Crystalline Ice Surface Research Articles

Ammonia Adsorption on Crystalline Ice Surface: Adsorption Structure and Site Selectivity

The structural heterogeneity of an ice surface at the atomic level is an important characteristic that influences the interactions between adsorbate molecules and the ice surface. In this study, we investigated the adsorption of ammonia on the basal (0001) surface of crystalline ice (CI) films to examine the adsorption selectivity of ammonia molecules on atomistically different sites of the ice surface. The adsorption structure and site-selectivity of ammonia adsorption on ice surfaces were examined by conducting temperature-programmed desorption, low energy sputtering, reactive ion scattering, reflection–absorption infrared spectroscopy, and surface voltage measurements. Ammonia adsorbs on ice surfaces without undergoing protonation, dissociation, or penetration into the bulk ice. At low coverages, adsorption occurs exclusively at the dangling H atom sites of the surface via N···H–O bond formation. The average orientation of the adsorbed molecular dipoles leans substantially toward the surface. After saturation of the dangling H sites, further adsorption of ammonia leads to the formation of a multilayer with isotropic molecular orientation. Titration experiment for the surface population of dangling H sites with ammonia reveals that the surface density of free O–H groups is 0.29 ± 0.04 monolayers for the CI films prepared by water vapor deposition at 140 K and postannealing at 160 K, indicating that the basal ice surface is not significantly reconstructed under vacuum conditions.

Description of the Interfacial Behavior of Benzonitrile at Icy Surfaces by Grand Canonical Monte Carlo Simulations.

The adsorption of benzonitrile at the surface of crystalline (Ih) and low-density amorphous (LDA) ice has been investigated by grand canonical Monte Carlo simulations at temperatures ranging from 50 to 200 K. It is found that, in spite of its rather large dipole moment of 4.5 D, benzonitrile molecules can only form a highly unsaturated monolayer on LDA ice, reaching not more than 50% of the surface concentration of the saturated monolayer even at the lowest temperature considered, and they practically do not adsorb on Ih ice. In spite of the observed weak ability of the benzonitrile molecules for being adsorbed, the estimated heat of adsorption at an infinitely low surface concentration of -66.8 ± 2.2 kJ/mol is rather large. This value includes the contribution of roughly -30 to -35 kJ/mol of a benzene ring, about -10 kJ/mol of a large molecular dipole moment, and about -20 to -25 kJ/mol of a benzonitrile-water H-bond, as estimated from comparisons with the heat of adsorption values of similar molecules. The surprisingly weak ability of benzonitrile for adsorption is thus attributed to the unusually strong cohesion between the molecules, considerably exceeding their adhesion to ice, as reflected in the 70-80 kJ/mol difference of the lateral and ice contributions to the binding energy of surface benzonitrile molecules in the presence of condensed benzonitrile.

H2 Formation on Interstellar Grains and the Fate of Reaction Energy

Abstract Molecular hydrogen is the most abundant molecular species in the universe. While no doubts exist that it is mainly formed on the interstellar dust grain surfaces, many details of this process remain poorly known. In this work, we focus on the fate of the energy released by the H2 formation on the dust icy mantles: how it is partitioned between the substrate and the newly formed H2, a process that has a profound impact on the interstellar medium. We carried out state-of-the-art ab initio molecular dynamics simulations of H2 formation on periodic crystalline and amorphous ice surface models. Our calculations show that up to two-thirds of the energy liberated in the reaction (∼300 kJ mol−1 ∼3.1 eV) is absorbed by the ice in less than 1 ps. The remaining energy (∼140 kJ mol−1 ∼1.5 eV) is kept by the newly born H2. Since it is 10 times larger than the H2 binding energy on the ice, the new H2 molecule will eventually be released into the gas phase. The ice water molecules within ∼4 Å from the reaction site acquire enough energy, between 3 and 14 kJ mol−1 (360–1560 K), to potentially liberate other frozen H2 and, perhaps, frozen CO molecules. If confirmed, the latter process would solve the long standing conundrum of the presence of gaseous CO in molecular clouds. Finally, the vibrational state of the newly formed H2 drops from highly excited states (ν = 6) to low (ν ≤ 2) vibrational levels in a timescale of the order of picoseconds.

