Influence of surface morphology on D2 desorption kinetics from amorphous solid water

The influence of surface morphology/porosity on the desorption kinetics of weakly bound species was investigated by depositing D2 on amorphous solid water (ASW) films grown by low temperature vapor deposition under various conditions and with differing thermal histories. A broad distribution of binding energies of the D2 monolayer on nonporous and porous ASW was measured experimentally and correlated by theoretical calculations to differences in the degree of coordination of the adsorbed H2 (D2) to H2O molecules in the ASW depending on the nature of the adsorption site, i.e., surface valleys vs surface peaks in a nanoscale rough film surface. For porous films, the effect of porosity on the desorption kinetics was observed to be a reduction in the desorption rate with film thickness and a change in peak shape. This can be partly explained by fast diffusion into the ASW pore structure via a simple one-dimensional diffusion model and by a change in binding energy statistics with increasing total effective surface area. Furthermore, the D2 desorption kinetics on thermally annealed ASW films were investigated. The main effect was seen to be a reduction in porosity and in the number of highly coordinated binding sites with anneal temperature due to ASW restructuring and pore collapse. These results contribute to the understanding of desorption from porous materials and to the development of correct models for desorption from and catalytic processes on dust grain surfaces in the interstellar medium.     

Crystallization kinetics of amorphous

The active sites of hydrogen-exchanged Y zeolite (HY) and dealuminated (HDY) zeolites are investigated by TPD of carbon monoxide. Only the high temperature TPD spectra of CO (TM»620-690°C) were observed, meaning that CO molecules interact with very strong acid sites. The amounts of CO bonded on these sites are small (less than 1 molecule per unit cell). The strong influence of dealumination on the coverage degree is found. The calculated values for kinetic parameters indicate chemisorption of CO in the investigated systems (Edes»240 kJ mol^-1, A»1011 s^-1).     

Crystallization kinetics of amorphous Se

Differential scanning calorimetry was used to study crystallization behavior in selenium glass under non-isothermal conditions. The crystallization kinetics were described in terms of the Johnson–Mehl–Avrami nucleation-growth model; activation energies and kinetic parameter m _JMA were determined. The study was performed in dependence with particle size, so that a novel approach to the evaluation of crystallization kinetics—the advanced interpretation of characteristic kinetic functions—could be employed. Extensive discussion of all aspects of a full-scale kinetic study for a complex crystallization process was performed within the framework of the introduced conception. The complexity of the crystallization process was found to be represented by very closely overlapping consecutive competing surface and bulk nucleation-growth mechanisms. Mutual interactions of both mechanisms as well as all other observed effects were explained in terms of thermal gradients, surface crystallization centers arising from the sample preparation treatments and a changing amount of volume nuclei originating from the combination of the pre-nucleation period and the actual glass preparation phase. The main objective of the study is to demonstrate the extent of so-far neglected information hidden in the characteristic kinetic functions and introduce a convenient tool for its acquisition.     

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.     

Ultrafast electron-induced desorption of water from nanometer amorphous solid water films

Laser-induced desorption of water molecules from nanometer amorphous solid water films supported on a single-crystal platinum substrate is reported. A femtosecond laser pulse creates hot substrate electrons, which are injected into the water layer, resulting in significant desorption at the water-vacuum interface. The dependence of the desorption yield on film thickness and results for isotopic spacer and capping layers reveal that the desorbing water originates from relatively deep down into the water layer, i.e., from several nanometers below the surface. This is proposed to be the result of cooperative electronic effects resulting from the high electron densities in the thin water film, which cause a transient destabilization of the water H-bonded network. Motion of excited water molecules through the layer is enabled by mixing within the layer on ultrafast timescales during the desorption process.     

