Proton transport in ice at 30-140 K: Effects of porosity.

We examined the role of porosity, a crucial characteristic of amorphous solid water (ASW), on electrostatic charging and discharging of ASW films with 500 eV He(+) and Xe(+) ions, by measuring the surface potentials with a Kelvin probe. When a charged ASW film is heated, its surface potential decreases sharply, at temperatures that depend on the maximum temperature the film was once subject to. This sharp decrease of the surface potential is not due to a large thermally induced increase of the dielectric constant ε as proposed in other studies, since measurements of ε yielded a value of ∼3 below ∼100 K.

Effect of microstructure on spontaneous polarization in amorphous solid water films.

Amorphous solid water (ASW) films grown by vapor deposition below 110 K develop negative surface voltages Vs with respect to the substrate. This polarization is due to a partial alignment of the water molecules during condensation. Kelvin probe measurements show that the magnitude of the surface potential, |Vs|, increases linearly with film thickness at a rate that decreases with increasing deposition temperature.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

Decomposition of solid amorphous hydrogen peroxide by ion irradiation.

We present laboratory studies of the radiolysis of pure (97%) solid H2O2 films by 50 keV H+ at 17 K. Using UV-visible and infrared reflectance spectroscopies, a quartz-crystal microbalance, and a mass spectrometer, we measured the absolute concentrations of the H2O, O2, H2O2, and O3 products as a function of irradiation fluence. Ozone was identified by both UV and infrared spectroscopies and O2 from its forbidden transition in the infrared at 1550 cm(-1).

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+.

Sputtering: survey of observations and derived principles.

We review the most salient observations and physical principles of knock-on and electronic sputtering and the role of sputtering in several astrophysical settings and applications in research and technology. In addition, we emphasize some unsolved problems, propose experiments and provide guides to representative literature reviews and significant recent publications.

Angular studies of potential electron emission in the interaction of slow ions with Al surfaces.

We report energy distributions of electrons emitted from Al surfaces under impact by 1 keV Ar+ and 1-5 keV Ne+ ions. The variation of the energy distributions with the angle of incidence is different for both ions and provides information on the mechanism responsible for electron emission. For Ar+ electron emission results mainly from Auger neutralization, while for Ne+ an important emission mechanism is the decay of plasmon excitations.

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.

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.