Grain Surface Chemistry

Catalytic reactions on the surfaces of interstellar dust grains, followed by release into the gas phase, can explain the formation of the most abnndant molecule, H2, and appears to be snccessful at explaining the formation of many of the more complex interstellar molecules. Unfortunately, grain chemistry processes are poorly understood and difficult to simulate. Grain surfaces are also recognized as repositories for many interstellar molecules that 

TABLE II Known Interstellar Molecules (January 2001) (Courtesy Barry Turner) 

Radicals Ions Rings Carbon chains Isomers 

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In the present section, we will present the different methods to model grain surface chemistry, the input parameters that go into these models, the experimental and theoretical methods to obtain these parameters, and the limitations of these models. Most gas-grain networks are predominantly designed for the low temperature regime and, as we will see in the following sections, the expressions that are used almost exclusively describe the Langmuir-Hinshelwood type of reaction. 

In the Monte Carlo methods, the evolution of (discrete) number densities is followed in time by randomly selecting a sequence of processes. The probability of selecting a process is proportional to its rate, which is determined in a similar manner to the rate equation method. Because Monte Carlo methods use random numbers and probabilities instead of analytical expressions, coupling between the two methods (rate equations for the gas and a Monte Carlo procedure for the grain) is harder to achieve. Different (kinetic) Monte Carlo implementations are used and usually a distinction between macroscopic and microscopic Monte Carlo is made. In the macroscopic simulations, only the number density is followed in time in the microscopic simulations the exact positions of the species are also considered. Recently, macroscopic Monte Carlo simulations of both the gas phase and grain surface chemistry have been carried out for a proto-planetary disk [52]. 

Stantcheva T, Shematovich VI, Herbst E (2002) On the master equation approach to diffusive grain-surface chemistry the H, O, CO system. Astron Astrophys 391 1069-1080... 

Strong shocks produced by the expanding ionized envelopes of massive stars and supernova remnants heat and compress the interstellar medium, leading to conditions ripe for many high-temperature chemical reactions. Like grain surface chemistry, reactions within the shock chemistry environment are difficult to simulate, but are progressing toward a physical framework that can be compared to observations. While OH and H2O are prominent products of ion-molecule chemistry as well as shock chemistry, SiO, and SiS are predominently produced in shocks. 

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

Other applications of REELM include monitoring variations like oxidation, segregation, and hydration in the surface chemistry of polycrystalline materials. Differences of 1 /10 of a monolayer in oxygen coverage due to variations in grain... 

The primary process initiating dust surface chemistry is the collision of a molecule from the ISM with the surface. The sticking probability is a measure of how often molecules will stick to the dust surface but this depends on the collision energy, the temperature of the grain surface and the nature of the chemical surface itself. The silicate surface is highly polar, at least for a grain of sand on Earth, and should attract polar molecules as well as atoms. The adsorption process can also be reversed, resulting in thermal desorption, both as the reverse of adsorption and by new molecules as the product of surface reactions. 

Surface chemistry increases the molecular diversity in the ISM, further enhanced by the presence of UV or far-UV radiation. Photolysis of molecules on the grain surfaces encourages radicals to be produced that undergo surface reactions but the rates of these fundamental processes are as yet unknown. Penetration of the radiation is controlled by the thickness and composition of the ice mantel and may protect larger molecules from photo-destruction. The dust surface is a fertile ground for organic synthesis. 

Ruffle D. P. and E. Herbst (2001). New models of interstellar grain chemistry - II. Surface chemistry in quiescent cores. Monthly Notices of the Royal Astronomical Society 322 770-778. 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

It is important to note that the layer thicknesses reported above were based strictly on solution chemistry analyses. Several reports have appeared on the thicknesses of leached layers using surface chemistry techniques. Petrovic et al. (1976) used XPS and analyzed K, Al, and Si content of altered K-feldspar grains and found the leached layer was <1.7 nm. Layer thicknesses for dissolution of enstatite, diopside, and tremolite based on XPS data are reported in Table 7.3. 

Abstract. Structural and adsorption characteristics of various adsorbents such as fumed silicas, silica gels, activated carbons and carbon/silicas were analyzed. The adsorption of a variety of compounds reveals the effects of adsorbent grain size, specific surface area, pore volume, pore size distribution, surface chemistry, conditions of adsorbent synthesis and pre-treatment. Both dynamic (nonequilibrium) and static (equilibrium) adsorption conditions are addressed. 

Models of irradiated disks predict four chemically distinct zones (see Fig. 4.1). (I) Zone of ices in the cold mid-plane opaque to incoming radiation. Chemistry in this region is dominated by cold gas-phase and grain-surface reactions. Here Infrared Space Observatory (ISO) and Spitzer observations confirmed the existence of ices, various silicates and PAHs (polycyclic aromatic hydrocarbons e.g. van den Ancker et al. 2000 van Dishoeck 2004 Bouwman et al. 2008). (II) Zone of molecules, a warm molecular layer adjacent to the mid-plane, dominated by ultraviolet/X-ray-driven photochemistry (III) the heavily irradiated zone of radicals, a hot dilute disk atmosphere deficient in molecules and (IV) the inner zone, inside of the ice line where terrestrial planets form. 

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