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Use of Similarity Profiles in Studying Cluster Formation in Molecular Beams: Evidence for the Role of Pre-Existing Dimers

机译:利用相似谱研究分子束中的团簇形成:预先存在的二聚体作用的证据

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At the present time there is widespread interest in the formation and properties of small aggregates of molecules 1,2. These systems constitute a form of matter having properties between those of an isolated gas-phase molecule and the bulk condensed state. Study of their formation and properties is an attractive method of following the continuous course of change of matter from the gaseous to the condensed phase, elucidating nucleation and solvation phenomena, and shedding light on certain problems in the field of surface science. Studies of the influence of source temperatures and pressures on the distributions of water, ammonia, and sulfur dioxide clusters are reported. The experiments reveal that nearly identical cluster distributions occur in cases where the pressure of dimer is maintained constant according to a simple euqation involving stagnation temperatre and pressure. In similar experiments covering the same range of pressures and temperatures, widely differing cluster distributions are obtained under conditions where the dimer concentration is not fixed. Our results suggest that large clusters proceed largely from pre-existing dimers, and that very few new ones are created early enough in the expansion to effect cluster growth. The gas-phase heat of dimerization of sulfur dioxide is determined to be 4.3 + or - 0.3 kcal/mole.

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