Dimer formation in supersonic water vapor molecular beams

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Abstract
An experimental and theoretical study of dimer formation in supersonic water vapor molecular beams is presented. Terminal dimer mole fractions, as a function of source pressure, temperature, and orifice diameter, were measured by sampling fully expanded jets with a quadrupole mass spectrometer employing a phase‐sensitive pulse counting technique. The results are in good agreement with a termolecular formation and bimolecular destruction kinetic model for the dimer if a sudden cessation is assumed for the reverse reaction at low reduced temperatures. The model makes use of an expression developed for the equilibrium dimer mole fraction of polar molecules, and takes into account the increase in collision diameter for water molecules due to the reduced kinetic energy of relative motion at low temperatures. The source pressure at which the terminal dimer mole fraction attains a maximum is found to be proportional to D−0.630 and T3.50, where D0 is the orifice diameter and T0 is the source temperature.

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