Alan B. Harker

The effect of atmospheric SO2 photochemistry upon observed nitrate concentrations in aerosols

In this study, X-ray photoelectron spectroscopy was used to observe the relationship between the nitrate and sulfate content of photochemically formed aerosol particles which were produced by exposing initially particle-free ambient air in a 14 m3 transparent Teflon chamber to sunlight with various amounts of NO2, SO2, and C3H6 added to the reaction mixture. It was observed that the photochemical oxidation of SO2 to SO42− decreased the amount of nitrates present in the aerosol significantly below that observed in the absence of SO2. It was also found that nitrate could be removed from an aerosol sample on a glass fiber filter by passing air with sulfate-containing aerosols through the filter. These observations indicate that H2SO4 formed from the photochemical oxidation of SO2 in the reaction chamber released NO3− from the surface of the aerosol particles, probably in the form of HNO3. An examination of analyses of ambient aerosols from the Los Angeles area showed a number of cases where a similar inverse relationship between particulate sulfate and nitrate content existed.

Fabrication and characterization of diamond moth eye antireflective surfaces on Ge

An antireflective coating on Ge which combines a polycrystalline diamond film with a surface relief (moth eye) structure has been designed and successfully fabricated. This progressive gradation in the effective refractive index between air and the substrate has reduced Fresnel reflection losses to below 1%. This provides a means of overcoming the high refractive index and surface roughness considerations that often limit optical application of polycrystalline diamond thin films.

Reactive ion beam deposition of thin films in the bismuth‐calcium‐strontium‐copper oxide ceramic superconductor system

Reactive ion beam deposition has been used to grow c‐axis‐oriented superconducting thin films in the Bi‐Ca‐Sr‐Cu‐O (BCSCO) system around the cation ratio 1:1:1:1 on single‐crystal (001) MgO. The films show a single superconducting transition with an initial onset near 85 K and a critical current of 5×104 A/cm2 at 10 K. Two different BCSCO‐containing phases have been identified in the thin films: one with a tetragonal pseudo‐body‐centered subcell, c=24.4 A, which is not superconducting above 28 K, and a second with c=30.6 A, which is responsible for the superconductivity. Electron diffraction measurements on the 30.6 A phase are consistent with those previously reported for the bulk ceramic.

Photodissociation quantum yield of NO2 in the region 375 to 420 nm

The absolute absorption cross section and photodissociation quantum yield of NO2 were determined over the wavelength region 375 to 420 nm at a temperature of 296 K. The quantum yield measurements were made at 1 nm intervals over this frequency range with an accuracy of ± 7%. The data show a significant decrease in the quantum yield between 380 and 390 nm indicating the possibility of a non-dissociating NO2 excited state in this region.

A kinetic study of the mercury sensitized luminescence of H2O and NH3

The transient species formed from the reactions of H2O and NH3 with excited Hg(6 3P0) atoms were studied by phase‐sensitive modulation spectroscopy. In the Hg/NH3 system, the (Hg⋅NH3) * exciplex was observed in emission, and its lifetime was found to be 1.7±0.1×10−6 sec; the third body stabilized reaction of NH3 with Hg(6 3P0) was found to proceed with a rate constant of k5b=2.2±0.2×10−31 (cm6/molecule2 sec) for M=N2. In the Hg/H2O system the (Hg⋅H2O) * exciplex was observed in emission and its lifetime found to be ? 3.3×10−8 sec; and the quenching of Hg(6 3P0) by water was found to have a rate constant of k4a + k4b[M] =9.4±0.5×10−15 (cm3/molecule sec) when M=1 atmosphere of N2. In the Hg/H2O system the OH radical was observed both in emission and absorption, and HgH was observed in absorption. Both of these species were shown to be formed by the nonradiative dissociation of the (Hg⋅H2O) * exciplex.

Microstructure and orientation effects in diamond thin films

The microstructure and orientation of diamond thin films grown by plasma assisted chemical vapor deposition have been studied as functions of growth temperature, substrate identity, and substrate pre‐treatment. Results indicate that for growth temperatures below 650 °C, competition between film growth and etching can lead to preferential (110) oriented films on a variety of substrate materials. This orientation can be globally sustained during growth by the occurrence of (111) planar defects.

The oriented growth of anatase in thin films of amorphous titania

The growth of an oriented anatase grain structure from the post-deposition annealing of amorphous reactive ion-beam-deposited thin films of titanium oxide has been demonstrated. The transformation has been observed directly by electron microscopy and the crystallization of the films occurs only when their thicknesses are greater than about 50 nm. Substrate effects were not found to be significant to the crystallization, which could also be induced by annealing freestanding films. The transformation processes can be described in terms of existing theories of crystal growth where the amorphous film is described as a supercooled liquid.

Measurements of the photochemical production of aerosols in ambient air near a freeway for a range of SO2 concentrations.

Abstract The formation and growth of photochemically produced aerosol particles were studied for initially particle-free ambient air collected downwind from a freeway in two 14 m3 transparent Teflon bags exposed to natural sunlight for SO2 concentrations ranging from 0.006 to 0.067 ppm, by volume. Using a Thermo Systems Electrical Aerosol Analyzer, the concentration and size distribution of aerosol particles from 0.0056 to 0.36 μm dia. was measured as a function of time. The volumetric conversion rate of material from the vapor to the condensed phase was found to be a linear function of the initial SO2 concentration and directly proportional to the initial non-methane hydrocarbon concentration in the bags. Filter samples of the photochemically produced aerosol particles were analyzed for sulfate, nitrate and nitrite. The sulfate and nitrate formation rates ranged from 0.72 to 2.6 μg m−3 h−1 and 0.76 to 3.2 μg m−3 h−1, respectively, while the nitrite formation rate ranged from 0.008 to 0.038 μg m−3 h−1. The overall rate of SO2 to sulfate conversion was estimated to range from 0.55 to 1.3% h−1. X-ray photoelectron spectroscopy was used to determine the chemical states of sulfur, nitrogen and carbon on the surface of the filter samples. Sulfur was found in the form of SO2−4, as well as chemisorbed SO2 and SO3. Nitrogen was present as NO−3 and as two reduced forms identified as amines or amides, and carbon was present as either elemental carbon or as paraffinic hydrocarbon.

MICROSTRUCTURAL CONTROL OF DIAMOND THIN FILMS BY MICROLITHOGRAPHIC PATTERNING

Microlithographic patterning has been used to elucidate the mechanisms controlling diamond film nucleation and grain growth. The approach is capable of establishing a degree of control over diamond nucleation on the substrate, which can be used to improve film uniformity and enhance fine grained microstructure. The observed microstructures in the patterned films are consistent with an intrinsic growth mechanism based upon defect‐initiated renucleation.

Planar weak-link devices from YBaCuO and BiSrCaCuO films

Weak‐link devices were fabricated from polycrystalline thin films of YBaCuO and BiSrCaCuO. Two distinct types of weak links have been found in these devices. The first type, found in all of the YBaCuO and most of the BiSrCaCuO devices, has relatively low dynamic resistances and critical currents with nearly linear temperature dependences. The second type of weak links, found in some BiSrCaCuO films, has much larger dynamic resistances and critical current temperature dependences similar to that of a tunnel junction. Superconductivity quantum interference devices (SQUID’s) fabricated from BiSrCaCuO showed a periodic response to applied fields that is consistent with the magnitude of the flux enclosed by the SQUID loop. Significant hysteresis was found in the SQUID response, apparently indicating flux penetration into the films at relatively low fields.