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The formation of dimers and trimers in free jet 4He cryogenic expansions

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Schöllkopf,  W.
Emeritus Group Molecular Interactions, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society;

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Toennies,  J. P.
Emeritus Group Molecular Interactions, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society;

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Bruch, L. W., Schöllkopf, W., & Toennies, J. P. (2002). The formation of dimers and trimers in free jet 4He cryogenic expansions. Journal of Chemical Physics, 117(4), 1544-1566.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0029-176B-9
Abstract
The formation of dimers, trimers, and tetramers in a free jet cryogenic expansion of 4He atoms has been studied by diffraction from a nanostructure transmission grating. The final average velocities, speed ratios and ambient temperatures of the expansions for source temperatures of 30, 12, and 6 K and source pressures between 0.1 and 80 bar were determined from time-of-flight measurements of the He atoms. The final mole fractions of the He2, He3, and He4 clusters in the beam were determined from the intensities of the corresponding first-order diffraction peaks for the same range of source conditions. For each source temperature, the final mole fractions of these small clusters first rise, pass through a maximum and then decrease with increasing source pressure. The processes leading to the formation of these clusters are simulated with a kinetic model that allows for density and temperature changes in the expanding beam. The best-fit three- body recombination rate constant for dimer formation increases by over three orders of magnitude as the thermal energy decreases from 1 K to 1 mK, in qualitative agreement with recent theories. (C) 2002 American Institute of Physics.