The effect of the incident collision energy on the porosity of vapor-deposited amorphous solid water films

Molecular beam techniques are used to grow water films on Pt(111) with various incident angles and collision energies from 5 to 205 kJ/mol. The effect of the incident angle and collision energy on the porosity and surface area of the vapor-deposited water films was studied using nitrogen physisorption and infrared spectroscopy. At low incident energy (5 kJ/mol), the infrared spectra, which provide a direct measure of the surface area, show that the surface area increases with incident angle and levels off at angles > 65 degrees . This is in contrast to the nitrogen uptake data, which display a maximum near 65 degrees because of the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor-deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition and for the structure of interstellar ices.     

Transport in amorphous solid water films: implications for self-diffusivity

Thermal desorption spectroscopy is employed to examine transport mechanisms in structured, nanoscale films consisting of labeled amorphous solid water (ASW, H(2)(18)O, H(2)(16)O) and organic spacer layers (CCl(4), CHCl(3)) prior to ASW crystallization (T approximately 150-160 K). Self-transport is studied as a function of both the ASW layer and the organic spacer layer film thickness, and the effectiveness of these spacer layers as a bulk diffusion "barrier" is also investigated. Isothermal desorption measurements of structured films are combined with gas uptake measurements (CClF(2)H) to investigate water self-transport and changes in ASW film morphology during crystallization and annealing. CCl(4) desorption is employed as a means to investigate the effects of ASW film thickness and heating schedule on vapor-phase transport. Combined, these results demonstrate that the interlayer mixing observed near T approximately 150-160 K is inconsistent with a mechanism involving diffusion through a dense phase; rather, we propose that intermixing occurs via vapor-phase transport through an interconnected network of cracks/fractures created within the ASW film during crystallization. Consequently, the self-diffusivity of ASW prior to crystallization (T approximately 150-160 K) is significantly smaller than that expected for a "fragile" liquid, indicating that water undergoes either a glass transition or a fragile-to-strong transition at a temperature above 160 K.     

Gas phase H/D exchange kinetics: DI versus D2O

The gas phase H/D exchange reactions of bradykinin (M + 3H)3+ ions with D2O and DI were monitored in a quadrupole ion trap mass spectrometer. The H/D exchange kinetics of both chemical probes (D2O and DI) indicate the presence of two noninterconverting reactive gas phase ion populations of bradykinin (M + 3H)3+ at room temperature. The H/D exchange involving DI, however, generally proceeds faster than that involving D2O. The rate observations described here can be rationalized on the basis of the "relay mechanism" (see Campbell et al. J. Am. Chem. Soc. 1995, 117, 12840-12854) recently proposed to account for H/D exchange between D2O and gaseous protonated polypeptides. The higher exchange rate with DI is believed to arise primarily as a result of its lower gas-phase acidity relative to that of D2O and, secondarily, as a result of the longer bond length of DI relative to that of OD in D2O.     

HCl adsorption and ionization on amorphous and crystalline H2O films below 50 K

Molecular beams were used to grow amorphous and crystalline H(2)O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm(-1) are superimposed. The band centered near 2530 cm(-1) is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous.     

Crystal growth kinetics in GeS2 amorphous thin films

The crystal growth kinetics of germanium disulfide in undercooled melts has been studied by optical microscopy under isothermal conditions. The linear growth kinetics of GeS₂ has been observed in the temperature range 672 ≤ T ≤ 711 K in thin film samples. The activation energy of crystal growth assuming Arrhenius behavior has been determined as E _G = 166 ± 8 kJ mol^?1 for thin film samples. From the dependence of reduced growth rate on undercooling, the interface driven 2-D surface nucleated model was estimated.     

Low energy charged particles interacting with amorphous solid water layers

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 μA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.     

Effect of temperature on the surface free energy of amorphous carbon films

Diamond-like carbon (DLC) and tetrahedral amorphous carbon (ta-C) have attracted much attention recently for biomedical and antifouling applications due to their excellent biocompatibility and inherent nonstick properties. It has been demonstrated that the solid surface free energy is a dominant factor in cellular or fouling adhesion. However, few data for the surface free energy of DLC and ta-C coatings at temperatures in the range 37-95 degrees C are available. In this study DLC and ta-C coatings on stainless steel 304 sheets were prepared using an unbalanced magnetron sputtering system and a filtered cathodic vacuum arc system, respectively. The contact angles of water, diiodomethane and ethylene glycol on the coated surfaces at temperatures in the range 20-95 degrees C were measured using a Dataphysics OCA-20 contact angle analyzer. The surface free energy of the coatings and their components (e.g., dispersion, polar or acid/base portions) were calculated using various methods. The experimental results showed that the total surface free energy and dispersive surface free energy of the ta-C coatings, DLC coatings, stainless steel 304 and titanium decreased with increasing surface temperature, while the acid-base SFE component increased with increasing temperature.     

