Laboratory for Atomic and Surface Physics

Research


Atomic Collisions	We study the interaction of energetic ions with solids to understand the fundamental mechanisms leading to electronic excitations and how these excitations evolve and lead to the emission of light (luminescence), electrons, atoms and molecules (sputtering), and to radiation damage, chemical changes or heat. We currently study metals, rare gas solids, condensed molecular gases, oxides and minerals. The studies are driven by interest in fundamental phenomena and by applications in semiconductor processing, nuclear fusion, gas discharges, and space exploration. Our studies include: Plasmon excitation by ion impact We are interested in many-electron excitations in solids. We discovered that quantized electron density oscillations (plasmons) can be excited in free-electron metals like Mg and Al by very slow ions (slower than the Fermi velocity of the solids). The source of the excitation is the potential energy carried by the ion, which is released when it captures an electron from the solid. A secondary excitation mechanism is caused when fast electrons can be energized in the solid by the incoming projectile. Some of our studies are done in collaboration with the Universidad Autonoma de Madrid (Spain), and the Univ. of Calabria in Cosenza (Italy). Ionization and excitation effects in condensed-gas solids Condensed gas solids are good model systems to study the effect of electronic excitations in insulators. We study by experiments and computer simulations how the electronic energy deposited by fast ions in solid films of Ar, Kr, Xe, O2, CO2, H2O, etc. is dissipated into sputtering, luminescence and electron emission. Irradiation results in an unusual state of matter, not accessible by other methods, where the material is taken extremely far away from thermal equilibrium. The results of the experiments have implications in radiation biology, the behavior of solids in space environments, the operation of detectors for nuclear and elementary particle physics experiments, the doping of semiconductor materials, etc. Example: Phys. Rev. Letters 74 (1995) 1474 Electron emission from solids In collaboration with Fujitsu (Japan), we studied the emission of electrons from thin MgO films induced by low energy ions (rare gases and sodium). Our goal is to understand the fundamental physical processes which will be useful in designing materials that can increase the energy efficiency of flat-panel displays, where oxide surfaces are subject to ion bombardment by a plasma. Example: Surface Sci. Lett. 571 (2004) L305 In collaboration with INTEC (Argentina) we study electron emission from graphite. We discovered structure in the electron energy distribution that signal the formation and decay of valence excitons. Example: Phys. Rev. Lett. 100 (2008) 227604 Charge transfer at surfaces The way in which electrons are transferred between surfaces and atoms or ions is one of the most basic problems in surface physics. We use electron spectroscopy to learn about basic processes. Besides the plasmon excitation mechanism mentioned above, we study Auger processes outside surfaces and also the formation of doubly-excited (autoionizing) states of Ne in collisions of Ne ions with surfaces. Example: Phys. Rev. Lett. 72 (1996) 4041 Understanding the basic excitation mechanisms allows us to apply it to study the effect of adsorption of atoms (like alkalis or oxygen) on the electronic structure of surfaces. Example: Phys. Rev. Lett. 77 (1996), 408 Electrostatic charging of surfaces We use a Kelvin probe and measurements of the energy of secondary ions to study the dynamics of electrostatic charging and discharging of thin insulator films. See Nucl. Instrum. Meth. B 268, 2888 (2010) for our work on water ice. Focused ion beam applications We use a FIB focused ion beam system to make nanostructured patterns in semiconductors to fix proteins Example: Langmuir 14 (1988) 6785	[top]

Surfaces	Low-energy electron-energy loss spectroscopy (EELS) We use EELS to study elementary electronic excitations in insulators. The information is then used to characterize defects and radiation damage in insulators such as metal oxides, polymers, diamond, and ice. Example: Phys. Rev. B, Rapid Comm. 6312 (2001) 1101. The work on diamond and oxide films is done in collaboration with Argonne National Laboratory. Example: Appl. Phys. Lett. 86 (2005) 042904.	[top]
Chemical Physics	We study the chemical-physical properties of condensed gases at low temperatures, such as sublimation, phase transformations, mixing, de-mixing, porosity, etc. Properties of low temperature water ice Water ice is a common material in the icy satellites of the giant planets, in comets, rings, and ice mantles in interstellar grains. The properties of ice at the low temperatures of these bodies are very interesting but differ from those of the common, and well studied hexagonal ice. Experiments at our laboratory found new effects in the sublimation of water ice films at temperatures. The rate of sublimation varies with time as amorphous ice is converted to crystalline ice. We found that the proportion of amorphous ice in the films depends on growth temperature (see Phys. Rev. B 48, 9973 (1993)). We discovered that the porosity of low temperature ice depends on the condition of growth from the gas phase (see J. Chem. Phys. 108 (1998) 3321), which explains the vast range of results reported by different researchers. A review of properties of low temperature water ice and its applications in astrophysics was published in Planet. Sp. Sci . 51, 953-961 (2003). Properties of solid mixtures of water and hydrogen peroxide We demonstrated a new method of growing pure hydrogen peroxide in ultrahigh vacuum 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 thermo gravimetric 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 H2O2 and H2O are shielded from radiolytic decomposition and warmed to temperatures where sublimation is significant, highly concentrated or even pure hydrogen peroxide may form. See J. Phys. Chem. A 115, 5324 (2011)	[top]

