Alan M. Frank

Ultrafast electron microscopy in materials science, biology, and chemistry

The use of pump-probe experiments to study complex transient events has been an area of significant interest in materials science, biology, and chemistry. While the emphasis has been on laser pump with laser probe and laser pump with x-ray probe experiments, there is a significant and growing interest in using electrons as probes. Early experiments used electrons for gas-phase diffraction of photostimulated chemical reactions. More recently, scientists are beginning to explore phenomena in the solid state such as phase transformations, twinning, solid-state chemical reactions, radiation damage, and shock propagation. This review focuses on the emerging area of ultrafast electron microscopy (UEM), which comprises ultrafast electron diffraction (UED) and dynamic transmission electron microscopy (DTEM). The topics that are treated include the following: (1) The physics of electrons as an ultrafast probe. This encompasses the propagation dynamics of the electrons (space-charge effect, Child’s law, Boersch effect) and extends to relativistic effects. (2) The anatomy of UED and DTEM instruments. This includes discussions of the photoactivated electron gun (also known as photogun or photoelectron gun) at conventional energies (60–200 keV) and extends to MeV beams generated by rf guns. Another critical aspect of the systems is the electron detector. Charge-coupled device cameras and microchannel-plate-based cameras are compared and contrasted. The effect of various physical phenomena on detective quantum efficiency is discussed. (3) Practical aspects of operation. This includes determination of time zero, measurement of pulse-length, and strategies for pulse compression. (4) Current and potential applications in materials science, biology, and chemistry. UEM has the potential to make a significant impact in future science and technology. Understanding of reaction pathways of complex transient phenomena in materials science, biology, and chemistry will provide fundamental knowledge for discovery-class science.

Single-shot dynamic transmission electron microscopy

A dynamic transmission electron microscope (DTEM) has been designed and implemented to study structural dynamics in condensed matter systems. The DTEM is a conventional in situ transmission electron microscope (TEM) modified to drive material processes with a nanosecond laser, “pump” pulse and measure it shortly afterward with a 30-ns-long probe pulse of ∼107 electrons. An image with a resolution of <20nm may be obtained with a single pulse, largely eliminating the need to average multiple measurements and enabling the study of unique, irreversible events with nanosecond- and nanometer-scale resolution. Space charge effects, while unavoidable at such a high current, may be kept to reasonable levels by appropriate choices of operating parameters. Applications include the study of phase transformations and defect dynamics at length and time scales difficult to access with any other technique. This single-shot approach is complementary to stroboscopic TEM, which is capable of much higher temporal resolution ...

Microstructure morphology of shock-induced melt and rapid resolidification in bismuth

With the growing importance of nanotechnology, there is increased emphasis on rapid solidification processing to produce materials microstructures with a finer length scale. However, few studies have focused on the question of how a material restructures itself on the microstructural scale when it refreezes at very high cooling rates. Here we report on the development of microstructures in pure bismuth metal as it is subjected to rapid shock-driven melting and subsequent resolidification (on release of pressure), where the estimated effective undercooling rates are on the order of 1010K∕s, orders of magnitude faster than any achieved before in bulk material. Microscopic examination of the recovered material indicates that the melting transformation was far from homogeneous, and substantial morphological changes are observed compared to the starting microstructure.

Investigation of thin laser-driven flyer plates using streak imaging and stop motion microphotography

The dynamic behavior of laser-accelerator flyers has been studied using high-speed streak imaging in combination with stop motion microphotography. With very thin targets, melting and plasma penetration of the flyer material occur in rapid sequence. The time delay from the onset of motion to flyer breakup increases with flyer thickness and decreasing incident energy. Flyer materials examined include pure aluminum (0.25-2.6 {mu}m thick) and composite targets (0.5-2.0 {mu}m thick) containing an insulating layer of aluminum oxide. While flyer breakup is observed in both types of material, the Al{sub 2}O{sub 3} barrier significantly delays the deleterious effects of deep thermal diffusion.

Spectral measurements and analysis of beam‐gas emissions

Spectral emissions of the Experimental Test Accelerator beam in 500‐Torr synthetic air have been measured in the wavelength range 250–700 nm for beam currents of 4.5 and 8 kA. Intense emissions are identified as radiation from nitrogen. Relative intensities agree well with Franck–Condon factors. Wet air results in a dramatic decrease (by a factor of 2–3) in intensity over the scanned range. This effect is presently attributed to increased hose motion. Near 700 nm, emissions that are not observed in dry air, are identified as emanating from water vapor, nitric oxide, and oxygen. The pressure dependence of emitted intensities at 337.1 and 391.4 nm were measured from 80 μ to 500 Torr of nitrogen as well as dry synthetic air. Results obtained for an 8‐kA beam are not explained by a time‐dependent Boltzmann air chemistry code which predicted very well previous measurements for a 1–kA beam.

Fabry-Perot Velocimetry Techniques: Is Doppler Shift Affected By Surface Normal Direction?

The Fabry-Perot laser velocimeter is used at LLNL for hydrodynamic, equation-of-state, surface-ejecta mass measurements and for other applications. Velocities of shocked surfaces can be measured to better than 1%, and multiple records can be superimposed on a single piece of film. Many phenomena are being investigated for possible sources of error. One concern was whether the direction of the surface normal could affect the measured Doppler shift, or whether the direction of the particle velocity was sufficient to determine the shift. A series of experiments with angles between the laser and particle velocity as small as 20° have shown that for the surface smoothnesses that we encounter, we see no effect caused by varying the direction of the surface normal.

Aluminum metal combustion in water revealed by high speed microphotography

In high explosives designed for air blast cratering fragmentation and underwater applications metallic additives chemically react with the oxidizer and are used to tailor the rate of energy delivery by the expansion medium. Although the specific mechanism for sustained metal combustion in the dense detonation medium remains in question it is generally accepted that the fragmentation of the molten particle and disruption of its oxide layer are a necessity. In this study we use high speed microphotography to examine the ignition and combustion of small 25-76 jim diameter and 23 mm long aluminum wires rapidly heated by a capacitor discharge system in water. Streak and framing photographs detailing the combustion phenomenon and the fragmentation of the molten aluminum were obtained over periods of 100 nsec - 100 j. tsec with a spatial resolution of 2 . im. The wire temperature was determined as a function of time by integrating the circuit equation together with the energy equation for an adiabatic wire and incorporating known aluminum electrical resistivity and temperature functions of energy density in the integration. In order for the aluminum to sustain a rapid chemical reaction with the water we found that the wire temperature has to be raised above the melting temperature of aluminum oxide. The triggering mechanism for this rapid reaction appears to be the fragmentation of the molten aluminum from the collapse of a vapor blanket about

Birefringent polarization-independent beam splitter

By adjusting both the thickness and angle of the incidence of a birefringent channeled spectrum plate, the extraordinary and ordinary reflectivities can be equalized, thereby providing a polarization-independent beam splitter.

Quick fast off-axis parabolas

The design and fabrication of four ultrafast, off-axis parabolas was required as quickly as possible.1 Each parabola couples a 10-cm diam beam of submillimeter radiation to a Schottky-Barrier detector.

MECHANISMS OF EBW HE INITIATION

Exploding bridgewire (EBW) initiation of high explosives (HE) has been used for many years without a clear understanding of the mechanisms involved. Data indicates that the shock pressures produced by the EBW may be insufficient for direct initiation. Evidence indicates that the electric field about the wire at the time of burst creates surface ionization of the HE. Our hypothesis is that the ionization pre-sensitizes the HE so that a weak shock can then initiate a detonation wave.