Infrared spectroscopy of solid hydrogen sulfide and deuterium sulfide

The infrared spectra of solid hydrogen sulfide (H2S) and deuterium sulfide (D2S) were collected at very low temperatures. Vapor deposition of thin films at the lowest temperature of 10 K produced amorphous solids while deposition at 70 K yielded the crystalline phase III. Infrared interference fringe patterns produced by the films during deposition were used to determine the film thickness. Careful measurement of the integrated absorbance peaks, along with the film thickness, allowed determination of the integrated band intensities. This report represents the first complete presentation of the infrared spectra of the amorphous solids. Observations of peaks near 3.915 and 1.982 microm (ca. 2554 and 5045 cm(-1), respectively) may be helpful in the conclusive identification of solid hydrogen sulfide on the surface of Io, a moon of Jupiter.     

Infrared spectroscopy of the solid phases of ammonia

Thin films of solid ammonia (NH(3) and ND(3)) have been characterized using low temperature (25-110 K) Fourier-transform infrared (FTIR) spectroscopy, and the three solid phase (amorphous, metastable, and crystalline) spectra are reported. This work has been motivated by confusion in the literature about the metastable and crystalline phases as a result of an early erroneous report by Staats and Morgan [(J. Chem. Phys. 31, 553 (1959)]. Although the crystalline phase has subsequently been reported correctly, the metastable phase has not been described in the literature in detail. The unique characteristics of the metastable phase, reported here for the first time, include multiple peaks in the nu(2) and nu(3) regions and peak intensities that are dependent on the deposition temperature. This behavior may be the result of (a) preferential molecular orientations in the solid, or (b) exciton splitting due to different crystal shapes in the solid. The amorphous and metastable phases of deuterated ammonia are also reported for the first time.     

Interaction of methanol with amorphous solid water

The interaction of methanol (MeOH) with amorphous solid water (ASW) composed of D2O molecules, prepared at 125 K on a polycrystalline Ag substrate, was studied with metastable-impact-electron spectroscopy, reflection-absorption infrared spectroscopy, and temperature-programmed desorption mass spectroscopy. In connection with the experiments, classical molecular dynamics (MD) simulations have been performed on a single CH3OH molecule adsorbed at the ice surface (T=190 K), providing further insights into the binding and adsorption properties of the molecule at the ice surface. Consistently with the experimental deductions and previous studies, MeOH is found to adsorb with the hydroxyl group pointing toward dangling bonds of the ice surface, the CH3 group being oriented upwards, slightly tilted with respect to the surface normal. It forms the toplayer up to the onset of the simultaneous desorption of D2O and MeOH. At low coverage the adsorption is dominated by the formation of two strong hydrogen bonds as evidenced by the MD results. During the buildup of the first methanol layer on top of an ASW film the MeOH-MeOH interaction via hydrogen-bond formation becomes of importance as well. The interaction of D2O with solid methanol films and the codeposition of MeOH and D2O were also investigated experimentally; these experiments showed that D2O molecules supplied to a solid methanol film become embedded into the film.     

Dewetting of thin amorphous solid water films and liquid-cubic ice coexistence in droplets studied using infrared-absorption and secondary-ion-mass spectroscopy

The infrared absorption band of decoupled OD stretching vibration (4 mol% HOD in 20-monolayer H 2O) of amorphous solid water is red-shifted and sharpened at around 160 K because of spontaneous nucleation. The crystal grows in a fluidized liquid that forms droplets on a Ni(111) substrate. The shape change and red-shift of a coupled OH band during crystallization are elucidated by a Mie particle scattering model, indicating that nanometer-size droplets are formed preferentially. The spontaneous nucleation at 160 K is bypassed when amorphous solid water is deposited on a crystallized water film; the crystals grow around nuclei at ca. 150 K, resulting in larger crystal grains that do not cause Mie scattering. However, the crystal grains behave like viscous droplets because their morphology changes continuously after the completion of crystallization. The coexisting liquid-like water is indistinguishable from cubic ice in local structure. This behavior resembles that of a quasiliquid formed during premelting.     

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.     