Astrochemistry Cosmochemistry	Objects in space, not protected by atmospheres, are subject to irradiation by stellar ultraviolet photons and energetic electrons and ions. We are interested in understanding how the surfaces react to this irradiation. Recent research has included laboratory irradiation of low temperature (4 - 160 K) ices of water, oxygen, carbon dioxide, and hydrogen peroxide with accelerated ions and hard-UV photons (Lyman-alpha) and ion irradiation of minerals. The laboratory participates in NASA's Cassini mission to Saturn. A summary of observations follow. Photodesorption of interstellar and circumsolar ice mantles We are studying the photodesorption of from ices of water and other gases induced by Lyman-alpha photons which simulate radiation from stars. We found that the photodesorption yields depend on irradiation dose and ice temperature. The existence of an incubation period before significant photodesorption occurs suggests that radicals produced by photolysis are important in desorption. The results where used to explain the puzzling absence of ice in diffuse interstellar clouds and to determine that photodesorption exceeds sublimation and erosion by energetic ions (sputtering) in the outer solar system. See our results in Nature 373, 205 (1995). Our new results for carbon dioxide and oxygen ices will be published soon. Gas absorption by planetary ices By applying an array of experimental techniques we characterized the porosity of vapor-deposited amorphous solid water. We observed microporosity and mesoporosity depending on the incidence angle of the water flux. The behavior of dangling OH bonds in the internal surface of micropores with methane uptake revealed that multilayer condensation 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. We discovered that microporosity is destroyed by ion irradiation, which implies that impacts from cosmic rays can cause compaction in the icy mantles of the interstellar grains, explaining the absence of dangling bond features in the infrared spectrum of molecular clouds. The implications for astrochemistry and cometary science are the subject of work in progress. Relevant publications: Compaction of microporous amorphous solid water by ion irradiation, J. Chem. Phys. 126, (2007) and Astrophys. J. (in press). In related work we determined conditions for formation of nanopores and mesopores, and obtained an upper bound of 1nm for the width of the nanopores (see J. Chem. Phys. 127 (2007) 204713). Irradiation effects on icy satellites We do experiments to determine the changes in the optical reflectance (albedo) of ices due to particle bombardment. The results show that the peculiar appearance of some satellites of Jupiter can result from bombardment by energetic magnetospheric ions. Recent experiments study properties of ice with embedded oxygen, to interpret recent observations by the Hubble Space Telescope which suggest the presence of oxygen and ozone in Ganymede. See our results in Science 276 (1997) 1839, J. Geophys. Res. E 103 (1998) 25865, and 104 (1999) 14,183. We recently studied in detail the synthesis and sputtering of O2 in ice by irradiation (Phys. Rev. B 72 (2005) 245422) and discovered a synergistic effect of ion irradiation and water co-deposition that should operate at icy satellites, favoring the synthesis of ozone (see Astrophys. J. Letters 644 (2006) L141). We explained the formation of hydrogen peroxide in the satellite Europa (Icarus 180, 265 (2006) ) by ion irradiation of water ice and determined the chemical environment of the molecule by infrared spectroscopy (Geophys. Res. Lett. 32, L172023 (2005) ). Related to this topic we studied quantitatively the ion-induced decomposition of H2O2 into different products (J. Chem. Phys. 124, 104702 (2006)) and the distillation kinetics of highly concentrated solid H2O2 and its dihydrate in ultrahigh vacuum ( J. Phys. Chem 110 (2006) 6911) To explain the spectacular plume of water vapor, ice particles and nitrogen gas of Enceladus we conducted experiments of irradiation of water-ammonia mixtures. During annealing, the irradiated samples eject particles, water, nitrogen and hydrogen in bursts and in slower outgassing. Microscopic observations lead to the proposal of micro-volcanoes consisting in bursting blisters containing radiolytic gas bubbles (see description in:: Enceladus: An Explanation for the Icy Outbursts and the Origin of Nitrogen, Astrophysics Journal Letters 649, L133 (2006) Influence of radicals in the evolution of comets We addressed the question of the evolution of ices that have been exposed to radiation from stellar sources and cosmic rays by studying the thermal evolution of a model ice sample: a mixture of water, hydrogen peroxide, dioxygen, and ozone produced by irradiating solid hydrogen peroxide with 50 keV protons at 17 K. Defects in the ice allow the normally forbidden infrared absorption in H 2 and O 2 , which disappears upon annealing. The temperature dependence of the formation of ozone indicates chemical reactions with radicals stored in the ice, triggered by the temperature increase. Relevant publications: Decomposition of Solid Amorphous Hydrogen Peroxide by Ion Irradiation, J. Chem. Phys. 124, (2005) 104702, A Model Study of Thermal Evolution of Astrophysical Ices, Astrophys. J. Lett. 639 (2006) L103, Ozone synthesis in the solid state To understand the observation of ozone in some satellites of Jupiter and Saturn we conduct laboratory experiments where we irradiate different molecular gases with fast protons and Lyman-alpha (10.2 eV) photons. Ozone formation is determined by observing the Hartley band in the ultraviolet reflectance spectrum and by mass spectrometry. Our results, to be published, provide information that is also useful to constrain models of ozone formation in the upper atmosphere of the Earth. See our results in Nucl. Instr. Meth. Phys. Res. B157 (1999)233, Nucl. Instr. Meth. Phys. Res. 193(2002)775, Astrophys. J. Letters 644, L141-L144 (2006), and J. Chem. Phys. 127 (2007) 204713. Carbon Dioxide Synthesis in Interstellar Grains To understand the formation of carbon dioxide on cold grains in interstellar clouds, we exposed solid CO to a beam of cold oxygen atoms, producing about a third of a monolayer of CO2 at 20 K. Heating a film with a water capped showed that carbon dioxide production increased by 30%, indicating additional reactions between CO and O diffusing in the ice layer. See our results in Astrophys. J. Letters 737, L14 (2011).	[top]

