The first layer of water on Rh(111): microscopic structure and desorption kinetics

The adsorption states and growth process of the first water (D2O) layer on Rh(111) were investigated using infrared reflection absorption spectroscopy, temperature programed desorption, and spot-profile-analysis low energy electron diffraction. Water molecules wet the Rh(111) surface intact. At the early stage of first layer growth, a (square root 3 x square root 3)R30 degrees commensurate water layer grows where "up" and "down" species coexist; the up and down species represent water molecules which have free OD, pointing to a vacuum and the substrate, respectively. The up domain was a flatter structure than an icelike bilayer. Water desorption from Rh(111) was a half-order process. The activation energy and the preexponential factor of desorption are estimated to be 60 kJ/mol and 4.8 x 10(16) ML(1/2)/s at submonolayer coverage, respectively. With an increase in water coverage, the flat up domain becomes a zigzag layer, like an ice bilayer. At the saturation coverage, the amount of down species is 1.3 times larger than that of the up species. In addition, the activation energy and the preexponential factor of desorption decrease to 51 kJ/mol and 1.3 x 10(14) ML(1/2)/s, respectively.     

Helium atom scattering study of the interaction of water with the BaF2(111) surface

The interaction of water with the BaF2(111) single crystal surface is investigated using the helium atom scattering technique. It is found that H2O forms a long-range ordered two-dimensional (2D) phase with (1 x 1) translational symmetry already after an exposure of 3 L (1 L=10(-6) Torr s) at temperatures below 150 K. The activation energy for desorption of the saturated 2D phase, which is assigned to a bilayer, is estimated to be 46+/-2 kJ mol(-1), corresponding to a desorption temperature of 165 K. The desorption of multilayers was observed at 150 K, consistent with a binding energy of 42+/-2 kJ mol(-1). Before completion and after desorption of the saturated 2D phase, a superstructure consistent with a disordered (square root of 3 x square root of 3)R30 degrees lattice has been observed, which is attributed to the first layer of water with a coverage of one molecule per surface unit cell, in accordance with recent theoretical studies. Desorption of this phase is observed at temperatures above 200 K, consistent with an unexpectedly strong bonding of the molecules to the substrate.     

The sticking probability of D(2)O-water on ice: Isotope effects and the influence of vibrational excitation

The present study measures the sticking probability of heavy water (D(2)O) on H(2)O- and on D(2)O-ice and probes the influence of selective OD-stretch excitation on D(2)O sticking on these ices. Molecular beam techniques are combined with infrared laser excitation to allow for precise control of incident angle, translational energy, and vibrational state of the incident molecules. For a translational energy of 69 kJ∕mol and large incident angles (θ ≥ 45°), the sticking probability of D(2)O on H(2)O-ice was found to be 1% lower than on D(2)O-ice. OD-stretch excitation by IR laser pumping of the incident D(2)O molecules produces no detectable change of the D(2)O sticking probability (<10(-3)). The results are compared with other gas∕surface systems for which the effect of vibrational excitation on trapping has been probed experimentally.     

Dewetting growth of crystalline water ice on a hydrogen saturated Rh(111) surface at 135 K

We investigated the water (D(2)O) adsorption at 135?K on a hydrogen pre-adsorbed Rh(111) surface using temperature programmed desorption and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. With increasing the hydrogen coverage, the desorption temperature of water decreases. At the saturation coverage of hydrogen, dewetting growth of water ice was observed: large three-dimensional ice grains are formed. The activation energy of water desorption from the hydrogen-saturated Rh(111) surface is estimated to be 51 kJ/mol. The initial sticking probability of water decreases from 0.46 on the clean surface to 0.35 on the hydrogen-saturated surface. In IRAS measurements, D-down species were not observed on the hydrogen saturated surface. The present experimental results clearly show that a hydrophilic Rh(111) clean surface changes into a hydrophobic surface as a result of hydrogen adsorption.     

Coverage-dependent sticking probability and desorption kinetics of water molecules on Rh(111)

The adsorption and desorption kinetics of water molecules on Rh(111) were investigated using temperature programed desorption (TPD). Water molecules on Rh(111) show coverage-dependent sticking probability; the initial sticking probability is estimated to be 0.46. In the desorption process, a dilute gaslike phase and two-dimensional islands of water coexist on the surface. Based on the model proposed by Kreuzer and Payne [Surf. Sci.200, L433 (1988)], the apparent fractional-order TPD spectra can be interpreted as first-order desorption from the coexistence of two phases on which the sticking probabilities are different. Based on this, the previous estimation of pre-exponential factors assuming half-order desorption [A. Beniya et al., J. Chem. Phys.125, 054717 (2006)] should be revised.     

