Manuel J. Manard

Chapter 4 Aun and Agn (n=1–8) nanocluster catalysts: gas-phase reactivity to deposited structures

Publisher Summary This chapter helps in understanding the size-dependent chemistry of metal clusters on two fronts: by probing the reactivity of mass-selected Au n + and Ag + n nanoclusters in the gas phase, and by studying the properties of mass-selected Au + n and Ag + n nanoclusters deposited on TiO 2 surfaces under ultrahigh vacuum (UHV) conditions. Detailed results on gas-phase clusters provide important thermodynamic information, both as data for testing theoretical models and to establish structural, energetic, and reactive properties for size-selected clusters. Depositing size selected clusters provides a platform for studying model nanocluster catalysts with a well-defined size. Surface structures, binding sites, and binding energies are determined by combining atomic-resolution scanning tunneling microscopy (STM) and density functional theory (DFT). In the event that a cluster should land in the immediate vicinity of a vacancy, interaction with a vacancy can occur, resulting in the appearance of a cluster over a bridging O atom row or between 5c-Ti atom and bridging O atom rows. Interestingly, Au7 are the first truly 3-D structures (according to DFT and supported by our STM data), often containing highly uncoordinated peak atoms, perhaps responsible for their relatively high catalytic activity in comparison to other cluster sizes.

Determining x-ray spectra of radiographic sources with a Compton spectrometer

Flash radiography is a diagnostic with many physics applications, and the characterization of the energy spectra of such sources is of interest. A Compton spectrometer has been proposed to conduct these measurements. Our Compton spectrometer is a 300 kg neodymium-iron magnet constructed by Morgan et al1, and it is designed to measure spectra in the <1 MeV to 20 MeV range. In this device, the x-rays from a radiographic source are collimated into a narrow beam directed on a converter foil. The forward-selected Compton electrons that are ejected from the foil enter the magnetic field region of the spectrometer. The electrons are imaged on a focal plane, with their position determined as a function of their energy. The x-ray spectrum is then reconstructed. Challenges in obtaining these measurements include limited dose of x-rays and the short pulse duration (about 50 ns) for time-resolved measurements. Here we present energy calibration measurements of the spectrometer using a negative ion source. The resolution of the spectrometer was measured in previous calibration experiments to be the greater of 1% or 0.1 MeV/c1. The reconstruction of spectra from a bremsstrahlung source and Co-60 source are also presented.