Crystallization kinetics of amorphous S20Se80

The effect of temperature on the crystallization kinetics of bulk amorphous S₂₀Se₈₀ has been studied using differential scanning calorimetry. Using the empricial Avrami expressions, a number of kinetic parameters has been determined. The order of the reactionn was found to be 2.3 for the range of temperature 340–365 K and 1.6 in the range of temperature between 370–400 K. This range ofn indicates that the crystallization process is diffusion controlled growth with decreasing nucleation rate. The activation energy was found to be 3, 1.1 ev depending on the same range of temperatures.A time-temperature-transformation (TTT) curves has been plotted from isothermal results over the temperature range 340 K to 400 K showing the presence of minimum at 374 K. From theTTT curves the critical cooling rate required to avoid 25, 50, 75 and 100% crystallization was found to be 2.4, 1.79, 0.72 and 0.42C/sec respectively.

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Zusammenfassung Mittels DSC wurde der Einflu\ der Temperatur auf die Kristallisierungskinetik von amorphem S₂₀Se₈₀ untersucht. Unter Verwendung der empirischen Avrami Gleichungen wurde eine Anzahl kinetischer Grö\en ermittelt. Danach ergab sich die Reaktionsordnung n für den Temperaturbereich 340–365 K mit 2.3 und für den Temperaturbereich 370–400 K mit 1.6. Dieses Intervall fürn zeigt, da\ es sich beim Kristallisationsproze\ um ein diffusionsbestimmtes Wachstum mit sinkender Kristallkeimbildungsgeschwindigkeit handelt. Für die Aktivierungsenergie wurde für die gleichen Temperaturbereiche ein Wert von 3 bzw. 1.1 eV gefunden.Ausgehend von den isothermen Ergebnissen im Temperaturbereich 340–400 K wurde eine Zeit-Temperatur-Transformationskurve (TTT) dargestellt, die bei 374 K die Gegenwart eines Minimum aufweist. Auf der Basis derTTT-Kurven konnte für eine 25-, 50-, 75-bzw. 100-prozentige Kristallisation eine kritische Kühlgeschwindigkeit von 2.4, 1.79, 0.72 bzw. 0.42C/s ermittelt werden.

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The author wish to thank Prof. Dr. M.F. Kotkata Department of Physics, at Ain Shams University in Cairo, for his valuable discussions, and also Dr. F. Sale the head of the Metallurgy department at UMIST in Manchester. Who gave the facility of doing the experimental work in his department.     

H2O and D2 mixtures under pressure: spectroscopy and proton exchange kinetics

We have investigated the pressure-induced spectral changes and the proton exchange reactions of D(2)-H(2)O mixtures to 64 GPa using micro-Raman spectroscopy. The results show the profound difference in the rotational and vibrational Raman spectra of hydrogen isotopes from those of the pure samples, showing the vibrational modes at higher frequencies and continuing to increase with pressure without apparent turnover. This indicates the repulsive nature of D(2)-H(2)O interaction without hydrogen bonds between the two and, thus, interstitial fillings of D(2) molecules into the bcc-like ice lattice. The spectral analysis using the Morse potential yields a hydrogen bond distance of 0.734 ? at 6 GPa--slightly shorter than that in pure--attributed to the repulsive interaction. The pressure-dependent spectral changes suggest that the proton-ordering transition in the ice lattice occurs over a large pressure range between 28 and 50 GPa, which is substantially lower than that of pure ice (40-80 GPa). This again indicates the presence of high internal pressure arising from the repulsive interaction. The Raman spectra show evidences that the proton exchange occurs in various phases including in solid D(2) and H(2)O mixtures. Based on the time-dependent spectral changes, we obtained the proton exchange rates of k ~ 0.085 h(-1) at 0.2 GPa in fluid D(2) and water mixtures, k ~ 0.03 h(-1) and 0.003 h(-1) at 2 GPa and 4 GPa, respectively, in fluid D(2)-ice mixtures, and k ~ 10(-3) h(-1) at 8 GPa in solid D(2) and ice mixtures.     