Mid-infrared extinction spectra and optical constants of supercooled water droplets

Complex refractive indices of supercooled liquid water have been retrieved at 269, 258, 252, and 238 K in the 4500-1100 cm(-1) wavenumber regime from series of infrared extinction spectra of micron-sized water droplets. The spectra collection was recorded during expansion experiments in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. A Mie inversion technique was applied to derive the low-temperature refractive index data sets by iteratively adjusting the room-temperature optical constants of liquid water until obtaining the best agreement between measured and calculated infrared spectra of the supercooled water droplets. The new optical constants, revealing significant temperature-induced spectral variations in comparison with the room-temperature refractive indices, proved to be in good agreement with data sets obtained in a recent study. A detailed analysis was performed to elaborate potential inaccuracies in the retrieval results when deriving optical constants from particle extinction spectra using an iterative procedure.     

Electron-stimulated production of molecular oxygen in amorphous solid water

The low-energy, electron-stimulated production of molecular oxygen from pure amorphous solid water (ASW) films and ASW films codosed with H(2)O(2) is investigated. Layered films of H(2)(16)O and H(2)(18)O are used to investigate the reaction mechanisms for O(2) production and the spatial profile of the reactions within the films. The O(2) yield is dose-dependent, indicating that precursors are involved in the O(2) production. For temperatures below approximately 80 K, the O(2) yield at steady state is relatively low and nearly independent of temperature. At higher temperatures, the yield increases rapidly. The O(2) yield is enhanced from H(2)O(2)-dosed water films, but the experiments show that H(2)O(2) is not the final precursor in the reactions leading to O(2). Instead, a stable precursor for O(2) is produced through a multistep reaction sequence probably involving the reactions of OH radicals to produce H(2)O(2) and then HO(2). The O(2) is produced in a nonthermal reaction from the HO(2). For relatively thick films, the reactions leading to O(2) occur at or near the ASW/vacuum interface. However, the electronic excitations that initiate the reactions occur over a larger range in the film. A kinetic model that qualitatively accounts for all of the observations is presented.     

Photochemistry of ethyl chloride caged in amorphous solid water

Caging and photo-induced decomposition of ethyl chloride molecules (EC) within a layer of amorphous solid water (ASW) on top of clean and oxygen-covered Ru(001) under ultra-high vacuum (UHV) conditions are presented. The caged molecules were estimated to reside 1.5 +/- 0.2 nm above the solid surface, based on parent molecule thermal decomposition on the clean ruthenium. Dissociative electron attachment (DEA) of the caged molecules following 193 nm laser irradiation, result in initial fragmentation to ethyl radical and chloride anion. It was found that photoreactivity on top of the clean ruthenium surface (Ru) is twenty times faster than on the oxygen-covered surface (O/Ru), with DEA cross sections: sigma(Ru) = (3.8 +/- 1) x 10(-19) cm(2) and sigma(O/Ru) = (2.1 +/- 0.3) x 10(-20) cm(2). This difference is attributed to the higher work function of oxygen-covered ruthenium, leading to smaller electron attachment probability due to mismatch of the ruthenium photo-electron energy with the adsorbed EC excited electron affinity levels. EC molecules fragmented within the cage, result in post-irradiation TPD spectra that reveal primarily C(4)H(8), C(3)H(5) and C(3)H(3), without any oxygen-containing molecules. Unique stabilization of the photoproducts has been observed with the first layer of water molecules in direct contact with the substrate, desorbing near 180 K, a significantly higher temperature than the desorption of fully caged molecules. This study may contribute for understanding stratospheric photochemistry and processes in interstellar space.     

Compaction of microporous amorphous solid water by ion irradiation

We have studied the compaction of vapor-deposited amorphous solid water by energetic ions at 40 K. The porosity was characterized by ultraviolet-visible spectroscopy, infrared spectroscopy, and methane adsorption/desorption. These three techniques provide different and complementary views of the structural changes in ice resulting from irradiation. We find that the decrease in internal surface area of the pores, signaled by infrared absorption by dangling bonds, precedes the decrease in the pore volume during irradiation. Our results imply that impacts from cosmic rays can cause compaction in the icy mantles of the interstellar grains, which can explain the absence of dangling bond features in the infrared spectrum of molecular clouds.     

