18. 4-fluorophenylthiolate-capped nanoparticles of cubic CdS, CdSe, ZnS, and PbS were prepared by a modified literature synthesis (17) and dispersed in acetone to a concentration of È0.05 to 0.5 M. Samples of 2 to 4 mL of the sols in polyethylene vials were treated with 0.1 to 0.2 mL of 3% H 2 O 2 (CdS, ZnS, PbS) or 3% tetranitromethane (CdSe) to induce gelation. Gels were aged for several days to a week, exchanged multiple times with acetone, and dried in a SPI-Dry (Structure Probe, Inc., West Chester, PA) critical-point dryer using CO 2. to 124 m 2 /g (mean 115 m 2 /g); adsorption average pore diameter (BJH), 23 to 28 nm; adsorption cumulative pore volume (BJH), 0.70 to 0.73 cm 3 /g. Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 Â 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude. In recent years, the invention of the femto-second frequency comb laser (1-3) has brought about a revolution in metrology. A frequency comb acts as a bridge between the radio frequency (RF) domain (typically tens of MHz) and the optical frequency domain (typically hundreds of THz). Thus, in precision spectroscopy, the optical cycles of a continuous wave (CW) ultrastable laser can be phase-locked and counted directly with respect to an absolute frequency standard such as an atomic clock (4, 5). The resultant frequency measurements approach a precision of 1 part in 10 15 in certain cases, potentially enabling the detection of possible drift in the fundamental constants (6, 7), among other quantum mechanical applications. Here, we perform precision metrology without the use of a CW laser. Instead, an atomic transition is excited directly with amplified and frequency-converted pulses from a femtosecond frequency comb laser. As a result of quantum interference effects in the atomic excitation process, we can achieve an accuracy that is about six orders of magnitude higher than the optical bandwidth of the individual laser pulses. The method used is related to Ramsey_s principle of separated oscillatory fields (8), which probes the phase evolution of an atom in spatially separated interaction zones. This technique is widely used in the RF domain for atomic fountain clocks (9). By extension, in the optical domain, excitation can be performed by pulses separated in time (rather than in space) to maintain phas...

In a previous paper, Germano, et al. (1991) proposed a method for computing coefficients of subgrid-scale eddy viscosity models as a function of space and time. This procedure has the distinct advantage of being self-calibrating and requires no a priori specification of model coefficients or the use of wall damping functions. However, the original formulation contained some mathematical inconsistencies that limited the utility of the model. In particular, the applicability of the model was restricted to flows that are statistically homogeneous in at least one direction. These inconsistencies and limitations are discussed and a new formulation that rectifies them is proposed. The new formulation leads to an integral equation whose solution yields the model coefficient as a function of position and time. The method can be applied to general inhomogeneous flows and does not suffer from the mathematical inconsistencies inherent in the previous formulation. The model has been tested in isotropic turbulence and in the flow over a backward-facing step.

Once believed to degrade into simple compounds, increasing evidence suggests plastics entering the environment are mechanically, photochemically, and/or biologically degraded to the extent that they become imperceptible to the naked eye yet are not significantly reduced in total mass. Thus, more and smaller plastics particles, termed microplastics, reside in the environment and are now a contaminant category of concern. The current study tested the hypotheses that microplastics concentration would be higher in proximity to urban sources, and vary temporally in response to weather phenomena such as storm events. Triplicate surface water samples were collected approximately monthly between July and December 2011 from four estuarine tributaries within the Chesapeake Bay, U.S.A. using a manta net to capture appropriately sized microplastics (operationally defined as 0.3-5.0 mm). Selected sites have watersheds with broadly divergent land use characteristics (e.g., proportion urban/suburban, agricultural and/or forested) and wide ranging population densities. Microplastics were found in all but one of 60 samples, with concentrations ranging over 3 orders of magnitude (<1.0 to >560 g/km(2)). Concentrations demonstrated statistically significant positive correlations with population density and proportion of urban/suburban development within watersheds. The greatest microplastics concentrations also occurred at three of four sites shortly after major rain events.

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