Photolysis of pure solid O3 and O2 films at 193 nm.

We studied quantitatively the photochemistry of solid O(3) and O(2) films at 193 nm and 22 K with infrared spectroscopy and microgravimetry. Photolysis of pure ozone destroyed O(3), but a small amount of ozone remained in the film at high fluence. Photolysis of pure O(2) produced O(3) in an amount that increased with photon fluence to a stationary level. For both O(2) and O(3) films, the O(3):O(2) ratio at large fluences is ∼0.07, about two orders of magnitude larger than those obtained in gas phase photolysis. This enhancement is attributed to the increased photodissociation of O(2) due to photoabsorption by O(2) dimers, a process significant at solid-state densities. We obtain initial quantum yield for ozone synthesis from solid oxygen, Φ(O(3)) = 0.24 ± 0.06, and quantum yields for destruction of O(3) and O(2) in their parent solids, Φ(-O(3)) = 1.0 ± 0.2 and Φ(-O(2)) = 0.36 ± 0.1. Combined with known photoabsorption cross sections, we estimate probabilities for geminate recombination of 0.5 ± 0.1 for O(3) fragments and 0.88 ± 0.03 for oxygen atoms from O(2) dissociation. Using a single parameter kinetic model, we deduce the ratio of reaction cross sections for an O atom with O(2) vs. O(3) to be 0.1-0.2. The general good agreement of the model with the data suggests the validity of the central assumption of efficient energy and spin relaxation of photofragments in the solid prior to their reactions with other species.

Isothermal decomposition of hydrogen peroxide dihydrate.

We present a new method of growing pure solid hydrogen peroxide in an ultra high vacuum environment and apply it to determine thermal stability of the dihydrate compound that forms when water and hydrogen peroxide are mixed at low temperatures. Using infrared spectroscopy and thermogravimetric analysis, we quantified the isothermal decomposition of the metastable dihydrate at 151.6 K. This decomposition occurs by fractional distillation through the preferential sublimation of water, which leads to the formation of pure hydrogen peroxide. The results imply that in an astronomical environment where condensed mixtures of H(2)O(2) and H(2)O are shielded from radiolytic decomposition and warmed to temperatures where sublimation is significant, highly concentrated or even pure hydrogen peroxide may form.

Photolysis of solid NH3 and NH3-H2O mixtures at 193 nm.

We have studied UV photolysis of solid ammonia and ammonia-dihydrate samples at 40 K, using infrared spectroscopy, mass spectrometry, and microgravimetry. We have shown that in the pure NH(3) sample, the main species ejected are NH(3), H(2), and N(2), where the hydrogen and nitrogen increase with laser fluence. This increase in N(2) ejection with laser fluence explains the increase in mass loss rate detected by a microbalance. In contrast, for the ammonia-water mixture, we see very weak signals of H(2) and N(2) in the mass spectrometer, consistent with the very small mass loss during the experiment and with a <5% decrease in the NH(3) infrared absorption bands spectroscopy after a fluence of ~3 × 10(19) photons/cm(2). The results imply that ammonia-ice mixtures in the outer solar system are relatively stable under solar irradiation.

Cassini finds an oxygen-carbon dioxide atmosphere at Saturn's icy moon Rhea.

The flyby measurements of the Cassini spacecraft at Saturn's moon Rhea reveal a tenuous oxygen (O(2))-carbon dioxide (CO(2)) atmosphere. The atmosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn's magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter's moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O(2) locked within Rhea's ice. The presence of CO(2) suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO(2) endogenic to Rhea's ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.

Radiation chemistry in ammonia-water ices.

We studied the effects of 100 keV proton irradiation on films of ammonia-water mixtures between 20 and 120 K. Irradiation destroys ammonia, leading to the formation and trapping of H(2), N(2), NO, and N(2)O, the formation of cavities containing radiolytic gases, and ejection of molecules by sputtering. Using infrared spectroscopy, we show that at all temperatures the destruction of ammonia is substantial, but at higher temperatures (120 K), it is nearly complete (approximately 97% destroyed) after a fluence of 10(16) ions/cm(2). Using mass spectroscopy and microbalance gravimetry, we measure the sputtering yield of our sample and the main components of the sputtered flux. We find that the sputtering yield depends on fluence. At low temperatures, the yield is very low initially and increases quadratically with fluence, while at 120 K the yield is constant and higher initially. The increase in the sputtering yield with fluence is explained by the formation and trapping of the ammonia decay products, N(2) and H(2), which are seen to be ejected from the ice at all temperatures.

Secondary electron emission spectra from clean and cesiated Al surfaces: the role of plasmon decay and data analysis for applications.

We report measurements of energy spectra of secondary electrons emitted from clean and cesiated aluminum surfaces under the impact of 130 eV electrons. Measurements show that the decay of bulk and surface plasmons dominates the electron emission. In contrast with theoretical calculations, our experiments indicate that the electron collision cascade inside the solid produced by electrons excited by plasmon decay do not contribute significantly to electron emission. A simple analysis of electron energy distributions measured as a function of Cs surface coverage allows separation of rediffused incident electrons from the continuum background of true secondary electrons. The result shows that yields of rediffused electrons used in several applications may have been significantly overestimated.

Formation, trapping, and ejection of radiolytic O2 from ion-irradiated water ice studied by sputter depth profiling.