The nucleation rate of crystalline ice in amorphous solid water

The kinetics of crystalline ice nucleation and growth in nonporous, molecular beam deposited amorphous solid water (ASW) films are investigated at temperatures near 140 K. We implement an experimental methodology and corresponding model of crystallization kinetics to decouple growth from nucleation and quantify the temperature dependence and absolute rates of both processes. Nucleation rates are found to increase from approximately 3x10(13) m(-3) s(-1) at 134 K to approximately 2x10(17) m(-3) s(-1) at 142 K, corresponding to an Arrhenius activation energy of 168 kJ/mol. Over the same temperature range, the growth velocity increases from approximately 0.4 to approximately 4 A s(-1), also exhibiting Arrhenius behavior with an activation energy of 47 kJ/mol. These nucleation rates are up to ten orders of magnitude larger than in liquid water near 235 K, while growth velocities are approximately 10(9) times smaller. Crystalline ice nucleation kinetics determined in this study differ significantly from those reported previously for porous, background vapor deposited ASW, suggesting the nucleation mechanism is dependent upon film morphology.     

Infrared optical constants of highly diluted sulfuric acid solution droplets at cirrus temperatures

Complex refractive indices for supercooled sulfuric acid solution droplets in the mid-infrared spectral regime (wavenumber range 6000-800 cm(-1)) have been retrieved for acid concentrations ranging from 33 to 10 wt % H2SO4 at temperatures between 235 and 230 K, from 36 to 15 wt % H2SO4 at temperatures between 225 and 219 K, and from 37 to 20 wt % H2SO4 at temperatures between 211 and 205 K. The optical constants were derived with a Mie inversion technique from measured H2SO4/H2O aerosol extinction spectra that were recorded during controlled expansion cooling experiments in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. The new data sets cover a range of atmospherically relevant temperatures and compositions in the binary sulfuric acid/water system for which infrared refractive indices have not been published so far, namely, the regime when supercooled H2SO4/H2O solution droplets at T < 235 K are subjected to an environment that is supersaturated with respect to the ice phase. With increasing ice supersaturation, the H2SO4/H2O aerosol particles will continuously dilute by the uptake of water vapor from the gas phase until freezing of the solution droplets eventually occurs when the acid concentration has dropped below a critical, temperature-dependent threshold value. With the aid of the new measurements, the homogeneous freezing process of supercooled H2SO4/H2O solution droplets at cirrus temperatures can be quantitatively analyzed by means of Fourier transform infrared spectroscopy, thereby overcoming a major drawback from previous studies: the need to use complex refractive indices that were measured at temperatures well above 235 K to deduce the composition of the low-concentrated H2SO4/H2O aerosol particles. As in the case of the complex refractive indices for sulfuric acid solutions with acid concentrations greater than 37 wt % H2SO4, the new low-temperature optical constants for highly diluted droplets also reveal significant temperature-induced spectral variations in comparison with the refractive indices for higher temperatures, which are associated with a change in the equilibrium between sulfate and bisulfate ions.     

Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting

Laser-induced breakdown spectroscopy of particles ejected by optical catapulting is discussed for the first time. For this purpose, materials deposited on a substrate were ejected and transported from the surface in the form of a solid aerosol by optical catapulting using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at 1064 nm. A Q-switched Nd:YAG laser at 532 nm was used for chemical characterization of the particles by laser-induced breakdown spectroscopy. Both lasers were synchronized in order to perform suitable spectral detection. The optical catapulting was optimized and evaluated using aluminum silicate particles, nickel spheres, and quartz and stainless steel particles. Experimental parameters such as the interpulse delay time, the sampling distance, the laser fluence, the sampling rate and the particle size have been studied. A correlation between these parameters and the particle size is reported and discussed.     

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.     

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.     

Electron stimulated reactions of methyl iodide coadsorbed with amorphous solid water

The electron stimulated reactions of methyl iodide (MeI) adsorbed on and suspended within amorphous solid water (ice) were studied using a combination of postirradiation temperature programmed desorption and reflection absorption infrared spectroscopy. For MeI adsorbed on top of amorphous solid water (ice), electron beam irradiation is responsible for both structural and chemical transformations within the overlayer. Electron stimulated reactions of MeI result principally in the formation of methyl radicals and solvated iodide anions. The cross section for electron stimulated decomposition of MeI is comparable to the gas phase value and is only weakly dependent upon the local environment. For both adsorbed MeI and suspended MeI, reactions of methyl radicals within MeI clusters lead to the formation of ethane, ethyl iodide, and diiodomethane. In contrast, reactions between the products of methyl iodide and water dissociation are responsible for the formation of methanol and carbon dioxide. Methane, formed as a result of reactions between methyl radicals and either parent MeI molecules or hydrogen atoms, is also observed. The product distribution is found to depend on the film's initial chemical composition as well as the electron fluence. Results from this study highlight the similarities in the carbon-containing products formed when monohalomethanes coadsorbed with amorphous solid water are irradiated by either electrons or photons.     