Microcanonical transition state theory for activated gas-surface reaction dynamics: application to H2/CU(111) with rotation as a spectator

A microcanonical unimolecular rate theory (MURT) model incorporating quantized surface vibrations and Rice-Ramsperger-Kassel-Marcus rate constants is applied to a benchmark system for gas-surface reaction dynamics, the activated dissociative chemisorption and associative desorption of hydrogen on Cu(111). Both molecular translation parallel to the surface and rotation are treated as spectator degrees of freedom. MURT analysis of diverse experiments indicates that one surface oscillator participates in the dissociative transition state and that the threshold energy for H2 dissociation on Cu(111) is E0 = 62 kJ/mol. The spectator approximation for rotation holds well at thermally accessible rotational energies (i.e., for Er less than approximately 40 kJ/mol). Over the temperature range from 300 to 1000 K, the calculated thermal dissociative sticking coefficient is ST = S0 exp(-Ea/kBT) where S0 = 1.57 and Ea = 62.9 kJ/mol. The sigmoid shape of rovibrational eigenstate-resolved dissociative sticking coefficients as a function of normal translational energy is shown to derive from an averaging of the microcanonical sticking coefficient, with threshold energy E0, over the thermal surface oscillator distribution of the gas-surface collision complexes. Given that H2/Cu(111) is one of the most dynamically biased of gas-surface reactive systems, the simple statistical MURT model simulates and broadly rationalizes the H2/Cu(111) reactive behavior with remarkable fidelity.     

Hydrogen peroxide formation following the vacuum ultraviolet photodissociation of water ice films at 90 K

The production of gaseous OH radicals from the 300-350 nm photodissociation of H(2)O(2) that was photolytically produced on a water ice surface following the 157 nm photolysis of water ice at 90 K was directly monitored using resonance-enhanced multiphoton ionization. The translational energy distribution estimated by the time-of-flight spectrum of the OH products is represented by a Maxwell-Boltzmann energy distribution with a translational temperature of 3750+/-250 K. The rotational temperature was estimated by a spectral simulation to be 225+/-25 K. Surface defects produced by HCl deposition on the water ice contributed to the higher production rate of H(2)O(2) in the 157 nm photoirradiation of water ice while surface coverage caused by CD(3)OH deposition decreased the H(2)O(2) production rate.     

The adsorption and desorption of ethanol ices from a model grain surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures >50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3+/-1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K.     

Heat of adsorption of naphthalene on Pt(111) measured by adsorption calorimetry

The heat of adsorption of naphthalene on Pt(111) at 300 K was measured with single-crystal adsorption calorimetry. The heat of adsorption on the ideal, defect-free surface is estimated to be (300 - 34 - 199(2)) kJ/mol. From this, a C-Pt bond energy for aromatic hydrocarbons on Pt(111) of approximately 30 kJ/mol is estimated, consistent with earlier results for benzene on Pt(111). There is higher heat of adsorption at very low coverage, attributed to step sites where the adsorption heat is >/=330 kJ/mol. Saturation coverage, = 1 ML, corresponds to 1.55 x 10(14) molecules/cm(2). Sticking probability measurements of naphthalene on Pt(111) give a high initial value of 1.0 and a Kisliuk-type coverage dependence that implies precursor-mediated sticking. The ratio of the hopping rate to the desorption rate of this precursor is approximately 51. Naphthalene adsorbs transiently on top of chemisorbed naphthalene molecules with a heat of adsorption of 83-87 kJ/mol.     

Dynamics of propene adsorption on Ag001

The interaction of propene with Ag(001) is investigated by high resolution electron energy loss spectroscopy and supersonic molecular beam methods under ultra high vacuum conditions. Propene adsorbs molecularly at 110 K and desorbs intact leaving a clean surface after annealing to 160 K. Two adsorption sites, characterized by slightly different vibrational modes, exist. The low frequency species is observed already at low coverage for molecules impinging at strongly hyperthermal energies while at lower translational energy it appears only at high coverage. The initial sticking probability S(0) decreases with increasing translational energy, as appropriate for nonactivated adsorption systems. The angle and energy dependence of S(0) indicate that scaling is intermediate between total and normal energy. From the coverage dependence of the sticking probability we infer that both a nonthermal intrinsic and a thermal extrinsic precursor exist.     

