A series of monodispersed oligo(p-phenyleneethynylene)s were synthesized bearing intramolecular hydrogen bonds between side chains of adjacent phenylene units in the backbone. Thus, all repeating units of the molecules are constrained in a coplanar orientation. Such planarized conformation is considered favorable for single-molecule conductance. Photophysical characterization results show narrowed bandgaps and extended conjugation lengths, consistent with a rigid, planar backbone framework as a result of intramolecular hydrogen bonding.
For the time being, Integrated Optics is able to produce a range of devices able of efficiently modulating phase, amplitude and frequency of guided waves in single mode structures implemented at the surface of active materials such as Lithium Niobate. In most of the cases, their basic principle relies on the interference pattern of two guided waves and the classical implementations of such modulators were mainly related to COBRA switch type directional couplers or to MACH ZEHNDER type interferometers. Only very recently a structure combining the coupled waveguides of a COBRA and the Y junction of the MACH ZEHNDER interferometer was designed. This paper is an analysis of the switching characteristics of this Y-fed directional coupler and of its high frequency modulation properties.
The yield of particle mass in secondary organic aerosol (SOA) formed by dark ozonolysis was measured for 0.3-22.8 ppbv of reacted alpha-pinene. Most experiments were conducted using a continuous- flow chamber, allowing nearly constant SOA concentration and chemical composition for several days. For comparison, some experiments were also conducted in batch mode. Reaction conditions were 25 degrees C, 40% RH, dry (NH4) SO4 seed particles, and excess 1-butanol. The organic particle loading was independently measured by an aerosol mass spectrometer and a scanning mobility particle sizer, and the two measurements agreed well. The observations showed that SOA formation occurred for even the lowest reacted alpha-pinene concentration of 0.3 ppbv. The particle mass yield was 0.09 at 0.15 mu gm(-3), increasing to 0.27 at 40 mu gm(-3). Compared to some results reported in the literature, the yields were 80 to 100% larger for loadings above 2 mu gm(-3). At lower loadings, the yields had an offset of approximately + 0.07 from those reported in the literature. To as low as 0.15 mu g m(-3), the yield curve had no inflection point toward null yield, implying the formation of one or several products having vapor pressures below this value. These observations of increased yields, especially for low loadings, are potentially important for accurate prediction by chemical transport models of organic particle concentrations in the ambient atmosphere.
Two-year measurements of particle number size distribution (3 nm-10 mu m) were conducted in Beijing, China since March 2004. Their seasonal, weekly and diurnal variations and dependencies on meteorological parameters were investigated. The annual average particle number concentrations of the nucleation mode (3-20 nm), Aitken mode (20100 nm), and accumulation mode (0.1-1 mu m) are 9000 cm(-3), 15,900 cm(-3), and 7800 cm(-3), respectively. Particle number concentrations in Beijing are generally higher than that in cities of developed countries, especially for the accumulation mode particles. Both the highest total particle number concentration and the lowest volume concentration occurred in spring due to the frequent nucleation events. However, the minimum particle number concentration was observed in summer, and the maximum volume concentration in fall. The diurnal variation of the nucleation mode particles was mainly influenced by nucleation events, primarily in spring and winter. The diurnal variation of number concentration of the Aitken mode particles closely correlates with the traffic densities in all the four seasons. At the same time, obvious contribution of the growth of the nucleation mode to the number concentration of the Aitken mode particle has been also found in spring, Summer, and fall. Significant differences in diurnal patterns of particle number concentrations between workdays and weekends are not observed in Beijing. Local wind speed plays an important role in shaping the particle number size distributions in the urban area of Beijing. With increasing wind speed, the nucleation and coarse mode particle number concentrations increase, while the number concentrations of the Aitken mode and accumulation mode particles decrease. A "U-shape relationship" between the total particle volume concentration and wind speed is observed. Frequently low wind speed (lower than 3 m s(-1)) in Beijing is one of key factors leading to the poor air quality and low visibility. (C) 2008 Elsevier Ltd. All rights reserved.
A new instrument, a 1 x 3 tandem differential mobility analyzer (1 x 3-TDMA), was deployed in June 2007 in the Southern Great Plains, Oklahoma, USA to study the phase of ambient particles. Its primary measurement, the irreversibility of the hygroscopic growth factor, is obtained by reversibly cycling relative humidity (RH) by +/- 8% and testing for irreversible changes in diameter. In 101 runs, efflorescence occurred 72% of the time for particles sampled at ambient RH. Deliquescence occurred in 13% of the runs. The more frequent occurrence of efflorescence compared to deliquescence was explained at least in part by the distribution of ambient RH, which had a median of 80% and quartiles of 65% and 93% RH. The deliquescence and efflorescence events were nearly exclusive from one another and could be separated by Min[RH Ambient, Inlet RH] < 40% for deliquescence and Max[RH Ambient, Inlet RH] > 50% for efflorescence. In outlook, the data set from the 1 x 3-TDMA regarding the phase and hence water content of ambient particles can be used for validating regional chemical transport models of particle phase. Citation: Martin, S. T., T. Rosenoern, Q. Chen, and D. R. Collins (2008), Phase changes of ambient particles in the Southern Great Plains of Oklahoma, Geophys. Res. Lett., 35, L22801, doi: 10.1029/2008GL035650.