Chemical Desorption versus Energy Dissipation: Insights from Ab Initio Molecular Dynamics of HCO· Formation

Abstract Molecular clouds are the cold regions of the Milky Way where stars form. They are enriched by rather complex molecules. Many of these molecules are believed to be synthesized on the icy surfaces of the interstellar submicron-sized dust grains that permeate the Galaxy. At 10 K thermal desorption is inefficient and, therefore, why these molecules are found in the cold gas has tantalized astronomers for years. The assumption of the current models, called chemical desorption, is that the molecule formation energy released by the chemical reactions at the grain surface is partially absorbed by the grain and the remaining energy causes the ejection of the newly formed molecules into the gas. Here we report accurate ab initio molecular dynamics simulations aimed at studying the fate of the energy released by the first reaction of the H· addition chain to CO, H· + CO HCO·, occurring on a crystalline ice surface model. We show that about 90% of the HCO· formation energy is injected toward the ice in the first picosecond, leaving HCO· with an energy content (10–15 kJ mol−1) of less than half its binding energy (30 kJ mol−1). As a result, in agreement with laboratory experiments, we conclude that chemical desorption is inefficient for this specific system, namely H· + CO on crystalline ice. We suspect this behavior to be quite general when dealing with hydrogen bonds, which are responsible for both the cohesive energy of the ice mantle and the interaction with adsorbates, as HCO·, even though ad hoc simulations are needed to draw specific conclusions on other systems.

Direct Experimental Evidence for Markedly Enhanced Surface Proton Activity Inherent to Water Ice

Autoionization and subsequent proton transfer processes determine the proton activity inherent to water molecular systems. In this study, we provide direct experimental evidence that the proton activity is markedly enhanced at the surface of crystalline ice, on the basis of simultaneous observation of H/D exchange of water molecules at the surface and in the interior of well-defined double-layer ice films composed of H2O and D2O. Thermal desorption mass spectrometry showed clear signatures derived from the surface H/D exchange equilibrium, whereas infrared absorption spectroscopy indicated no appreciable H/D exchange progress in the interior. Detailed kinetic analyses revealed that the rate of H/D exchange at the surface is at least three orders of magnitude higher than in the interior. This drastic enhancement of the proton activity suggests an extremely high concentration of surface hydrated protons in comparison with those in the bulk. Our results also highlight the impact of the local hydrogen-bond structure on the autoionization event of water molecules.

Adsorption of Formamide at the Surface of Amorphous and Crystalline Ices under Interstellar and Tropospheric Conditions. A Grand Canonical Monte Carlo Simulation Study

The adsorption of formamide is studied both at the surface of crystalline (Ih) ice at 200 K and at the surface of low density amorphous (LDA) ice in the temperature range of 50-200 K by grand canonical Monte Carlo (GCMC) simulation. These systems are characteristic of the upper troposphere and of the interstellar medium (ISM), respectively. Our results reveal that while no considerable amount of formamide is dissolved in the bulk ice phase in any case, the adsorption of formamide at the ice surface under these conditions is a very strongly preferred process, which has to be taken into account when studying the chemical reactivity in these environments. The adsorption is found to lead to the formation of multimolecular adsorption layer, the occurrence of which somewhat precedes the saturation of the first molecular layer. Due to the strong lateral interaction acting between the adsorbed formamide molecules, the adsorption isotherm does not follow the Langmuir shape. Adsorption is found to be slightly stronger on LDA than Ih ice under identical thermodynamic conditions, due to the larger surface area exposed to the adsorption. Indeed, the monomolecular adsorption capacity of the LDA and Ih ice surfaces is found to be 10.5 ± 0.7 μmol/m2 and 9.4 μmol/m2, respectively. The first layer formamide molecules are very strongly bound to the ice surface, forming typically four hydrogen bonds with each other and the surface water molecules. The heat of adsorption at infinitely low surface coverage is found to be -105.6 kJ/mol on Ih ice at 200 K.

