J. Thomas Dickinson

Hydrogen in rocks: an energy source for deep microbial communities

To survive in deep subsurface environments, lithotrophic microbial communities require a sustainable energy source such as hydrogen. Though H2 can be produced when water reacts with fresh mineral surfaces and oxidizes ferrous iron, this reaction is unreliable since it depends upon the exposure of fresh rock surfaces via the episodic opening of cracks and fissures. A more reliable and potentially more voluminous H2 source exists in nominally anhydrous minerals of igneous and metamorphic rocks. Our experimental results indicate that H2 molecules can be derived from small amounts of H2O dissolved in minerals in the form of hydroxyl, OH- or O3Si-OH, whenever such minerals crystallized in an H2O-laden environment. Two types of experiments were conducted. Single crystal fracture experiments indicated that hydroxyl pairs undergo an in situ redox conversion to H2 molecules plus peroxy links, O3Si/OO\SiO3. While the peroxy links become part of the mineral structure, the H2 molecules diffused out of the freshly fractured mineral surfaces. If such a mechanism occurred in natural settings, the entire rock column would become a volume source of H2. Crushing experiments to facilitate the outdiffusion of H2 were conducted with common crustal igneous rocks such as granite, andesite, and labradorite. At least 70 nmol of H2/g diffused out of coarsely crushed andesite, equivalent at standard pressure and temperature to 5,000 cm3 of H2/m3 of rock. In the water-saturated, biologically relevant upper portion of the rock column, the diffusion of H2 out of the minerals will be buffered by H2 saturation of the intergranular water film.

Pulsed laser ablation and deposition of fluorocarbon polymers

Thin films of polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) have been formed by the pulsed-laser deposition technique. The structure of the PTFE films depends upon the substrate temperature during deposition. At substrate temperatures from room temperature to 200°C the films were determined to be amorphous. Films formed at higher substrate temperatures contain both amorphous and crystalline components. Transmission IR spectroscopy indicates that the amorphous and the semi-crystalline variants of PTFE are chemically equivalent. The PVDF films were found to be amorphous for all substrate temperatures studied in the range 25°C to 200°C. For both materials it was found that the use of higher substrate temperatures results in the formation of smoother films.

In situ photoelectron emission microscopy of a thermally induced martensitic transformation in a CuZnAl shape memory alloy

We report photoelectron emission microscope observations of the thermal martensitic transformation in a CuZnAl shape memory alloy. The phase transformation appears at 48°C during heating and at 42°C upon cooling. The transformation is marked by a sharp change in photoelectron intensity, as well as a significant displacement and reorientation of surface features. The difference in the photoelectron intensity before and after the transformation is attributed to a change in work function of about 0.2eV. Photoemission electron microscopy provides real-time information on microstructural changes and phase-dependent electronic properties.

The role of defects in the laser ablation of wide bandgap materials

Abstract In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent), the absorption of laser radiation at 248 nm (5 eV photons) at modest fluences is defect dominated. We describe a technique for imaging the initial defect densities by their luminescence at low laser fluences and apply it to cleaved MgO. High defect densities are observed along many cleavage steps, consistent with previous observations of strong point-to-point variations in the ablative response of cleaved MgO surfaces. The composition of neutral particles emitted from the surface can be strongly influenced by defects (here, impurities), as shown by the intense emission of carbon oxides and the correspondingly weak emissions of atomic and molecular oxygen from arc-fused MgO at fluences below that required for sustained emission. We also present evidence for defect-mediated ion emission at these low fluences. Mass selected time-of-flight measurements of Mg + emission show a well defined kinetic energy in the range of 4–5 eV, consistent with electrostatic ejection of weakly bound ions following the ionization of a nearby surface F-center.

