J. T. Dickinson

Adsorption and desorption of ammonia, hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH3 molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ⩽ 2 × 10−4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.

Interactions of wide band‐gap single crystals with 248 nm excimer laser radiation. II. NaCl

The emission of charged and neutral particles from single‐crystal MgO irradiated with pulsed 248 nm excimer laser light is studied by means of quadrupole mass spectrometry, time‐resolved emission spectroscopy, luminescence spectroscopy, and scanning electron microscopy (SEM) observations. The role of the initial distribution of near‐surface defects plus defects which result from repeated application of laser pulses is explored. This increase in defect density eventually leads to formation of a visible plume and rapid material vaporization. SEM observations after irradiation indicate that substantial surface fracture is present prior to the onset of rapid vaporization. Defect production during irradiation is attributed to mechanical processes involving deformation and fracture with accompanying dislocation motion. The accumulation of these defects increases laser absorption in the near‐surface region resulting in rapid thermal etching and cluster emission.

The Emission of Electrons and Positive Ions from Fracture of Materials.

The emission of electrons and positive ions from materials undergoing fracture is investigated. We present a survey of charged particle emission from a number of materials including crystalline insulators, glass, graphite, polymers and composites. Particular attention is given to fibre-reinforced epoxy systems which yield unique forms of charge emission. Energy distributions of the emitted particles are given for E-glass-epoxy strands, polybutadiene filled with glass beads, and mica. Evidence is presented that interfacial failure and charge separation play important roles in the observed emission.

Single asperity tribochemical wear of silicon nitride studied by atomic force microscopy

Nanometer scale single asperity tribochemical wear of silicon nitride was examined by measuring the wear of atomic force microscope tips translated against a variety of substrates in aqueous solutions. We show that the chemical nature of the substrate plays an important role: significant wear was observed only when the substrate surface is populated with appropriate metal-hydroxide bonds. Mica and calcite substrates, whose water-exposed cleavage surfaces lack these bonds, produced little if any tip wear. As a function of contact force FN and scan duration t, the length of the tips in this work decreases approximately as (FNt)0.5. We propose that pressure-induced intermediate states involving hydroxyl groups form on both the tip and the substrate; chemical reactions subsequently form transient bridging chemical bonds that are responsible for tip wear.

Atomic layer wear of single‐crystal calcite in aqueous solution using scanning force microscopy

Scanning force microscopy (SFM) is employed to study nm‐scale wear of single‐crystal calcite in an aqueous solution. When the SFM tip is drawn back and forth in a linear fashion across the edge of a preexisting single atomic layer etch pit, dissolution is strongly enhanced at the point where the tip crosses the step. The wear rate as a function of contact force is consistent with a thermally activated wear process, where the activation energy is locally reduced in the strain field of the SFM tip. The activation volume for the strain dependence is on the order of the average volume per ion in the CaCO3 lattice. This study provides further support for strain enhanced nucleation of double kinks along preexisting steps.

Simultaneous measurements of the electron and photon emission accompanying fracture of single‐crystal MgO

We investigate the simultaneous emission of electrons and photons during the deformation and fracture of two types of high‐purity single‐crystal MgO. These crystals exhibit significant differences in optical opacity due to differences in void and precipitate concentrations. Measurements of the emission of visible photons during deformation and prior to failure of the crystals are presented, along with the time dependencies of the photon and electron emission during and after fracture. Correlations with fracture strength and fractographic features are also discussed.

Fracto‐emission: The role of charge separation

Fracto‐emission is the emission of particles (e.g., electrons, ions, ground state and excited neutrals, and photons) during and following fracture. We have found that during fracture in vacuum of adhesive bonds and crystalline materials involving large amounts of charge separation on the surface the emission of charged particles, excited neutrals, light, and radio waves occurs with unique and revealing time dependencies. In this paper we report simultaneous fracto‐emission measurements on several systems. We interpret the results in terms of a conceptual model involving the following steps: (1) charge separation due to fracture, (2) desorption of gases from the material into the crack tip, (3) a gas discharge in the crack, (4) energetic bombardment of the freshly created crack walls, and (5) thermally stimulated electron emission, accompanied by electron stimulated desorption of ions and excited neutrals. In addition to evidence from fracture experiments, we present results from studies of electron bombardment of a polymer surface.

Color center formation in soda-lime glass with femtosecond laser pulses

We show that exposure of soda-lime glass to ultrafast laser pulses at 800 nm causes coloration (darkening). We have characterized this coloring with time-resolved measurements of the transmission of 633 nm light through the glass during laser exposure. Reverse processes (partial bleaching) operate on time scales of μs to seconds. The competition between coloration after the femtosecond pulse and the subsequent transmission recovery limits the darkening that can be achieved at a given femtosecond pulse energy and repetition rate. The response of soda-lime glass to 400 and 267 nm ultrafast pulses is quite similar, although much lower pulse energies are required for darkening. We argue that darkening is due to absorption processes that produce mobile charge carriers, which then interact to produce trapped hole centers (H3+) that absorb strongly at 633 nm. Trapped electrons (that form E centers) are the likely cause of the accompanying loss of transmission in the near ultraviolet. Finally, we show that diffra...

Interaction of wide band gap single crystals with 248 nm excimer laser radiation. IV. Positive ion emission from MgO and NaNO3

We report quadrupole mass‐selected, time‐of‐flight measurements of Mg+ from polished, single crystal MgO and Na+ from cleaved, single crystal NaNO3 exposed to 248 nm (5 eV) laser radiation. A large fraction of the ions emitted from these materials have energies well above the energy of the incident photon. As the fluence is raised from low values, the ion intensities show thresholdlike behavior with a high‐order fluence dependence (roughly sixth order). At still higher fluences, the fluence dependence of Mg+ from MgO decreases to roughly second order. We attribute these emissions to weakly bound ions adsorbed atop surface electron traps; when the underlying vacancy is photoionized, the adsorbed ion is electrostatistically ejected at high energy. We argue that several photons are required to ionize a surface electron trap beneath an adsorbed ion, accounting for the high‐order fluence dependence and satisfying conservation of energy. (Several 5 eV photons are required to produce a 10 eV ion.) We show that a...

Interactions of wide band gap single crystals with 248 nm excimer laser radiation. III: The role of cleavage-induced defects in MgO

When single‐crystal, arc‐fused MgO is cleaved and exposed to single pulses of 248 nm, 3–10 J/cm2 excimer laser light (in vacuum), micron‐sized holes are produced in the irradiated area. Cleavage produces micron‐sized sites which exhibit highly localized absorption, resulting in decomposition, melting, and vaporization of the crystal at these sites. At fluences between 1 and 2 J/cm2, single laser pulses can produce localized, superficial melting and decomposition, leaving clusters of Mg‐rich droplets on the surface. The mass‐resolved charged and neutral particle emission produced by irradiation of the cleaved MgO surface with single laser pulses are examined. There are considerable variations in these emissions when single laser pulses strike different regions of the cleavage surface; these variations are strongly correlated with the production of conical holes in the irradiated region. A phenomenological model for the creation of these sites involving dislocation driven processes during cleavage and the r...