Haumea’s thermal emission revisited in the light of the occultation results

A recent multi-chord occultation measurement of the dwarf planet (136108) Haumea (Ortiz et al., 2017) revealed an elongated shape with the longest axis comparable to Pluto’s mean diameter. The chords also indicate a ring around Haumea’s equatorial plane, where its largest moon, Hi’iaka, is also located. The Haumea occultation size estimate (size of an equal-volume sphere11Dequ = 2 · (a · b · c)1/3. Dequ = 1595 km) is larger than previous radiometric solutions (equivalent sizes in the range between 1150 and 1350 km), which lowers the object’s density to about 1.8 g/cm3, a value closer to the densities of other large TNOs. We present unpublished and also reprocessed Herschel and Spitzer mid- and far-infrared measurements. We compare 100 and 160 µm thermal lightcurve amplitudes - originating from Haumea itself - with models of the total measured system fluxes (ring, satellite, Haumea) from 24–350 µm. The combination with results derived from the occultation measurements allows us to reinterpret the object’s thermal emission. Our radiometric studies show that Haumea’s crystalline water ice surface must have a thermal inertia of about 5 J K−1 m−2s−1/2 (combined with a root mean square of the surface slopes of 0.2). We also have indications that the satellites (at least Hi’iaka) must have high geometric albedos ≳ 0.5, otherwise the derived thermal amplitude would be inconsistent with the total measured system fluxes at 24, 70, 100, 160, 250, and 350 µm. The high albedos imply sizes of about 300 and 150 km for Hi’iaka and Namaka, respectively, indicating unexpectedly high densities > 1.0 g cm−3 for TNOs this small, and the assumed collisional formation from Haumea’s icy crust. We also estimated the thermal emission of the ring for the time period 1980–2030, showing that the contribution during the Spitzer and Herschel epochs was small, but not negligible. Due to the progressive opening of the ring plane, the ring emission will be increasing in the next decade when JWST is operational. In the MIRI 25.5 µm band it will also be possible to obtain a very high-quality thermal lightcurve to test the derived Haumea properties.

Self-Similar Mode of Ice Surface Softening During Friction

Softening of ice surface under friction is explored in terms of the rheological model for viscoelastic matter approximation. The non-linear relaxation of strain and fractional feedbacks are allowed. Additive non-correlated noise associated with shear strain, stress as well as with temperature of ice surface layer, is introduced, and a phase diagram is built where the noise intensities of the stress and temperature define the domains of crystalline ice, softened ice, and two types of their mixture (stick-slip friction). Conditions are revealed under which crystalline ice and stick-slip friction proceed in the self-similar mode. Corresponding strain power-law distribution is provided by temperature fluctuations that are much larger than noise intensities of strain and stress. This behavior is fixed by homogenous probability density in which characteristic strain scale is absent. Since the power-type distribution is observed at minor strains, it meets self-similar crystalline ice surface. Constructed friction force time series are investigated for all rubbing regimes using fast Fourier transformation and autocorrelation function analysis. It is revealed that these series represent “pink”-colored noise and weak correlations are realized in the system. The conclusion is drawn that the results of tribological experiment can be forecasted.

Efficient Thermal Reactions of Sulfur Dioxide on Ice Surfaces at Low Temperature: A Combined Experimental and Theoretical Study

The interaction of sulfur dioxide (SO2) gas with a crystalline ice surface at low temperature was studied by analyzing the surface species with low energy sputtering (LES) and reactive ion scattering methods and the desorbing gases with temperature-programmed desorption mass spectrometry. The study gives direct evidence for the occurrence of efficient hydrolysis of SO2 with low energy barriers on the ice surface. Adsorbed SO2 molecules react with the ice surface at temperatures above ∼90 K to form anionic molecular species, which can be detected by OH–, SO2–, and HSO3– emission signals in the LES experiments. Heating the sample above ∼120 K causes the desorption of SO2 gas from the surface-bound hydrolysis products. As a result, the hydrolysis of SO2 on an ice surface is most efficient at 100–120 K. The surface products formed at these temperatures correspond to metastable states, which are kinetically isolated on the cold surface. Quantum mechanical calculations of a model ice system suggest plausible me...