Vertical diffusion of water molecules near the surface of ice

We studied diffusion of water molecules in the direction perpendicular to the surface of an ice film. Amorphous ice films of H(2)O were deposited on Ru(0001) at temperature of 100-140 K for thickness of 1-5 bilayer (BL) in vacuum, and a fractional coverage of D(2)O was added onto the surface. Vertical migration of surface D(2)O molecules to the underlying H(2)O multilayer and the reverse migration of H(2)O resulted in change of their surface concentrations. Temporal variation of the H(2)O and D(2)O surface concentrations was monitored by the technique of Cs(+) reactive ion scattering to reveal kinetics of the vertical diffusion in depth resolution of 1 BL. The first-order rate coefficient for the migration of surface water molecules ranged from k(1)=5.7(+/-0.6) x 10(-4) s(-1) at T=100 K to k(1)=6.7(+/-2.0) x 10(-2) s(-1) at 140 K, with an activation energy of 13.7+/-1.7 kJ mol(-1). The equivalent surface diffusion coefficients were D(s)=7 x 10(-19) cm(2) s(-1) at 100 K and D(s)=8 x 10(-17) cm(2) s(-1) at 140 K. The measured activation energy was close to interstitial migration energy (15 kJ mol(-1)) and was much lower than diffusion activation energy in bulk ice (52-70 kJ mol(-1)). The result suggested that water molecules diffused via the interstitial mechanism near the surface where defect concentrations were very high.     

Dynamically biased RRKM model of activated gas-surface reactivity: vibrational efficacy and rotation as a spectator in the dissociative chemisorption of CH4 on Pt(111)

The reactivity of CH(4) impinging on a Pt(111) surface was examined using a precursor-mediated microcanonical trapping model of dissociative chemisorption wherein the effects of rotational and vibrational energy could be explored. Dissociative sticking coefficients for a diverse range of non-equilibrium effusive beam, supersonic beam, and eigenstate-resolved experiments were simulated and an average relative discrepancy between theory and experiment of better than 50% was achieved by treating molecular rotations and translation parallel to the surface as spectator degrees of freedom, and introducing a dynamically-biased vibrational efficacy. The model parameters are {E(0) = 57.9 kJ mol(-1), s = 2, η(v) = 0.40} where E(0) is the apparent threshold energy for reaction, s is the number of surface oscillators participating in energy exchange within each gas-surface collision complex formed, and η(v) is the mean vibrational efficacy for reaction relative to normal translational energy which figures in the assembly of the active exchangeable energy which is available to surmount the activation barrier to dissociative chemisorption. GGA-DFT electronic structure calculations provided vibrational frequencies for the transition state for dissociative chemisorption. The asymmetry of the rotational state populations in supersonic and effusive molecular beam experiments allowed kinetic analysis to establish that taking rotation as a spectator degree of freedom is a good approximation. Surface phonons, rather than the incident molecules, are calculated to play the dominant role in supplying the energy required to overcome the activation barrier for dissociative chemisorption under the thermal equilibrium conditions relevant to high pressure catalysis. Over the temperature range 300 K ≤T≤ 1000 K, the thermal dissociative sticking coefficient is predicted to be well described by S(T) = S(0) exp(-E(a)/RT) where S(0) = 0.62 and E(a) = 62.6 kJ mol(-1).     

State-resolved dynamics of oxygen atom recombination on polycrystalline Ag

Rotationally resolved, velocity distributions for desorbed O2 molecules formed by O-atom recombination on the surface of a polycrystalline Ag surface are reported. Surface O atoms are generated by oxygen permeation through a 0.25-mm-thick Ag foil heated to 1020 K. Desorbing O2 molecules are probed by (2 + 1) resonant multiphoton ionization via the C 3Pig (3ssigma), v' = 2 <-- <-- X 3Sigmag-, v" = 0 transition and time-of-flight mass spectrometry. Measured velocity distributions are near Maxwell-Boltzmann and yield average translational energies which are significantly lower than the surface temperature ([Et]/2kB approximately 515 K) and essentially independent of rotational excitation. Comparison of the observed C-X (2,0) resonantly enhanced multiphoton ionization spectrum with spectral simulations suggests that the v" = 0 rotational state distribution is more consistent with the surface temperature, but spectral congestion and apparent intensity perturbations prevent a more quantitative analysis. The calculated, sticking curves show a small barrier energy barrier (approximately 10 meV) beyond which sticking decreases. These observations are consistent with low energy desorption and adsorption pathways involving a weakly bound molecular O2 precursor.     