Interaction of benzene with amorphous solid water adsorbed on polycrystalline Ag

The interaction of benzene with polycrystalline Ag and amorphous solid water (D(2)O) deposited thereupon at 124 K was investigated. Metastable impact electron spectroscopy, Reflection-absorption infrared spectroscopy, and temperature programmed desorption were utilized to obtain information on the electronic structure and the relative contribution to the bonding properties of the aromatic molecules among themselves and with D(2)O. On Ag, the benzene molecular plane is oriented parallel to the surface in the first layer. The second layer is tilted with respect to the first one. A total work function decrease of 0.8 eV takes place during the buildup of the first two layers. On amorphous solid water, the orientational distribution of the benzene molecular planes is initially peaked at an angle parallel to the water surface. During the completion of the first adlayer a coverage-induced reorientation takes place, inducing a tilt of the benzene molecules of the first adlayer. Still larger benzene exposures appear to lead to the formation of three-dimensional benzene clusters. Films produced by codepositing benzene and D(2)O or by postdepositing D(2)O layers on benzene films display "volcano like" benzene desorption during ice crystallization.     

Infrared spectroscopy and photochemistry of NCCN+ and CNCN+ trapped in solid neon

When a Ne:NCCN sample is codeposited at 4.3 K with neon atoms that have been excited in a microwave discharge, the infrared and near infrared spectra of the resulting deposit include a prominent peak at 1799.5 cm-1, previously assigned to nu3 of NCCN+, and several new absorptions at higher frequencies which are contributed by combination bands of ground-state NCCN+. The exposure of the deposit to near infrared and red light results in the appearance of two new absorptions which are attributed to CNCN+. The reverse isomerization occurs when the sample is exposed to near ultraviolet radiation, but the two new absorptions are regenerated upon subsequent irradiation with near infrared and red light.     

Characterization of porosity in vapor-deposited amorphous solid water from methane adsorption

We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques such as quartz crystal microgravimetry, UV-visible interferometry, and infrared reflectance spectrometry in tandem with methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films condensed from both diffuse and collimated water vapor sources. Films deposited from a diffuse source show a step in the isotherms and much less adsorption at low pressures than films deposited from a collimated source with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77 degrees incidence are mesoporous, in addition to having micropores. Remarkably, mesoporosity is retained upon warming to temperatures as high as 140 K where the ice crystallized. The binding energy distribution for methane adsorption in the micropores of ice films deposited from a collimated source peaks at approximately 0.083 eV for deposition at normal incidence and at approximately 0.077 eV for deposition at >45 degrees incidence. For microporous ice, the intensity of the infrared bands due to methane molecules on dangling OH bonds on pore surfaces increases linearly with methane uptake, up to saturation adsorption. This shows that the multilayer condensation of methane does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. For ice deposited at 77 degrees incidence, the increase in intensity of the dangling bond absorptions modified by methane adsorption departs from linearity at large uptakes.     

Infrared optical constants, dielectric constants, molar polarizabilities, transition moments, dipole moment derivatives and Raman spectrum of liquid cyclohexane

Previous studies have been done in this laboratory focusing on the optical properties of several liquid aromatic and aliphatic hydrocarbons in the infrared. The current study reports the infrared and absorption Raman spectra of liquid cyclohexane. Infrared spectra were recorded at 25 °C over a wavenumber range of 7400–490 cm⁻¹. Infrared measurements were taken using transmission cells with pathlengths ranging from 3 to 5000 μm. Raman spectra were recorded between 3700 and 100 cm⁻¹ at 25 °C using a 180° reflection geometry. Ab initio calculations of the vibrational wavenumbers at the B3LYP/6311G level of theory were performed and used to help assign the observed IR and Raman spectra. Extensive assignments of the fundamentals and binary combinations observed in the infrared imaginary molar polarizability spectrum are reported. The imaginary molar polarizability spectrum was curve fitted to separate the intensity from the various transitions and used to determine the transition moments and magnitudes of the derivatives of the dipole moment with respect to the normal coordinates for the fundamentals.