Long-Time Scale Simulations of Tunneling-Assisted Diffusion of Hydrogen on Ice Surfaces at Low Temperature

Hydrogen (H) atom diffusion on dust grain surfaces is the rate-limiting step in many hydrogenation reactions taking place in interstellar clouds. In cold (10–30 K) molecular clouds, the dust grains are coated by amorphous water ice. Therefore, H adatom mobility on ice surfaces is of fundamental importance in this context. We have calculated H atom adsorption and diffusion on both crystalline and amorphous ice surfaces using an analytic interaction potential for H2O–H. Tunneling rates for H atom hops between adsorption sites are explicitly calculated, the kinetic Monte Carlo method is used to simulate long time scale evolution and the diffusion coefficient, D, is evaluated for the temperature range 5–120 K. For ice Ih, we find D = 1.6 × 10–7 cm2/s at 10 K and below that temperature tunneling becomes the dominant diffusion mechanism. On the amorphous ice surface, the mobility of H is much slower than for ice Ih, D = 5.8 × 10–11 cm2/s at 25 K. Below 25 K, the H adatom becomes trapped in the deepest adsorptio...

Molecular Simulations of Halomethanes at the Air/Ice Interface.

Halogenated organics are emitted into the atmosphere from a variety of sources of both natural and anthropogenic origin. Their uptake at the surface of aerosols can affect their reactivity, for example, in processes that take part in ozone destruction due to production of reactive chlorine, bromine, and iodine radicals. Classical molecular dynamics (MD) simulations are carried out to investigate the interaction of small halomethane molecules of atmospheric relevance with a crystalline ice surface. The following halomethanes were studied: CH3Cl, CH2Cl2, CHCl3, CH3Br, CH2Br2, and CHBr3. MD simulations provide an invaluable insight into the adsorption behavior of halomethanes species. The adsorption energy is increasing as the number of halogen atoms is increasing. Moreover, brominated methanes exhibit a stronger interaction with the ice than their chlorinated analogs. Implications for the atmospheric chemistry are discussed.

Diffusion-desorption ratio of adsorbed CO and CO2on water ice

Diffusion of atoms and molecules is a key process for the chemical evolution in the star forming regions of the interstellar medium. Accurate data on the mobility of many important interstellar species is however often not available and this provides a serious limitation for the reliability of models describing the physical and chemical processes in molecular clouds. Here we aim to provide the astrochemical modeling community with reliable data on the ratio between the energy barriers for diffusion and desorption for adsorbed CO and CO$_2$ on water ices. To this end, we use a fully atomistic, off-lattice kinetic Monte Carlo technique to generate dynamical trajectories of CO and CO$_2$ molecules on the surface of crystalline ice at temperatures relevant for the interstellar medium. The diffusion to desorption barrier ratios are determined to be 0.31 for CO and 0.39 for CO$_2$ . These ratios can be directly used to improve the accuracy of current gas-grain chemical models.

Excess electrons in ice: a density functional theory study

We present a density functional theory study of the localization of excess electrons in the bulk and on the surface of crystalline and amorphous water ice. We analyze the initial stages of electron solvation in crystalline and amorphous ice. In the case of crystalline ice we find that excess electrons favor surface states over bulk states, even when the latter are localized at defect sites. In contrast, in amorphous ice excess electrons find it equally favorable to localize in bulk and in surface states which we attribute to the preexisting precursor states in the disordered structure. In all cases excess electrons are found to occupy the vacuum regions of the molecular network. The electron localization in the bulk of amorphous ice is assisted by its distorted hydrogen bonding network as opposed to the crystalline phase. Although qualitative, our results provide a simple interpretation of the large differences observed in the dynamics and localization of excess electrons in crystalline and amorphous ice films on metals.

Amorphous on the surface

Crystalline ice surfaces are found to exhibit an unusually large spread of vacancy formation energies, akin to an amorphous material. The finding has implications for the fundamental understanding of electrostatically frustrated surfaces and for the reactivity and catalytic properties of atmospheric ice.