Solar System	Effects of ion irradiation in space An important part of our research aims at characterizing and understanding the effect of irradiation with energetic ions, electrons and ultraviolet photons on surfaces in space. We are interested in chemical and optical alterations, erosion, and electrostatic charging. Sputtering of ice by low energy ions and astronomical implications The magnetosphere of Saturn has ions with much lower average energies than the Jovian magnetosphere. This finding by Cassini was surprising and implied a revision of our understanding of plasma interactions with icy grains and satellites. Recently, we measured the sputtering of ice for low energy ions and obtained a scaling with ion type, energy, angle of incidence and temperature. The results were used to calculate erosion rates of ices in the rings and icy satellites. Relevant publications: Nucl. Instr. Methods. Phys. Res. B 209 (2003) 294, Surface Sci. 588 (2005) 1, Surface Sci. 602 (2008) 156; Planet. Space Sci. 56 (2008) 1238 CASSINI mission research Recent publications of our team CAPS (Cassini Plasma Spectrometer) include Science 307 (2005) 1262, Geophys. Res. Lett. 32 (2005) L14S04, Geophys. Res. Lett. 32 (2005) L14S09, Planet. Space Sci. 56 (2008) 3.	[top]

Radiation of Rocky Surfaces	Ion irradiation of minerals Current experiments at our laboratory simulate the effect of the solar wind on the surface composition of asteroids and their appearance (spectral reflectance). Low energy ion bombardment results in oxygen loss as determined by X-ray photoelectron spectroscopy (see results in J. Geophys. Res. 104, 1865 (1999)). Other experiments look at the effect of irradiation on interplanetary and interstellar dust to interpret observations on collected dust which may bring important clues on the origin of our solar system. Recently we have completed research on forsterite and on troilite. See Icarus 195 (2008) 622. In collaboration with Lawrence Livermore National Laboratory, we have studied in the lab the process of amorphization of interstellar grains. See Astrophysics Journal 662 (2007) 372 Simulation of micrometeorite impacts We use a nanosecond-pulsed excimer laser to simulate the effect of the impact of hypervelocity (tens of km/s) micrometeorites on surfaces that produce very high temperatures locally at the impact point. See results in Icarus 196 (2008) 285	[top]

Radiation Chemistry	Related to applications to different astronomical problems (see different sections) we study the fundamental processes of radiation chemistry in the physical (prompt) and chemical (delayed) stages.
Geoscience	Alteration of glass encapsulation of nuclear waste material by ground water. Ozone production from rock fracture. See our recent paper in and press release.	[top]

Techniques	We developed a new low-energy plasma gun that allows to clean surfaces from hydrocarbons without sputtering. See Surface Interf. Analysis 42, 40 (2009)	[top]

Other	Spacecraft instrumentation We participate in the team involved with the design of the plasma instruments that are flying in the Cassini mission to Saturn. We study the factors that affect the operation and determine the sensitivity of the ion-mass spectrometer (IMS). This is a time-of-flight (TOF) device where the clock is started by electrons ejected when the ion to be analyzed is sent through an ultra thin (~8 nm) carbon foil. The description of the instrument can be found in Space Sci. Rev., 114, 1 (2004). Our experimental work on carbon foils was published in Physical Review B58, 2529-2538 (1998).	[top]

Our research is supported by NSF and NASA.