Stephen C. Langford

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.

Consequences of simultaneous exposure of inorganic solids to excimer laser light and an electron beam

In previous work we examined the changes in surface topography of sodium trisilicate glass (Na2O⋅3SiO2) with exposure to pulsed 248‐nm excimer laser light at fluences of 2.6–5 J/cm2 (per laser pulse), as well as the character of the products emitted from the glass surface (e.g., ± ions, electrons, ground state and excited neutral atoms and molecules). At fluences above a threshold, ablative etching occurs only after a number of preliminary laser pulses. In this work, we show a dramatic synergism in the ablation process by simultaneous bombardment of the glass surface with 0.5–2 keV electrons and laser pulses. Extensive etching can be initiated immediately and sustained indefinitely at subthreshold fluences. Reflection electron energy loss spectroscopy performed on electron‐bombarded surfaces shows band gap states growing with exposure. We propose that surface and near‐surface defects produced by inelastic scattering of the electron beam provide single‐photon absorption centers, facilitating etching at sub...

Photoinduced Formation of Zinc Nanoparticles by UV Laser Irradiation of ZnO

Simple exposure of single-crystal ZnO to 193 nm excimer laser radiation at room temperature results in unexpected coloration. The gray to nearly black colored material, seen principally in the irradiated laser spot, is superficial. We present unambiguous evidence that this coloration is due to high densities of metallic Zn nanoparticles growing on the exposed surface of the crystal. Higher fluence laser exposure generates accumulated surface metal just outside of the irradiated spot. We suggest that the near surface bulk is photodecomposing; thermally driven diffusion leads to surface Zn metal aggregation.

The interaction of ultraviolet excimer laser light with sodium trisilicate

We describe the results of irradiating sodium trisilicate glass with 248‐nm excimer laser light at fluences from 1 to 5 J/cm2. A threshold for the onset of etching occurs at 3 J/cm2. We investigate the changes in surface topography as a function of laser fluence. We also identify the ionic and neutral species contained in the emission plume. A clear correlation is observed between the etching threshold and (i) onset of emission of fast excited neutrals, as well as (ii) the appearance of atomic Na D resonance radiation. The high velocities (10–15 km/s) of the excited neutrals and ions (H+, Na+, and Si+) are attributed to laser/plume interactions. The character and origin of lower velocity neutral species (atomic O, Si and molecular NaO) are presented. The possible role of surface fracture in the onset of etching is also discussed.

Electrical transients during interfacial debonding and pullout of a metal rod from an epoxy matrix

We examine electrical transients which are produced during the propagation of a debonding crack along the interface of an axially loaded stainless‐steel rod embedded in epoxy. Time‐resolved correlations with fracto‐emission signals and tensile strain in the rod are made to assist interpretation. We show that on microsecond time scales, the motion of the debonding crack as well as the initial pullout motion of the rod following debonding can be detected and followed. A model is developed which consistently describes the sign and amplitude of the detected electrical signals. These experimental data provide time‐resolved information on the sequence of events leading to interfacial fracture and initial pullout of the rod from the epoxy.

Negative charge emission due to excimer laser bombardment of sodium trisilicate glass

We describe measurements of negative charge emission accompanying irradiation of sodium trisilicate glass (Na2O⋅3SiO2) with 248‐nm excimer laser light at fluences on the order of 2 J/cm2 per pulse, i.e., at the threshold for ablative etching of the glass surface. The negative charge emission consists of a very prompt photoelectron burst coincident with the laser pulse, followed by a much slower plume of electrons and negative ions traveling with a high density cloud of positive ions, previously identified as primarily Na+. Using combinations of E and B fields in conjunction with time‐of‐flight methods, the negative ions were successfully separated from the plume and tentatively identified as O−, Si−, NaO−, and perhaps NaSi−. These negative species are probably formed by gas phase collisions in the near‐surface region which result in electron attachment.

Alkali emission accompanying fracture of sodium silicate glasses

Measurements of atomic Na emission accompanying the fracture of sodium trisilicate glass and a soda lime glass in vacuum were made by quadrupole mass spectroscopy and surface ionization techniques. Peak Na° emission intensities occur some 3-6 ms after the fracture event and decay over tens of milliseconds. This behavior is attributed to the diffusion of Na + ions into a layer of damaged material at the surface where the ions are subsequently neutralized and thermally emitted as Na°

Scanning tunneling microscope observations of MgO fracture surfaces

Scanning tunnel microscope (STM) observations of surfaces of single crystal MgO fractured in three‐point bend and coated with thin gold films are reported. Macroscopically, the fracture geometry employed yields a smooth, approximately (027) surface along the tensile side of the sample, and a heavily stepped (001) surface along the compressive side. STM observations of the tensile side show smooth regions approximately (027) in orientation with relatively little relief. These smooth regions are separated by rough regions which often could only be partially imaged with the present instrument. The compressive side showed much more varied structures, with many (001) features as well as features approximately (720) in orientation. Fractal dimension measurements were made on topographs of tensile and compressive side features. Observations of triangular features on the compressive side of one MgO sample show evidence of interaction between the growing crack and dislocation structures. The significance of these ...

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.

Anisotropy effects on fracto‐emission from MgF2 single crystals

An important component of fracto‐emission is the emission of electrons (EE) accompanying the fracture of materials. In this letter we present measurements of EE accompanying the fracture of MgF2 single crystals loaded in three‐point bend. In particular, we examine the effect of changing the crystal orientation relative to the loading direction on emission intensity. We find that fracture surfaces with different crystal orientations yield significantly different EE. We propose that this is due to differences in the density of defects produced by such a fracture.