James E. Patterson

Shock Wave Induced Decomposition of RDX: Time-Resolved Spectroscopy

Time-resolved optical spectroscopy was used to examine chemical decomposition of RDX crystals shocked along the [111] orientation to peak stresses between 7 and 20 GPa. Shock-induced emission, produced by decomposition intermediates, was observed over a broad spectral range from 350 to 850 nm. A threshold in the emission response of RDX was found at about 10 GPa peak stress. Below this threshold, the emission spectrum remained unchanged during shock compression. Above 10 GPa, the emission spectrum changed with a long wavelength component dominating the spectrum. The long wavelength emission is attributed to the formation of NO2 radicals. Above the 10 GPa threshold, the spectrally integrated intensity increased significantly, suggesting the acceleration of chemical decomposition. This acceleration is attributed to bimolecular reactions between unreacted RDX and free radicals. These results provide a significant experimental foundation for further development of a decomposition mechanism for shocked RDX (following paper in this issue).

Shock wave induced decomposition of RDX: quantum chemistry calculations.

Quantum chemical calculations on single molecules were performed to provide insight into the decomposition mechanism of shocked RDX. These calculations complement time-resolved spectroscopy measurements on shock wave compressed RDX crystals (previous paper, this issue). It is proposed that unimolecular decomposition is the primary pathway for RDX decomposition in its early stages and at stresses lower than approximately 10 GPa. This decomposition leads to the generation of broadband emission from 350 to 850 nm. Chemiluminescence from (2)B1 and (2)B2 excited states of NO2 radicals is associated with a major portion of the experimentally observed emission spectrum (>400 nm). The remaining portion (<400 nm) of the emission spectrum primarily results from excited HONO intermediates. It is proposed that for stresses higher than 10 GPa, bimolecular reactions between radical decomposition products and unreacted RDX molecules become the dominant pathway. This radical assisted homolysis pathway is cyclic and leads to the acceleration of decomposition, with increased production of low energy NO2 radicals. These radicals produce emission that is stronger in the long wavelength portion of the spectrum. Finally, a comprehensive chemical decomposition mechanism is put forward that is consistent with the experimental observations of shock-induced emission in RDX crystals.

Multilayer polymer microchip capillary array electrophoresis devices with integrated on-chip labeling for high-throughput protein analysis.

It is desirable to have inexpensive, high-throughput systems that integrate multiple sample analysis processes and procedures, for applications in biology, chemical analysis, drug discovery, and disease screening. In this paper, we demonstrate multilayer polymer microfluidic devices with integrated on-chip labeling and parallel electrophoretic separation of up to eight samples. Microchannels were distributed in two different layers and connected through interlayer through-holes in the middle layer. A single set of electrophoresis reservoirs and one fluorescent label reservoir address parallel analysis units for up to eight samples. Individual proteins and a mixture of cancer biomarkers have been successfully labeled on-chip and separated in parallel with this system. A detection limit of 600 ng/mL was obtained for heat shock protein 90. Our integrated on-chip labeling microdevices show great potential for low-cost, simplified, rapid, and high-throughput analysis.

Vibrational energy in molecules probed with high time and space resolution

This article reviews experimental measurements of vibrational energy in condensed-phase molecules that simultaneously provide time resolution of picoseconds and spatial resolution of ångströms. In these measurements, ultrashort light pulses are used to input vibrational energy and probe dynamical processes. High spatial resolution is obtained using vibrational reporter groups in known locations on the molecules. Three examples are discussed in detail: (1) vibrational energy flow across molecules in a liquid from an OH–group to a CH3–group; (2) vibrational energy flow across a molecular surfactant monolayer that separates an aqueous and a non-polar phase in a suspension of reverse micelles; and (3) vibrational energy input by laser-driven shock waves to a self-assembled monolayer of long-chain alkane molecules. These experiments provide new insights into the movement of mechanical energy over short length and time scales where ordinary concepts of heat conduction no longer apply, where the concepts of quantum mechanical energy transfer reign supreme.

Optically determined velocity distributions of metastable argon in the second stage of an inductively coupled plasma mass spectrometer

Velocity profiles of argon metastable atoms have been measured in the second stage of an inductively coupled plasma mass spectrometer (ICP-MS). These profiles were obtained from laser-induced fluorescence excitation spectra of the Ar I 4s[3/2]° to 4p[5/2] transition. Velocities were determined from the Doppler shift of the exciting radiation. An argon-filled hollow cathode lamp served as a stationary wavelength reference. The velocity distribution at the skimmer orifice is bimodal, indicative of a flow disturbance at or slightly upstream of the skimmer orifice.

