Femtosecond Visualization of hcp-Iron Strength and Plasticity under Shock Compression.

Iron is a key constituent of planets and an important technological material. Here, we combine in situ ultrafast x-ray diffraction with laser-induced shock compression experiments on Fe up to 187(10) GPa and 4070(285) K at 10^{8} s^{-1} in strain rate to study the plasticity of hexagonal-close-packed (hcp)-Fe under extreme loading states. {101[over ¯]2} deformation twinning controls the polycrystalline Fe microstructures and occurs within 1 ns, highlighting the fundamental role of twinning in hcp polycrystals deformation at high strain rates. The measured deviatoric stress initially increases to a significant elastic overshoot before the onset of flow, attributed to a slower defect nucleation and mobility. The initial yield strength of materials deformed at high strain rates is thus several times larger than their longer-term flow strength. These observations illustrate how time-resolved ultrafast studies can reveal distinctive plastic behavior in materials under extreme environments.

The degradation of acetaldehyde in estuary waters in Southern California, USA.

Acetaldehyde plays an important role in oxidative cycles in the troposphere. Estimates of its air-water flux are important in global models. Biological degradation is believed to be the dominant loss process in water, but there have been few measurements, none in estuaries. Acetaldehyde degradation rates were measured in surface waters at the inflow to the Upper Newport Back Bay estuary in Orange County, Southern California, USA, over a 6-month period including the rainy winter season. Deuterated acetaldehyde was added to filtered and unfiltered water samples incubated in glass syringes, and its loss analyzed by purge and trap gas chromatography mass spectrometry. Filtered samples showed no significant degradation, suggesting that particle-mediated degradation is the dominant removal process. Correlation between measured degradation rate constants in unfiltered incubations and bacteria counts suggests the loss is due to microorganisms. Degradation in unfiltered samples followed first-order kinetics, with rate constants ranging from 0.0006 to 0.025 min-1 (k; average 0.0043 ± 0.006 min-1). Turnover (1/k) ranged from 40 to 1667 min, consistent with prior studies in coastal waters. Acetaldehyde concentrations in the estuary are estimated to range from 30 to ~500 nM (average ~250 nM). Results suggest the estuary is a source of acetaldehyde to the atmosphere.

Photochemical degradation of oil products in seawater monitored by 3D excitation emission matrix (EEM) fluorescence spectroscopy: implications for coloured dissolved organic matter (CDOM) studies.

Fluorescence 3D excitation emission matrix (EEM) spectra of oil products in artificial seawater were monitored as a function of irradiation time in a solar simulator. EEMs were obtained for an excitation range of 240-400 nm and an emission range of 248-830 nm; this is the wavelength range typically used in chromophoric dissolved organic matter (CDOM) EEM studies in natural waters. This allows for comparison to prior work on CDOM in an oil-contaminated salt marsh that attributed a fluorescent component in the tryptophan/tyrosine protein-region to oil. For comparison, EEMs were also measured for a broader excitation range of 220-400 nm typically used in oil related studies to capture the primary oil peak at lower excitation wavelengths. Fluorescence intensities in both excitation wavelength ranges decayed exponentially with irradiation time consistent with first-order kinetics. There was little change in wavelength for primary oil peaks. However, in the CDOM, wavelength range peaks typically shifted to longer excitation and shorter emission wavelengths, moving into the protein peak region of the CDOM EEM spectrum. This is consistent with a decrease in the complexity of the structure of the organic material. Half-lives for photodegradation ranged from 0.36 to 7.2 days for the oil wavelength range and 0.14 to 28 days for the CDOM wavelength range. Higher density oils typically had higher degradation rate constants. Peak locations and peak behaviour are consistent with the primary fluorophore in the oil products being PAH-related.