Amorphous solid water films: transport and guest-host interactions with CO2 and N2O dopants

Guest-host interactions have been examined experimentally for amorphous solid water (ASW) films doped with CO2 or N2O. The main diagnostics are Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). ASW films deposited at 90 K are exposed to a dopant, and the first molecules that attach to a film enter its bulk until it is saturated with them. Subsequent dopant adsorption results in crystal growth atop the ASW film. There are distinct spectral signatures for these two cases: LO and TO vibrational modes for the crystal overlayer, and an easily distinguished peak for dopant molecules that reside within the ASW film. Above 105 K, the dopant surface layer desorbs fully. Some dopants residing within the ASW film remain until 155 K, at which point the ASW-to-crystalline-ice transition occurs, expelling essentially all of the dopant. No substantial differences are observed for CO2 versus N2O. It is shown that annealing an ASW film to 130 K lowers the film's capacity to include dopants by a factor of approximately 3, despite the fact that the ASW spectral feature centered at approximately 3250 cm(-1) shows no discernible change. Sandwiches were prepared: ASW-dopant-ASW etc., with the dopant layer displaying crystallinity. Raising these samples past 105 K resulted in the expulsion of essentially all of the crystalline dopant. What remained displayed the same spectral signature as the molecules that entered the bulk following adsorption at the surface. It is concluded that the adsorption sites, though prepared differently, have a lot in common. Dangling OH bonds were observed. When they interacted with a dopant, they underwent a red shift of approximately 50 cm(-1). This is in qualitative agreement with studies that have been carried out with weakly bound binary complexes. As a result of this study, a fairly complete, albeit qualitative, picture is in place for the adsorption, binding, and transport of CO2 and N2O in ASW films.     

Electron-stimulated sputtering of thin amorphous solid water films on Pt(111)

The electron-stimulated sputtering of thin amorphous solid water films deposited on Pt(111) is investigated. The sputtering appears to be dominated by two processes: (1) electron-stimulated desorption of water molecules and (2) electron-stimulated reactions leading to the production of molecular hydrogen and molecular oxygen. The electron-stimulated desorption of water increases monotonically with increasing film thickness. In contrast, the total sputtering--which includes all electron-stimulated reaction channels--is maximized for films of intermediate thickness. The sputtering yield versus thickness indicates that erosion of the film occurs due to reactions at both the water/vacuum interface and the Pt/water interface. Experiments with layered films of D2O and H2O demonstrate significant loss of hydrogen due to reactions at the Pt/water interface. The electron-stimulated sputtering is independent of temperature below approximately 80 K and increases rapidly at higher temperatures.     

Crystallization kinetics of amorphous Fe75-xCuxSi9B16

In this work the influence of Cu admixtures on the crystallization process of amorphous Fe-Si-B alloys is studied, based on measurements of differential thermal calorimetry of the series Fe_75-xCu_xSi₉B₁₆ (x=0, 1, 2, 2.8 and 3.5) during their heating with different heating rates. The first crystallization stage can not be traced for any of the amounts of Cu content examined, while the second stage is observed only when the Cu content is 1 at%. The activation energy as estimated with Kissinger's method for the third crystallization stage has a mean value of 326 kJ mol^-1 and with the isoconversional Flynn, Wall and Ozawa method is almost constant when 0.05<a<0.6 and exhibits a small monotical decrease when a>0.6. The main crystallization peak can not be described by means of a single JMA-type function. This revised version was published online in August 2006 with corrections to the Cover Date.