Investigation of Metal and Metal Oxide Clusters Small Enough to Constitute the Critical Size for Gas Phase Nucleation in Combustion Processes

摘要

Over the course of this contract a variety of techniques have been employed to study the properties of small atomic and molecular clusters formed in the gas phase via homogeneous nucleation. The clustering occurs either in an adiabatic expansion of a condensable species (e.g. argon, krypton, xenon, or sulfur hexafluoride) in an inert carrier gas (e.g. helium), or as a mixing process using a hot condensable (e.g. lead, silver, copper, indium or bismuth) and a cold carrier gas (e.g. argon, helium, carbon dioxide or sulfur hexafluoride). A continuous development several types of cluster sources has been carried out and includes free jets, very small hypersonic laval nozzles, and a series of metal ovens with carrier gas mixing. Any one, of these sources, constitutes the first stage of a differentially pumped, molecular beam system which the produces a continuous beam of clusters. The denisty in the beam is so low that it is collisionless and thus the clusters do not interact with each other or with any other foreign molecule or surface. The study of these isolated clusters is carried out primarily using high energy electron beams (40 to 75 KeV). The resulting diffraction patterns are obtained either on film or through use of a single channel, scintillation, pulse counting system employing synchronous detection.