Role of Nonresonant Sum-Frequency Generation in the Investigation of Model Liquid Chromatography Systems

In vibrationally resonant sum-frequency generation (VR-SFG) spectra, the resonant signal contains information about the molecular structure of the interface, whereas the nonresonant signal is commonly treated as a background and has been assumed to be negligible on transparent substrates. The work presented here on model chromatographic stationary phases contradicts this assumption. Model stationary phases, consisting of functionalized fused-silica windows, were investigated with VR-SFG spectroscopy, both with and without experimental suppression of the nonresonant response. When samples are moved from CD(3)OD to D(2)O, the VR-SFG spectrum was found to change over time when the nonresonant signal was present but not when the nonresonant signal was suppressed. No effect was seen when the solvent was changed and pressurized to 900 psi. These results suggest that the response to the new solvent manifests primarily in the nonresonant response, not the resonant response. Any structural changes caused by the new solvent environment appear to be minor. The nonresonant signal is significant and must be properly isolated from the resonant signal to ensure a correct interpretation of the spectral data. Curve-fitting procedures alone are not sufficient to guarantee a proper interpretation of the experimental results.

Plasma treatment of polystyrene thin films affects more than the surface.

Plasma treatment of polymer materials introduces chemical functionalities and modifies the material to make the native hydrophobic surface more hydrophilic. It is generally assumed that this process only affects the surface of the material. We used vibrationally resonant sum-frequency generation spectroscopy to observe changes in the orientation of phenyl groups in polystyrene (PS) thin films on various substrates before and after plasma treatment. VR-SFG selectively probes regions of broken symmetry, such as surfaces, but can also detect the emergence of anisotropy. On dielectric substrates, such as fused silica, the spectroscopic peak corresponding to the symmetric stretching (ν2) mode of the phenyl rings was undetectable after plasma treatment, showing that surface phenyl rings were altered. This peak also diminished on conducting substrates, but the intensity of another peak corresponding to the same mode in a bulklike environment increased significantly, suggesting that plasma treatment induces partial ordering of the bulk polymer. This ordering is seen on conducting substrates even when the polymer is not directly exposed to the plasma. Annealing reverses these effects on the polystyrene bulk; however, the surface phenyl rings do not return to the orientation observed for untreated films. These results call into question the assumption that the effects of plasma treatment are limited to the free surface and opens up other possibilities for material modification with low-temperature plasmas.

Back-surface gold mirrors for vibrationally resonant sum-frequency (VR-SFG) spectroscopy using 3-mercaptopropyltrimethoxysilane as an adhesion promoter.

Back-surface mirrors are needed as reference materials for vibrationally resonant sum-frequency generation (VR-SFG) probing of liquid-solid interfaces. Conventional noble metal mirrors are not suitable for back-surface applications due to the presence of a metal adhesion layer (chromium or titanium) between the window substrate and the reflective metal surface. Using vapor deposited 3-mercaptopropyltrimethoxysilane (MPTMS) as a bi-functional adhesion promoter, gold mirrors were fabricated on fused silica substrates. These mirrors exhibit excellent gold adhesion as determined by the Scotch® tape test. They also produce minimal spectroscopic interference in the C–H stretching region (2800–3000 cm−1), as characterized by VR-SFG. These mirrors are thus robust and can be used as back-surface mirrors for a variety of applications, including reference mirrors for VR-SFG.

Shock compression spectroscopy with high time and space resolution

Femtosecond laser‐driven shock compression experiments are described using nonlinear coherent vibrational sum‐frequency generation spectroscopy (SFG) probing of molecular materials. SFG selectively monitors molecular groups at surfaces and interfaces, providing a high degree of spatial resolution. In initial experiments a self‐assembled monolayer of long‐chain alkane molecules is studied, where SFG sees only the methyl (−CH3) head groups. The plane of methyl groups is just 1.5A thick. Shock‐induced bending of the chain and shock‐induced rotations around carbon‐carbon bonds are observed. Possible future directions are discussed.

Static and shock compression of RDX single crystals: Raman spectroscopy

We report an examination of the polymorphic α - γ transformation for the energetic crystal RDX under static and shock compression. Raman spectroscopy was utilized as a probe of the molecular level response. We observed splitting of several Raman modes with the occurrence of the phase transition. From the response of the lattice and internal modes to static compression, we propose that the γ phase structure is isomorphous with the D2h point group with eight molecules per unit cell occupying C1 symmetry sites. The phase transition was observed to occur under shock wave loading with a transition time of ~100 ns for a peak stress of 5.5 GPa. The occurrence of the phase transition under shock wave loading has important implications for the onset of chemical decomposition in RDX.