Large variation of vacancy formation energies in the surface of crystalline ice

Resolving the atomic structure of the surface of ice particles within clouds, over the temperature range encountered in the atmosphere and relevant to understanding heterogeneous catalysis on ice, remains an experimental challenge. By using first-principles calculations, we show that the surface of crystalline ice exhibits a remarkable variance in vacancy formation energies, akin to an amorphous material. We find vacancy formation energies as low as ~0.1-0.2 eV, which leads to a higher than expected vacancy concentration. Because a vacancy's reactivity correlates with its formation energy, ice particles may be more reactive than previously thought. We also show that vacancies significantly reduce the formation energy of neighbouring vacancies, thus facilitating pitting and contributing to pre-melting and quasi-liquid layer formation. These surface properties arise from proton disorder and the relaxation of geometric constraints, which suggests that other frustrated materials may possess unusual surface characteristics.

Formation of H2+ by Ultra-Low-Energy Collisions of Protons with Water Ice Surfaces

The molecular ion of dihydrogen (H2+) is produced by 1 eV collisions of protons (H+) on amorphous water ice surfaces. The reaction is also observed on crystalline ice surfaces, but with lower efficiency. Collisions of D+ on amorphous H2O and D2O ices yield D2+ on the former, subsequent to isotope exchange on the H2O surface. Ultra-low-energy collision-induced dihydrogen ion production is also observed from alkanol surfaces, with decreasing efficiency as the alkyl chain length increases. There is no corresponding reaction on solid hexane. This endothermic reaction, with implications for interstellar chemistry and plasma etching processes, is proposed to occur as a result of stabilization of the other reaction product, a hydroxyl radical (OH•), on water surfaces through hydrogen-bonding interactions with the surface. These results point to an interesting chemistry involving ultra-low-energy ions on molecular solids.

A computational study on the structures of methylamine–carbon dioxide–water clusters: evidence for the barrier free formation of the methylcarbamic acid zwitterion (CH3NH2+COO−) in interstellar water ices

We investigated theoretically the interaction between methylamine (CH(3)NH(2)) and carbon dioxide (CO(2)) in the presence of water (H(2)O) molecules thus simulating the geometries of various methylamine-carbon dioxide complexes (CH(3)NH(2)/CO(2)) relevant to the chemical processing of icy grains in the interstellar medium (ISM). Two approaches were followed. In the amorphous water phase approach, structures of methylamine-carbon dioxide-water [CH(3)NH(2)/CO(2)/(H(2)O)(n)] clusters (n = 0-20) were studied using density functional theory (DFT). In the crystalline water approach, we simulated methylamine and carbon dioxide interactions on a fragment of the crystalline water ice surface in the presence of additional water molecules in the CH(3)NH(2)/CO(2) environment using DFT and effective fragment potentials (EFP). Both the geometry optimization and vibrational frequency analysis results obtained from these two approaches suggested that the surrounding water molecules which form hydrogen bonds with the CH(3)NH(2)/CO(2) complex draw the carbon dioxide closer to the methylamine. This enables, when two or more water molecules are present, an electron transfer from methylamine to carbon dioxide to form the methylcarbamic acid zwitterion, CH(3)NH(2)(+)CO(2)(-), in which the carbon dioxide is bent. Our calculations show that the zwitterion is formed without involving any electronic excitation on the ground state surface; this structure is only stable in the presence of water, i.e. in a methyl amine-carbon dioxide-water ice. Notably, in the vibrational frequency calculations on the methylcarbamic acid zwitterion and two water molecules we find the carbon dioxide asymmetric stretch is drastically red shifted by 435 cm(-1) to 1989 cm(-1) and the carbon dioxide symmetric stretch becomes strongly infrared active. We discuss how the methylcarbamic acid zwitterion CH(3)NH(2)(+)CO(2)(-) might be experimentally and astronomically identified by its asymmetric CO(2) stretching mode using infrared spectroscopy.

