spectroscopy. Otherwise inaccessible information about the dewetting and crystallization of ASW is revealed by monitoring desorption of the CCl(4) underlayer. The desorption maximum of CCl(4) on graphene occurs at ~140 K. When ASW wets the CCl(4) no desorption below 140 K is observed. However, the mobility of the water molecules increases with ASW deposition temperature, leading to a thermodynamically driven dewetting of water from the hydrophobic CCl(4) surface. This dewetting exposes some CCl(4) to the ambient environment, allowing unhindered desorption of CCl(4) below 140 K. When ASW completely covers the underlayer, desorption of CCl(4) is delayed until crystallization induced cracking of the ASW overlayer opens an escape path to the surface. The subsequent rapid episodic release of CCl(4) is termed a "molecular volcano". Reflection absorption infrared spectroscopy (RAIRS) measurements indicate that the onset and duration of the molecular volcano is directly controlled by the ASW crystallization kinetics.

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) are used to investigate the crystallization kinetics and measure the excess free energy of metastable amorphous solid water films (ASW) of H(2)O and D(2)O grown using molecular beams. The desorption rates from the amorphous and crystalline phases of ASW are distinct, and as such, crystallization manifests can be observed in the TPD spectrum. The crystallization kinetics were studied by varying the TPD heating rate from 0.001 to 3 K/s. A coupled desorption-crystallization kinetic model accurately simulates the desorption spectra and accurately predicts the observed temperature shifts in the crystallization. Isothermal crystallization studies using RAIRS are in agreement with the TPD results. Furthermore, highly sensitive measurements of the desorption rates were used to determine the excess free energy of ASW near 150 K. The excess entropy obtained from these data is consistent with there being a thermodynamic continuity between ASW and supercooled liquid water.

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.

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.

The adsorption and desorption of HCl on Pt(111) is investigated by temperature programmed desorption, infrared reflection absorption spectroscopy, and low energy electron diffraction. Five peaks are identified in the desorption spectra prior to the onset of multilayer desorption. At low coverage (0.25 monolayers (ML)), desorption peaks at approximately 135 and 200 K are observed and assigned to recombinative desorption of dissociated HCl. At higher coverages, desorption peaks at 70, 77, and 84 K are observed. These peaks are assigned to the desorption of molecularly adsorbed HCl. The infrared spectra are in agreement with these assignments and show that HCl deposited at 20 K is amorphous but crystallizes when heated above 60 K. Kinetic analysis of the desorption spectra reveals a strong repulsive coverage dependence for the desorption energy of the low coverage features (0.25 ML). The diffraction data indicate that at low temperature the adsorbed HCl clusters into ordered islands with a (3 x 3) structure and a local coverage of 4/9 with respect to the Pt(111) substrate.

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.

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 (typically100 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.

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.

Layered nanoscale amorphous solid films of methanol and ethanol undergo complete intermixing prior to the onset of measurable desorption at 120 K. This intermixing precedes and inhibits crystallization. Subsequent desorption of the film is described quantitatively by a kinetic model describing evaporation from a continuously mixed ideal binary liquid solution. This occurs at temperatures below the melting point of the binary mixture, indicating ideal behavior for the supercooled liquid solution. This approach provides a new method for preparing and examining deeply supercooled solutions.

We show that N2 monolayer desorption from ice surfaces is a quantitative, highly sensitive method for following the surface crystallization kinetics at low temperatures. Vapor deposited water films on a crystalline ice substrate exhibit amorphous growth at temperatures below ∼110 K. The rate of crystallization for these amorphous films is dramatically accelerated compared to the rate of crystallization observed for the amorphous films deposited directly on Pt(111). We find that the crystalline ice substrate acts as a two-dimensional nucleus for the growth of the crystalline phase, thereby accelerating the crystallization kinetics.