We report experimental studies of 100 keV Ar(+) ion irradiation of ice leading to the formation of molecular oxygen and its trapping and ejection from the surface, at temperatures between 80 and 150 K. The use of a mass spectrometer and a quartz-crystal microbalance and sputter depth profiling at 20 K with low energy Ar ions allowed us to obtain a consistent picture of the complex radiolytic mechanism. We show that the dependence of O(2) sputtering on ion fluence is mainly due to the buildup of trapped O(2) near the surface. A small proportion of the O(2) is ejected above 130 K immediately upon creation from a precursor such as OH or H(2)O(2). The distribution of trapped oxygen peaks at or near the surface and is shallower than the ion range. Measurements of sputtering of H(2) help to elucidate the role of this molecule in the process of O(2) formation: out-diffusion leading to oxygen enrichment near the surface. The competing phenomena of OH diffusion away from the ion track and hydrogen escape from the ice and their temperature dependence are used to explain the finding of opposite temperature dependencies of O(2) and H(2)O(2) synthesis. Based on the new data and understanding, we discuss the application of our findings to ices in the outer solar system and interstellar space.

Physical and chemical effects on crystalline H2O2 induced by 20 keV protons.

We present laboratory studies on radiation chemistry, sputtering, and amorphization of crystalline H(2)O(2) induced by 20 keV protons at 80 K. We used infrared spectroscopy to identify H(2)O, O(3), and possibly HO(3), measure the fluence dependence of the fraction of crystalline and amorphous H(2)O(2) and of the production of H(2)O and destruction of H(2)O(2). Furthermore, using complementary techniques, we observe that the sputtering yield depends on fluence due to the buildup of O(2) radiation products in the sample. In addition, we find that the effective cross sections for the destruction of hydrogen peroxide and the production of water are very high compared to radiation chemical processes in water even though the fluence dependence of amorphization is nearly the same for the two materials. This result is consistent with a model of fast cooling of a liquid track produced by each projectile ion rather than with the disorder produced by the formation of radiolytic products.

Exciton autoionization in ion-induced electron emission.

We report on measurements of electron emission spectra from surfaces of highly oriented pyrolytic graphite (HOPG) excited by 1-5 keV He+ and Li+ which, for He+, exhibit a previously unreported high-energy structure. Through a full quantum dynamic description that allows for the calculation of neutralization and electron-hole pair excitation, we show that these high-energy electrons can arise from autoionization of excitons formed by electron promotion to conduction band states close to the vacuum level. The same calculation explains the observed absence of high-energy excitons for Li+ on HOPG.

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.

Low density solid ozone.

We report a very low density ( approximately 0.5 g/cm(3)) structure of solid ozone. It is produced by irradiation of solid oxygen with 100 keV protons at 20 K followed by heating to sublime unconverted oxygen. Upon heating to 47 K the porous ozone compacts to a density of approximately 1.6 g/cm(3) and crystallizes. We use a detailed analysis of the main infrared absorption band of the porous ozone to interpret previous research, where solid oxygen was irradiated by UV light and keV electrons.

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.

Distillation kinetics of solid mixtures of hydrogen peroxide and water and the isolation of pure hydrogen peroxide in ultrahigh vacuum.

We present results of the growth of thin films of crystalline H2O2 and H2O2*2H2O (dihydrate) in ultrahigh vacuum by distilling an aqueous solution of hydrogen peroxide. We traced the process using infrared reflectance spectroscopy, mass loss on a quartz crystal microbalance, and in a few cases ultraviolet-visible reflectance. We find that the different crystalline phases-water, dihydrate, and hydrogen peroxide-have very different sublimation rates, making distillation efficient to isolate the less volatile component, crystalline H2O2.

Composition and dynamics of plasma in Saturn's magnetosphere.

During Cassini's initial orbit, we observed a dynamic magnetosphere composed primarily of a complex mixture of water-derived atomic and molecular ions. We have identified four distinct regions characterized by differences in both bulk plasma properties and ion composition. Protons are the dominant species outside about 9 RS (where RS is the radial distance from the center of Saturn), whereas inside, the plasma consists primarily of a corotating comet-like mix of water-derived ions with approximately 3% N+. Over the A and B rings, we found an ionosphere in which O2+ and O+ are dominant, which suggests the possible existence of a layer of O2 gas similar to the atmospheres of Europa and Ganymede.

Oxygen on Ganymede: laboratory studies.

To test proposals for the origin of oxygen absorption bands in the visible reflectance spectrum of Ganymede, the reflectance of condensed films of pure oxygen (O2) and O2-water mixtures and the evolution of O2 from the films as a function of temperature were determined. Absorption band shapes and positions for oxygen at 26 kelvin were similar to those reported for Ganymede, whereas those for the mixtures were slightly shifted. The band intensity dropped by more than two orders of magnitude when the ice mixture was warmed to 100 kelvin, although about 20 percent of the O2 remained trapped in the ice, which suggested that at these temperatures O2 molecules dissolve in the ice rather than aggregate in clusters or bubbles. The experiments suggest that the absorption bands in Ganymede's spectrum were not produced in the relatively warm surface of the satellite but in a much colder source. Solid O2 may exist in a cold subsurface layer or in an atmospheric haze.

Photodesorption from low-temperature water ice in interstellar and circumsolar grains.

Dust grains in the interstellar medium and the outer Solar System commonly have a coating of water ice, which affects their optical properties and surface chemistry. The thickness of these icy mantles may be determined in part by the extent of photodesorption (photosputtering) by background ultraviolet radiation. But this process is poorly understood, with theoretical estimates of the photodesorption rate spanning several orders of magnitude. Here we report measurements of the absolute ultraviolet photodesorption yield of low-temperature water ice. Our results indicate that the rate of photodesorption is appreciable. In particular, it can account for the absence of icy mantles on grains in diffuse interstellar clouds, it exceeds solar-wind ion erosion and sublimation in the outer Solar System, and it is important in determining the lifetimes of icy mantles in dense molecular clouds.