Surface Modification of Polytetrafluoroethylene and the Deposition of Copper Films

Enhancement of the adhesion of thin copper films on polytetrafluoroethylene substrates was found when the substrate surface was irradiated with a pulsed UV excimer laser prior to metal deposition. The interaction between the laser and the polymer was examined by characterizing the neutral and charged species emitted from the surface during irradiation. The nature of the species emitted indicates that significant chemical modification of the polymer surface occurs. In addition to chemical modification, the interaction with the laser also alters the surface morphology of the polymer. Irradiation at fluences of ∼ 0.6 J/cm 2 results in an overall planarization of the surface, whilst irradiation at higher fluences results in the formation and enlargement of voids and localized melting.

3. Low Fluence Laser Desorption and Plume Formation from Wide Bandgap Crystalline Materials

Publisher Summary This chapter presents a study on low fluence laser desorption and plume formation from wide bandgap crystalline materials. It presents an analysis of the interaction of photo-emitted and thermally emitted particles from exposure to pulsed laser irradiation of surfaces of wide bandgap ionic crystals. At sub-bandgap photon energies, these emissions include photoelectrons, energetic positive ions, and neutral metal atoms. The chapter discusses experimentally that significant portions of the distributions of these particles overlap in space and time in the near-surface region. It also presents a model of the collective motion of these particles and shows that as laser fluence is increased, sufficient densities overlap and kinetic energies are achieved to result in plume formation. The features examined in the chapter include the excitation of neutral atoms to generate plume fluorescence and eventual atomic ionization at fluences far below any inverse bremsstrahlung or catastrophic breakdown process. Over a range of fluences below breakdown, a wide range of particle emission phenomena are observed. The chapter provides a description of photodesorption, which is a relatively gentle process, removing sub-monolayer quantities of material/laser pulse. The processes can fall into nonthermal (for example, photoelectronic) and thermal categories. For semiconductors and insulators, the incident light may be sub-bandgap or may involve single-photon band-to-band transitions, depending on the laser photon energy relative to E g .

Generation of color centers by femtosecond laser pulses in wide-bandgap materials

The high instantaneous powers associated with femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long term behavior of coloration. In this work, we probe the evolution of color centers produced by femtosecond laser radiation in soda lime glass and single crystal sodium chloride on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we can follow the changes in coloration due to individual or multiple femtosecond pulses, and follow the evolution of that coloration for long times after femtosecond laser radiation is terminated. For the soda lime glass, the decay of color centers is well described in terms of bimolecular annihilation reactions between electron and hole centers. Similar processes appear to operate in single crystal sodium chloride. We report also fabrication of permanent periodic patterns in soda lime glass by two time coincident femtosecond laser pulses.

Organic molecules released from olivine by impact fracture

To evaluate the possible inventory of organic matter on the prebiotic Earth efforts have been concentrated on reactions in the atmosphere. Examples are the classic Miller-Urey experiments, using electric discharges, and photochemistry. Minerals have been considered only with respect to their surfaces which provide potential sites for catalytic activity, or as templates for the synthesis of complex organic molecules out of simple precursors.

Physical and chemical aspects of laser-materials interactions relevant to laser processing

The use of lasers in packaging and materials processing is an increasingly attractive choice for high technology manufacturing. As we push for more demanding materials processing tasks and smaller dimensions, an understanding of the underlying physical and chemical aspects of problems becomes important. Here we discuss some of these issues relevant to materials processing.

Pulse-width influence on laser induced desorption of positive ions from ionic solids

We have compared the desorption of positive ions, including Mg+ and MgO+, form ionic magnesium oxide single crystals following pulsed laser excitation using either nanosecond or femtosecond sources. Following optical excitation, desorbed ions are rapidly extracted and mass analyzed using standard time-of-flight techniques. Ion yields and velocities are determined as a function of laser fluence. The threshold similarity is a surprising result, as sub-band gap nanosecond pulses are only likely to excite defect states efficiently, while the ultrahigh peak-power femtosecond pulses could in principle induce multiphoton and avalanche excitation. We argue that at least in this specific case, the important factor appears to be merely the number of photons and not the pulse duration. However, it is observed that femtosecond excitation yields considerable H+ and less interference from impurity alkali ions than does nanosecond excitation. The source of the protons is presumably the hydroxylated MgO surface.