Infrared Spectroscopic Study of Methane Ice, Pure and in Mixtures with Polar (H2O) and Nonpolar (N2) Molecules.

Mid-infrared studies of fundamental modes of ices of pure CH4 and its mixtures with polar (H2O) and nonpolar (e.g., N2) molecules are essential in order to learn the state of aggregation and thermal history of ices present in the interstellar medium and outer solar system bodies. Such data will be useful in the interpretation of observational data from the James Webb Space Telescope. Using an ultrahigh vacuum apparatus, we conducted reflection-absorption infrared spectroscopy measurements in the mid-IR range of pure methane ice and methane-containing ice mixtures of interest to interstellar and solar system ice chemistry, e.g., with H2O and N2 molecules. We found that nuclear spin conversion (NSC) in solid methane and its crystalline structures is affected─in different ways─by the presence of H2O and N2. Specifically, we found a relationship between the thickness and the solid-state ordering transformation in methane thin films. This new study of the NSC of pure CH4 ice and of the CH4:H2O ice mixture at 7 K is carried out in relation to the segregation of H2O using the ν1 and ν2 IR inactive modes of methane. The diffusion of N2 and CH4 in the CH4:N2 ice mixture with temperature cycling has been studied to obtain the relationship between IR features and the state of aggregation of the ice.

Infrared Spectroscopic Study of Solid Methane: Nuclear Spin Conversion of Stable and Metastable Phases.

Infrared spectroscopy was employed to study thin films of solid methane at low temperatures. We report new measurements of temporal changes of infrared spectra of methane ice in the ν3 and ν4 bands due to nuclear spin conversion upon rapid cooling from 30 to 6.0-11.0 K. The relaxation rates of the nuclear spin were found to be a function of temperature. The activation energy associated with the relaxation has been determined over an extended temperature range. We also found a new metastable phase of methane ice upon deposition at T < 7 K. After the deposition at 6 K and annealed to a higher temperature, a phase transition from the metastable phase to a stable crystalline phase takes place. We found that the relaxation has different activation energies below and above 8.5 K. From a quantitative analysis of the ν3 and ν4 IR bands, we suggest that the metastable phase is a crystalline phase with a degree of orientational disorder between the two known stable solid phases.

Formation of ozone by solid state reactions.

We studied the isotopic composition of ozone formed at low (3-10 K) temperature via O + O2 solid state reactions using a partially dissociated 16O/16O2 : 18O/18O2 = 1 : 1 mixture. The ozone ice has an isotopic abundance that differs from the statistical one and from gas phase studies. Ozone formation is influenced by the competition of the production of O2 (O + O or O + O3) vs. O3 (O + O2) and by the energy released in the O + O reaction. The exothermicity of the O + O reaction helps to overcome the barrier of the O + O2 reaction. Heating the ozone ice past 50 K brings about a transformation from amorphous to crystalline ice. The formation of ozone on water ice yields a blue shift of IR bands, and the yield of formed O3 increases up to the sample temperature of 100 K. When 18O/18O2 is deposited on H216O ice, formation of 18O18O16O is detected. We propose that the exothermicity of the reaction 18O + 18O drives water dissociation (16O + H2) followed by ozone formation (16O + 18O2 → 16O18O18O).

Application of a diffusion-desorption rate equation model in astrochemistry.

Desorption and diffusion are two of the most important processes on interstellar grain surfaces; knowledge of them is critical for the understanding of chemical reaction networks in the interstellar medium (ISM). However, a lack of information on desorption and diffusion is preventing further progress in astrochemistry. To obtain desorption energy distributions of molecules from the surfaces of ISM-related materials, one usually carries out adsorption-desorption temperature programmed desorption (TPD) experiments, and uses rate equation models to extract desorption energy distributions. However, the often-used rate equation models fail to adequately take into account diffusion processes and thus are only valid in situations where adsorption is strongly localized. As adsorption-desorption experiments show that adsorbate molecules tend to occupy deep adsorption sites before occupying shallow ones, a diffusion process must be involved. Thus, it is necessary to include a diffusion term in the model that takes into account the morphology of the surface as obtained from analyses of TPD experiments. We take the experimental data of CO desorption from the MgO(100) surface and of D2 desorption from amorphous solid water ice as examples to show how a diffusion-desorption rate equation model explains the redistribution of adsorbate molecules among different adsorption sites. We extract distributions of desorption energies and diffusion energy barriers from TPD profiles. These examples are contrasted with a system where adsorption is strongly localized--HD from an amorphous silicate surface. Suggestions for experimental investigations are provided.

Atomic oxygen diffusion on and desorption from amorphous silicate surfaces.