甲烷在Ni表面及La薄膜上激活解离化学吸附

利用分子束技术改变甲烷的平动能E_k来研究E_k及其法向分量E_n对甲烷在Ni表面及La薄膜上激活解离吸附的影响。对CH_4/Ni及CH_4/La系统, 当甲烷的平动能E_k分别低于58.5 kJ·mol~(-1)及52.3 kJ·mol~(-1)时, 没观察到甲烷的解离吸附。当甲烷的平动能超过此阈值时, 即对CH_4/Ni系统, 当Ek=58.5增至63.8 kJ·mol~(-1)时, 初始沾着几率s_0由0至0.54线性增加; 对CH_4/La系统, 当E_k=52.3增至63.8 kJ·mol~(-1)时, S_0由0至0.49线性增加。这些结果表明, 两个系统的化学吸附是不经过前趋态的直接化学吸附。最后求出CH_4/Ni, CH_4/La系统的表观活化能分别为46.8 kJ·mol~(-1)和38.1 kJ·mol~(-1)。     

Site-dependent electron-stimulated reactions in water films on TiO2(110)

Electron-stimulated reactions in thin [<3 ML (monolayer)] water films adsorbed on TiO(2)(110) are investigated. Irradiation with 100 eV electrons results in electron-stimulated dissociation and electron-stimulated desorption (ESD) of adsorbed water molecules. The molecular water ESD yield increases linearly with water coverage theta for 0< or =theta< or =1 ML and 11 ML, the water ESD yield per additional water molecule adsorbed (i.e., the slope of the ESD yield versus coverage) is 3.5 times larger than for theta<1 ML. In contrast, the number of water molecules dissociated per incident electron increases linearly for theta< or =2 ML without changing slope at theta=1 ML. The total electron-stimulated sputtering rate, as measured by postirradiation temperature programmed desorption of the remaining water, is larger for theta>1 ML due to the increased water ESD for those coverages. The water ESD yields versus electron energy (for 5-50 eV) are qualitatively similar for 1, 2, and 40 ML water films. In each case, the observed ESD threshold is at approximately 10 eV and the yield increases monotonically with increasing electron energy. The results indicate that excitations in the adsorbed water layer are primarily responsible for the ESD in thin water films on TiO(2)(110). Experiments on "isotopically layered" films with D(2)O adsorbed on the Ti(4+) sites (D(2)O(Ti)) and H(2)O adsorbed on the bridging oxygen atoms (H(2)O(BBO)) demonstrate that increasing the water coverage above 1 ML rapidly suppresses the electron-stimulated desorption of D(2)O(Ti) and D atoms, despite the fact that the total water ESD and atomic hydrogen ESD yields increase with increasing coverage. The coverage dependence of the electron-stimulated reactions is probably related to the different bonding geometries for H(2)O(Ti) and H(2)O(BBO) and its influence on the desorption probability of the reaction products.     

Co-adsorption of water and hydrogen on Ni(111)

We have studied the surface coverage dependence of the co-adsorption of D and D(2)O on the Ni(111) surface under UHV conditions. We use detailed temperature-programmed desorption studies and high resolution electron energy loss spectroscopy to show how pre-covering the surface with various amounts of D affects adsorption and desorption of D(2)O. Our results show that the effects of co-adsorption are strongly dependent on D-coverage. In the deuterium pre-coverage range of 0-0.3 ML, adsorption of deuterium leaves a fraction of the available surface area bare for D(2)O adsorption, which shows no significant changes compared to adsorption on the bare surface. Our data indicate phase segregation of hydrogen and water into islands. At low post-coverages, D(2)O forms a two-phase system on the remaining bare surface that shows zero-order desorption kinetics. This two phase system likely consists of a 2-D solid phase of extended islands of hexamer rings and a 2-D water gas phase. Increasing the water post-dose leads at first to 'freezing' of the 2-D gas and is followed by formation of ordered, multilayered water islands in-between the deuterium islands. For deuterium pre-coverages between 0.3 and 0.5 ML, our data may be interpreted that the water hexamer ring structure, (D(2)O)(6), required for the formation of an ordered multilayer, does not form anymore. Instead, more disordered linear and branched chains of water molecules grow in-between the extended, hydrophobic deuterium islands. These deuterium islands have a D-atom density in agreement with a (2x2)-2D structure. The disordered water structures adsorbed in-between form nucleation sites for growth of 3-D water structures. Loss of regular lateral hydrogen bonding and weakened interaction with the substrate reduces the binding energy of water significantly in this regime and results in lowering of the desorption temperature. At deuterium pre-coverages greater than 0.5 ML, the saturated (2x2)-2D structure mixes with (1x1)-1D patches. The mixed structures are also hydrophobic. On such surfaces, submonolayer doses of water lead to formation of 3-D water structures well before wetting the entire hydrogen-covered surface.     