Reactivity of water–electron complexes on crystalline ice surfaces

The interactions between long-living electrons trapped in defects of crystalline D2O and electronegative molecules have been investigated using two-photon photoemission spectroscopy. When covered by a Xe adlayer, the spectroscopic signature of the trapped electrons vanishes, which provides evidence that the trapping sites are located on the surface of the crystalline ice. The reactive character of these surface-trapped electrons with molecules has been studied. In the case of CFCl3 adsorbed on top of the ice, we show that the trapped electrons induce the dissociation of the molecules, via a dissociative electron attachment process, resulting in *CFCl2 and Cl(-) formation. The latter species are responsible for the observed increase of the work function and presumably for the deactivation of the surface trapping sites with respect to subsequent light-induced population by excited electrons. This process is thought to be of high efficiency since it is observed for a very low CFCl3 coverage of only approximately 0.004 monolayer (ML). In the case of exposure of the crystalline ice to a partial pressure of gaseous O2, the deactivation of the trapping site has also been observed. The mechanism is thought to involve the formation of the O2*(-) transient anion by electron attachment, followed by its reactive interaction with the water molecules of the defect. In both cases, the mechanisms are triggered by negative ion resonances which are known from experiments using a primary electron beam to be effective for isolated molecules for ballistic electrons of approximately 0 eV. We thereby demonstrate a similarity between the processes induced by these primary, very low kinetic-energy electrons and by the long-living surface electrons on the crystalline ice surface. These results suggest that the photoexcited trapped electrons can play an important role in the heterogeneous chemical processes on condensed water surfaces and could be relevant in the polar stratosphere chemistry.

A Dynamic Landscape from Femtoseconds to Minutes for Excess Electrons at Ice−Metal Interfaces

Stabilization of excess electrons is studied at crystalline ice−metal interfaces by femtosecond time-resolved two-photon photoelectron spectroscopy and ab initio calculations. Following optical excitation into delocalized image potential states, electrons localize at pre-existing defects which are located at the ice−vacuum interface. Remarkably, the stabilization of these trapped electrons is monitored continuously from femtoseconds up to minutes. By first principle calculations we identify defect structures, that support excess electrons in front of crystalline ice surfaces, and suggest that the stabilization proceeds through subsequent conformational substates. The excess electron wave functions are efficiently screened from the underlying bulk and explain the long residence times observed in the experiment. Thereby, we shed light on the collective character of the nuclear rearrangement that determines the energetics over all timescales.

Hydrogen adsorption and diffusion on amorphous solid water ice

Results of classical trajectory calculations on the adsorption of H atoms to amorphous solid water (ASW) ice, at a surface temperature Ts of 10 K are presented. The calculations were performed for incidence energies Ei ranging from 10 to 1000 K, at random incidence. The adsorption probability Ps can be fitted to a simple decay function: Ps = 1.0e −Ei(K )/300 . Our calculations predict similar adsorption probabilities for H atoms to crystalline and ASW ice, although the average binding energy Eb of the trapped H atoms calculated for ASW of 650 ± 10 K is higher than that found for crystalline ice of 400 ± 5 K. The binding energy distributions were fitted to Gaussian functions with full width half-maximum of 111 and 195 K for crystalline and amorphous ice surfaces, respectively. The variation of the H atom binding sites in the case of the ASW surface leads to broadening of the distribution of Eb compared to that of crystalline ice. We have also calculated the ‘hot-diffusion’ distance travelled by the impinging atom over the surface before being thermalized, which is found to be about 30 A long at Ei = 100 K and increases with Ei. The diffusion coefficient D of thermally trapped H atoms is calculated to be 1.09 ± 0.04 × 10 −5 cm 2 s −1 at T s = 10 K. The residence time τ of H atoms adsorbed on ASW is orders of magnitude longer than that of H atoms adsorbed on crystalline ice for the same ice Ts, suggesting that H2 formation on crystalline and non-porous ice is quite limited compared to that on porous ice. This is in good agreement with the results of experiments on H2 formation on porous and non-porous ASW surfaces. At low Ts, the long values of τ , the high values of D and the large hot distance travelled on the ASW surface before trapping the impinging H atom ensure that Langmuir‐Hinshelwood and hot-atom mechanisms for H2 formation will be effective. The data presented here will be important ingredients for models to describe the formation of H2 on interstellar ices and reactions of H atoms with other species at the ice surface.