Surface reactions involving atomic oxygen have attracted much attention in astrophysics and astrochemistry, but two of the most fundamental surface processes, desorption and diffusion, are not well understood. We studied diffusion and desorption of atomic oxygen on or from amorphous silicate surfaces under simulated interstellar conditions using a radio-frequency dissociated oxygen beam. Temperature programmed desorption (TPD) experiments were performed to study the formation of ozone from reaction of atomic and molecular oxygen deposited on the surface of a silicate. It is found that atomic oxygen begins to diffuse significantly between 40 K and 50 K. A rate equation model was used to study the surface kinetics involved in ozone formation experiments. The value of atomic oxygen desorption energy has been determined to be 152 ± 20 meV (1764 ± 232 K). The newly found atomic oxygen desorption energy, which is much higher than the well-accepted value, might explain the discrepancy in abundance of molecular oxygen in space between observations and chemical models.

Diffusion, adsorption, and desorption of molecular hydrogen on graphene and in graphite.

The diffusion of molecular hydrogen (H2) on a layer of graphene and in the interlayer space between the layers of graphite is studied using molecular dynamics computer simulations. The interatomic interactions were modeled by an Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential. Molecular statics calculations of H2 on graphene indicate binding energies ranging from 41 meV to 54 meV and migration barriers ranging from 3 meV to 12 meV. The potential energy surface of an H2 molecule on graphene, with the full relaxations of molecular hydrogen and carbon atoms is calculated. Barriers for the formation of H2 through the Langmuir-Hinshelwood mechanism are calculated. Molecular dynamics calculations of mean square displacements and average surface lifetimes of H2 on graphene at various temperatures indicate a diffusion barrier of 9.8 meV and a desorption barrier of 28.7 meV. Similar calculations for the diffusion of H2 in the interlayer space between the graphite sheets indicate high and low temperature regimes for the diffusion with barriers of 51.2 meV and 11.5 meV. Our results are compared with those of first principles.

Sputtering effects and water formation on an amorphous silicate surface.

We studied the formation of deuterated water on an amorphous silicate surface held at low temperature (10 K < T < 40 K). The surface is first characterized by using Ar(+) ion bombardment, and preferential sputtering of oxygen is found. Sputtering creates oxygen vacancies in the surface region that can be filled by deposition of atomic oxygen. The conditions used in the experiment are meant to make it relevant to the study of the initial stages of water formation on dust grains in interstellar space. By changing the D/O ratio of atomic beams of deuterium and oxygen at thermal energy and the temperature of the sample during deposition, we show that the routes to the formation of D2O2 can be untangled and, under certain circumstances, the net yield of D2O2 can be suppressed. The formation efficiency for water and other molecules is then estimated.

Interaction of hydrogen with surfaces of silicates: single crystal vs. amorphous.

We have studied how the formation of molecular hydrogen on silicates at low temperature is influenced by surface morphology. At low temperature (<30 K), the formation of molecular hydrogen occurs chiefly through weak physical adsorption processes. Morphology then plays a role in facilitating or hindering the formation of molecular hydrogen. We studied the formation of molecular hydrogen on a single crystal forsterite and on thin films of amorphous silicate of general composition (Fe(x)Mg((x-1)))(2)SiO(4), 0 < x < 1. The samples were studied ex situ by Atom Force Microscopy (AFM), and in situ using Thermal Programmed Desorption (TPD). The data were analysed using a rate equation model. The main outcome of the experiments is that TPD features of HD desorbing from an amorphous silicate after its formation are much wider than the ones from a single crystal; correspondingly typical energy barriers for diffusion and desorption of H, H(2) are larger as well. The results of our model can be used in chemical evolution codes of space environments, where both amorphous and crystalline silicates have been detected.

Interaction of atomic and molecular hydrogen with tholin surfaces at low temperatures.

We study the interaction of atomic and molecular hydrogen with a surface of tholin, a man-made polymer considered to be an analogue of aerosol particles present in Titan's atmosphere, using thermal programmed desorption at low temperatures below 30 K. The results are fitted and analyzed using a fine-grained rate equation model that describes the diffusion, reaction, and desorption processes. We obtain the energy barriers for diffusion and desorption of atomic and molecular hydrogen. These barriers are found to be in the range of 30-60 meV, indicating that atom/molecule-surface interactions in this temperature range are dominated by weak adsorption forces. The implications of these results for the understanding of the atmospheric chemistry of Titan are discussed.

The formation of interstellar molecules via reactions on dust grain surfaces.

The formation of molecules in interstellar space via reactions on the surface of dust grains has been the subject of numerous theoretical studies, since a very important reaction, the formation of molecular hydrogen, is thought to occur primarily on dust. Beginning with the very first experiments of the formation of H2 on a dust grain analogue by Pirronello et al., in 1997, and continuing today, it has been possible to study and characterize the processes of molecular hydrogen formation on realistic analogues of dust grains with increasing sophistication. In our laboratory, we use state-of-the-art surface science techniques to come as closely as technically possible to the conditions of molecule formation in the ISM. Here, we present the results of studies on the mechanisms and rate of formation of molecular hydrogen on dust grain analogues.