The scattering of nitric oxide (NO) from a clean single-crystal of silver THE (111) FACE has been studied. The rotational and fine structure distributions of NOXsq pi have been determined as a function of surface temperature and of incident kinetic energy normal to the surface plane. The analysis technique is that of laser-induced fluorescence using a Nd:YAG pumped dye laser which is both frequency doubled and Raman shifted to overlap the NO(gamma) band system in the UV. It is found that the energy transferred into the NO rotational degree of freedom in a single gas-surface encounter is expressed by where E sub r is the mean rotational energy, E sub n the normal component of the NO kinetic energy, E sub s the surface temperature, E wub w a parameter characterizing the NO/Ag(111) well depth, and a and b are coefficients that express the fraction of kinetic and surface energy appearing as rotational excitation.
Vols. for 1977- consist of two parts: Chemistry, biological sciences, engineering sciences, metallurgy and materials science (issued in the spring); and Physics, electronics, mathematics, geosciences (issued in the fall).
We have also measured the kinetics and products of a reaction between gas-phase ozone and anthracene adsorbed at the air-aqueous interface. The reactions at the "uncoated" air-water interface and at an interface consisting of a monolayer of various organic compounds were studied. In all the cases, the reaction follows a Langmuir-Hinshelwood mechanism, in which ozone first adsorbs to the air-aqueous interface, and then reacts with adsorbed anthracene. For typical atmospheric ozone concentrations, the estimated reactive uptake coefficient ranges from 2 x 10-8 to 3 x 10-7 depending on the nature of the air-aqueous interface. Smaller (C4, C6) carboxylic acids at the interface inhibit the reaction (compared to the "clean" water surface); 1-octanol enhances it. Under some circumstances, oxidation by ozone on aqueous surfaces may be more important in the atmosphere than gas phase oxidation by OH radicals. The fate of atmospheric semi-volatile organic compounds (SOCs) such as polycyclic aromatic hydrocarbons (PAHs) depends, in part, on their distribution between the gas phase and particulate phases, and oxidation reactions. In the daytime troposphere, photochemically produced OH radicals dominate these reactions, whereas chlorine atoms may contribute to the oxidation of certain PAHs in coastal areas. In the dark, oxidation by nitrate radicals (NO3) may also be important, whereas ozone can contribute to the oxidation both during the day and at night. We have developed a laser-induced fluorescence (LIF) method to study adsorption and oxidation of PAHs at the air-aqueous interface. This method allows to directly probe the mechanisms and extent of PAH uptake on aqueous surfaces. The kinetics of the adsorption of anthracene and pyrene onto "pure" water and water coated with an organic film were measured. The surface uptake coefficients of both PAHs are estimated to be on the order of 10-5, and increase by a factor of 2--3 for uptake to the 1-octanol coated water surface. However, the surface uptake of pyrene to a hexanoic-acid-coated aqueous surface does not display this enhancement. Resolved fluorescence spectra of pyrene adsorbed onto 1-octanol-coated surfaces indicate that pyrene is in a less polar environment than when adsorbed at the hexanoic-acid-coated surfaces.
In a rarefied gas, heat transfer in the transition regime (i.e., Knudsen number range of 0.1 to 10) is affected by molecular as well as gas-surface interactions. Theoretical results for the heat transfer can be obtained through computational solutions of the Boltzmann equation and appropriate intermolecular and gas-surface interaction models. As with all computational and phenomenological models, experimental verification is required. In this work, a low-pressure laser induced fluorescence (LIF) technique is investigated for measuring temperature profiles between parallel flat plates, and the results are compared to theoretical predictions. Iodine vapor is used as the gas medium due to its attractive spectral properties. Two surfaces of a closed, short cylindrical iodine cell are employed as a parallel flat plate geometry and are maintained at different temperatures. Cold and hot plate temperature combinations of (1) 20°C and 70°C, respectively, and (2) 20°C and 115°C, respectively, were used. Calibration measurements were performed at uniform temperatures of 45°C and 70°C, Solid iodine stem temperatures of 0°C and -20°C, corresponding to pressures of 30 mtorr and 3 mtorr, respectively, and Knudsen numbers of 0.05 and 0.5, respectively, were investigated. The data shows the expected trends, indicating observable temperature jumps at the surfaces and matching well with the theoretical predictions in the bulk gas. Deviations from the theory were largest near the surface, possibly a result of the limitations of the theoretical models for this particular experimental case. In addition, problems stemming from fluctuations of the chamber pressure as well as attenuation of the excitation laser beam affected collection of definitive temperature profile measurements. However, much progress with the low-pressure LIF technique has been made, and the technique continues to look promising for obtaining accurate and reliable temperature profile measurements in the transition regime.
