Infrared perfect absorber based on nanowire metamaterial cavities

An infrared perfect absorber based on gold nanowire metamaterial cavities array on a gold ground plane is designed. The metamaterial made of gold nanowires embedded in alumina host exhibits an effective permittivity with strong anisotropy, which supports cavity resonant modes of both electric dipole and magnetic dipole. The impedance of the cavity modes matches the incident plane wave in free space, leading to nearly perfect light absorption. The incident optical energy is efficiently converted into heat so that the local temperature of the absorber will increase. Simulation results show that the designed metamaterial absorber is polarization-insensitive and nearly omnidirectional for the incident angle.Comment: 3 pages, 4 figure

Laboratory Studies of Gas-Grain Processes on Cosmic Dust Analogues

The formation of molecules in the interstellar medium (ISM) takes place both in the gas phase and on surfaces of cosmic dust grains. Gas phase reactions alone are found to be insufficient to account for the observed abundance of molecules such as molecular hydrogen, water, carbon dioxide, methanol as well as many other complex molecules; grain surfaces must be involved as catalysts to explain their formation. In this thesis we study the physical and chemical processes on surfaces of cosmic dust grain analogues in simulated ISM environments. Oxygen is the third most abundant element in the universe and is present in many astrobiologically important molecules. The desorption energy of atomic oxygen is a fundamental parameter that enters ISM models because it controls the residence time of this atom on a surface. However, it has not been measured in the laboratory. In this thesis, this parameter is measured by using both an indirect and a direct method. The measured value agrees with model predictions based on astronomical observations. The formation of two oxygen-containing molecules, water and hydroxylamine, is studied next. Water is the main component of ice mantles in dense clouds and is indispensable for the origin of life. Its formation via ozone hydrogenation on an analog of a warm dust grain is studied experimentally. The desorption energy of an important intermediate product in the reaction, the OH radical, is also inferred. Hydroxylamine is a precursor to the formation of glycine, which is the simplest amino acid. The formation of hydroxylamine via the oxidation of ammonia is studied by sequential deposition of ammonia and atomic oxygen and is followed by temperature programmed desorption experiments. The measured high reaction efficiency predicts that ammonia oxidation on grain surfaces could be an important route to hydroxylamine formation. The last chapter of this thesis introduces a rate equation model to simulate surface kinetics, including diffusion and desorption, of atoms and molecules on non-uniform surfaces

The 12^{12}CO2_2 and 13^{13}CO2_2 Absorption Bands as Tracers of the Thermal History of Interstellar Icy Grain Mantles

Analyses of infrared signatures of CO$_2$ in water dominated ices in the ISM can give information on the physical state of CO$_2$ in icy grains and on the thermal history of the ices themselves. In many sources, CO$_2$ was found in the `pure' crystalline form, as signatured by the splitting in the bending mode absorption profile. To a large extent, pure CO$_2$ is likely to have formed from segregation of CO$_2$ from a CO$_2$:H$_2$O mixture during thermal processing. Previous laboratory studies quantified the temperature dependence of segregation, but no systematic measurement of the concentration dependence of segregation is available. In this study, we measured both the temperature dependence and concentration dependence of CO$_2$ segregation in CO$_2$:H$_2$O mixtures, and found that no pure crystalline CO$_2$ forms if the CO$_2$:H$_2$O ratio is less than 23%. Therefore the segregation of CO$_2$ is not always a good thermal tracer of the ice mantle. We found that the position and width of the broad component of the asymmetric stretching vibrational mode of $^{13}$CO$_2$ change linearly with the temperature of CO$_2$:H$_2$O mixtures, but are insensitive to the concentration of CO$_2$. We recommend using this mode, which will be observable towards low mass protostellar envelopes and dense clouds with the James Webb Space Telescope, to trace the thermal history of the ice mantle, especially when segregated CO$_2$ is unavailable. We used the laboratory measured $^{13}$CO$_2$ profile to analyze the ISO-SWS observations of ice mantles towards Young Stellar Objects, and the astrophysical implications are discussed.Comment: 11 pages, 12 figures, ApJ accepte

Formation of hydroxylamine on dust grains via ammonia oxidation

The quest to detect prebiotic molecules in space, notably amino acids, requires an understanding of the chemistry involving nitrogen atoms. Hydroxylamine (NH$_2$OH) is considered a precursor to the amino acid glycine. Although not yet detected, NH$_2$OH is considered a likely target of detection with ALMA. We report on an experimental investigation of the formation of hydroxylamine on an amorphous silicate surface via the oxidation of ammonia. The experimental data are then fed into a simulation of the formation of NH$_2$OH in dense cloud conditions. On ices at 14 K and with a modest activation energy barrier, NH$_2$OH is found to be formed with an abundance that never falls below a factor 10 with respect to NH$_3$. Suggestions of conditions for future observations are provided.Comment: 9 pages, 9 figure