This page contains all entries posted to Physics Announcements in November 2013. They are listed from oldest to newest.
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November 2013 Archives
Isoprene: A Chemical Link between the Biosphere and the Atmosphere
Glenn M. Wolfe
Air quality and climate change, two of the most pervasive environmental issues of the 21st century, are inextricably linked to our atmosphere. Near the surface, atmospheric composition is defined by the emission and transformation of a host of chemical species, including nitrogen oxides (NOx) and volatile organic compounds (VOC). While NOx is primarily a byproduct of anthropogenic activities (e.g. fossil fuel combustion), the vast majority (~90%) of global non-methane VOC emissions originate from the biosphere. Isoprene, a highly reactive hydrocarbon emitted by oak trees and other vegetation, comprises a full third of this budget. In many regions, isoprene and NOx fuel the photochemical processes responsible for production of secondary pollutants like ozone and organic aerosol, thereby impacting air quality and climate on a global scale.
Despite decades of research, our understanding of isoprene is still evolving. Emission inventories remain highly uncertain, and models struggle to reproduce observations in low-NOx, high-isoprene regions; however, much progress has been made. Recent laboratory studies have unveiled a wealth of novel reaction mechanisms, while field observations continue to challenge canonical chemistry and raise new questions. In this seminar I will highlight several such advances and demonstrate how new perspectives and measurement capabilities are helping to elucidate the details of isoprene chemistry. Completing this picture is critical for predicting and mitigating future impacts of anthropogenic activities on both the atmosphere and other Earth systems
Location: Physics Bldg., Room 401
Date: Monday, November 11, 2013
Time: 2:00 pm
Location: PHYS 401
Investigation of Atomic Layer Deposition of Metal Oxides on III-V Semiconductors
Atomic layer deposition (ALD) is becoming a leading technique for the fabrication of nanoscale materials because of its precise thickness control and unprecedented capability of film uniformity and surface conformality. Applications for such materials are diverse, ranging from Li-ion batteries and dye-sensitized solar cells to coatings on 3D structures and replacing SiO2 with high dielectric constant materials in metal oxide semiconductor field effect transistors (MOSFETs).
Our current understanding of ALD is based on ideal, one-pathway mechanisms. However, recent studies have shown we know very little about what reactions actually take place during ALD. The purpose of this research is to widen our understanding of the fundamentals of ALD by characterizing specific ALD processes such as the deposition Ta2O5 on InAs(100), post-deposition annealing effects on TiO2 thin films, the diffusion phenomenon of III-V semiconductor substrate atoms through metal oxide overlayers and explore the diffusion barrier capabilities of Al2O3 in these thin film systems.
You're invited to attend Malachi's dissertation defense.
Date: Thursday, November 14, 2013
Time: 9:45 am
Location: PHYS 401
Examining the Role of the Compton-thick, X-ray reprocessor in Type 1 Active Galactic Nuclei
Studies have indicated that black holes and their host galaxies must co-evolve, although the mechanism linking the two is not yet clear. X-ray observations of the actively accreting systems (active galactic nuclei, hereafter AGNs) over a valuable probe of conditions in the inner environs of the supermassive black hole, as X-ray production within these systems comprises a significant fraction (5%-40%) of the bolometric luminosity and originates close to the nucleus. Detailed spectroscopy in this bandpass has established that the inner environs comprise multiple X-ray absorbing zones with column densities extending into the Compton-thick regime (N_H > 10^24 atoms cm^-2). Compton-thick absorbers are known to have outflowing velocities up to 0.3c. The kinetic energy of oufltowing material with velocities > 0.1c may possibly be comparable to the gravitational binding energy of the stellar bulge and serve as a link between the black hole and the host galaxy. In addition, weak outflows may have a signicant effect on star formation in the host galaxy.
In this study, we use X-ray observations of radio-quiet AGN to examine some aspects of the flow of material between the black hole and the host galaxy. First, we model a small sample of unabsorbed Seyfert galaxies, finding their X-ray spectra to be consistent with arising as reflection from tens to hundreds of r_g in a Compton-thick, accretion disk wind of solar abundances seen face-on. Then, we explore properties of the local AGN population in the very hard band, above 10 keV, where there has been scant data available to date. We finnd a high flux for local AGN above 10 keV, and thus a very hard spectral shape over the Suzaku bandpass. Taken together, the spectral hardness and equivalent width of Fe K emission are consistent with reprocessing by an ensemble of Compton-thick clouds that partially cover the continuum source. Simple considerations place the distribution of Compton-thick clouds at or within 10^17 cm.
This work demonstrates not only that a Compton-thick wind can have a profound effect on the observed X-ray spectrum of an AGN, even when the system is not viewed through the flow, but that at least 50% of the continuum in type 1 AGNs is partially-covered by Compton-thick gas, suggesting that type 1 AGNs may not offer a completely direct view of the primary continuum, as once thought.
Date: Monday, November 25, 2013
Time: 12:30 pm
Location: PHYS 401
New measurement scheme for second-order correlation measurements of thermal light
Experiments using second order correlations have proven to be important for many applications, such as the Hanbury Brown Twiss interferometer which measures the angular size of stars, three dimensional LIDAR imaging, ghost imaging, quantum computing, and lithography. Of these experiments, only one regularly uses thermal light: the Hanbury Brown Twiss interferometer. The other experiments usually use lasers or entangled light sources, which are less than practical for many real-world applications. In addition, entangled experiments are typically performed at the single photon level and prone to decoherence. However, all of these experiments can also be performed with thermal light, such as sunlight, if one condition is met: the temporal coherence is increased allowing many measurements to be made within the coherence time. This is problematic for thermal light, as its temporal coherence is the inverse of its bandwidth, on the order of 10^-15. Our goal is to investigate the use of linear and nonlinear optics to stretch the time-scale of thermal light, reducing the bandwidth and increasing the temporal coherence without drastically reducing the intensity, allowing Hui Chen's Positive Negative Fluctuation Correlation Protocol to be used. This would result in a new method to achieve high intensity, high contrast second-order correlation measurements with broadband thermal light.
Date: Wednesday, November 27, 2013
Time: 10:00 am
Location: PHYS 401
Study of the Relative Humidity Impact on Atmospheric Aerosols by Phase Function and Polarization Measurements Using the Polarized Imaging Nephelometer PI-Neph
Atmospheric aerosols influence the Earth's radiation budget by scattering and absorbing sunlight radiation. Aerosols also modify the microphysical and radiative properties, as well as the water content and lifetime of clouds. In atmosphere conditions, aerosol particles experience hygroscopic growth due to the relative humidity (RH) influence. Wet aerosols particles are larger than their dry equivalents; therefore, they scatter more light. The quantitative knowledge of the RH effect on aerosols and its influence on the ability to scatter light is of substantial importance when comparing ground-based observations with other optical aerosol measurements techniques such satellite and photometry retrievals as well as for climate forcing calculations. This study focuses on measurements of aerosol optical properties under the effect of RH by performing different experiments where aerosols are humidified. Our next goal is to investigate the effect of water on aerosol phase function and polarized phase function using the UMBC Polarized Imaging Nephelometer (PI-Neph) designed and built by the LACO group. This study will produce the first reports of laboratory generated and ambient aerosol polarization measurements at high levels of RH.