FACTORS GOVERNING THE PHASE OF CLUSTERS NUCLEATED IN SUPERSONIC-FLOW - CRYSTALLINE ACETYLENE

The size and solid-state structure of molecular clusters of acetylene generated in supersonic expansions with a carrier gas depend upon conditions in the flow. Available evidence suggests that, in their initial stages of growth, clusters are above the bulk transition temperature (133 K, from cubic to orthorhombic) but that they are appreciably below it when they are probed by electron diffraction after they exit the Laval nozzle. Conditions favoring the production of the higher temperature cubic form are high subject mole fraction, low initial pressure, and helium carrier. Increasing the pressure and carrier molecular weight and decreasing the concentration of seeded vapor raised the proportion of orthorhombic phase observed. Clusters were approximately 10 nm in diameter under typical expansion conditions. Contrary to conventional expectations, clusters grown in helium were somewhat larger than those grown in neon. As found earlier by X. Shi for another system, mixed cluster beams are inhomogeneous with a central core rich in clusters of the colder phase surrounded by a diffuse fringe of clusters of the warmer phase. Gas-dynamic and kinetic factors responsible for the observed response to changes in flow conditions are discussed. © 1990 American Chemical Society.