Infrared spectroscopy of oxygen adsorbed on hydrogen covered Pt(111)

Using infrared reflection absorption spectroscopy we have investigated how preadsorbed hydrogen affects the adsorption of O(2) on the Pt(111) surface at temperatures below the onset of the water formation reaction. On the fully hydrogen covered surface, Theta(H)=1, O(2) physisorbs at temperatures below 45 K, the weakly dipole active internal stretch vibration is observed at 1548 cm(-1). Unlike on the clean Pt(111) surface, this adsorption state does not act as a precursor for O(2) chemisorption. The physisorbed molecules simply desorb above 45 K and no chemisorbed O(2) state is populated directly from the gas phase in the temperature range 45-90 K. When the surface is approximately half covered, Theta(H) approximately 0.4, with preadsorbed hydrogen, O(2) chemisorbs on the clean Pt(111) surface regions in the characteristic peroxolike and superoxolike states with vibration frequencies around 700 cm(-1) and 870 cm(-1). These values correspond to dense O(2) islands which develop already at low O(2) coverages. At this hydrogen coverage, we find that the initial sticking probability of chemisorbed O(2) is drastically reduced at 90 K and the general uptake also proceeds slowly when compared with observations for the clean surface. We suggest that this is due to a change in the behavior of the physisorbed O(2) precursor.     

Hydrogen molecule in the small dodecahedral cage of a clathrate hydrate: quantum five-dimensional calculations of the coupled translation-rotation eigenstates

We report quantum five-dimensional (5D) calculations of the energy levels and wave functions of the hydrogen molecule, para-H2 and ortho-H2, confined inside the small dodecahedral (H2O)20 cage of the sII clathrate hydrate. All three translational and the two rotational degrees of freedom of H2 are included explicitly, as fully coupled, while the cage is treated as rigid. The 5D potential energy surface (PES) of the H2-cage system is pairwise additive, based on the high-quality ab initio 5D (rigid monomer) PES for the H2-H2O complex. The bound state calculations involve no dynamical approximations and provide an accurate picture of the quantum 5D translation-rotation dynamics of H2 inside the cage. The energy levels are assigned with translational (Cartesian) and rotational quantum numbers, based on calculated root-mean-square displacements and probability density plots. The translational modes exhibit negative anharmonicity. It is found that j is a good rotational quantum number, while the threefold degeneracy of the j = 1 level is lifted completely. There is considerable translation-rotation coupling, particularly for excited translational states.     

Determination of the sticking coefficient and scattering dynamics of water on ice using molecular beam techniques

The sticking coefficient for D(2)O impinging on crystalline D(2)O ice was determined for incident translational energies between 0.3 and 0.7 eV and for H(2)O on crystalline H(2)O ice at 0.3 eV. These experiments were done using directed molecular beams, allowing for precise control of the incident angle and energy. Experiments were also performed to measure the intensity and energy of the scattered molecules as a function of scattering angle. These results show that the sticking coefficient was near unity, slightly increasing with decreasing incident energy. However, even at the lowest incident energy, some D(2)O did not stick and was scattered from the ice surface. We observe under these conditions that the sticking probability asymptotically approaches but does not reach unity for water sticking on water ice. We also present evidence that the scattered fraction is consistent with a binary collision; the molecules are scattered promptly. These results are especially relevant for condensation processes occurring under nonequilibrium conditions, such as those found in astrophysical systems.     

Entropy and dynamics of water in hydration layers of a bilayer

We compute the entropy and transport properties of water in the hydration layer of dipalmitoylphosphatidylcholine bilayer by using a recently developed theoretical scheme [two-phase thermodynamic model, termed as 2PT method; S.-T. Lin et al., J. Chem. Phys. 119, 11792 (2003)] based on the translational and rotational velocity autocorrelation functions and their power spectra. The weights of translational and rotational power spectra shift from higher to lower frequency as one goes from the bilayer interface to the bulk. Water molecules near the bilayer head groups have substantially lower entropy (48.36 J/mol/K) than water molecules in the intermediate region (51.36 J/mol/K), which have again lower entropy than the molecules (60.52 J/mol/K) in bulk. Thus, the entropic contribution to the free energy change (TΔS) of transferring an interface water molecule to the bulk is 3.65 kJ/mol and of transferring intermediate water to the bulk is 2.75 kJ/mol at 300 K, which is to be compared with 6.03 kJ/mol for melting of ice at 273 K. The translational diffusion of water in the vicinity of the head groups is found to be in a subdiffusive regime and the rotational diffusion constant increases going away from the interface. This behavior is supported by the slower reorientational relaxation of the dipole vector and OH bond vector of interfacial water. The ratio of reorientational relaxation time for Legendre polynomials of order 1 and 2 is approximately 2 for interface, intermediate, and bulk water, indicating the presence of jump dynamics in these water molecules.