Measurements of relative concentrations and velocities of small clusters (n⩽40) in expanding water vapor flows

The concentrations of water clusters $\text{(2⩽n⩽40)}$ relative to the monomer concentration in expanding water vapor flows have been determined over a range of source pressures and temperatures by mass spectrometry. The corresponding directed flow velocities and thermal velocity distributions of the monomers and clusters of each size have been determined by mass‐filtered time‐of‐flight (velocity) spectrometry. Relative concentration generally decreases exponentially with n. Translational thermal equilibrium $\text{∼65\hspace{0.17em}}\mathrm{K}$ prevails for the monomers and clusters of $\text{n≲5}$ under all source conditions. Translational temperatures of larger clusters generally increase with increasing n. The results suggest a kinetic process in which surviving smaller clusters are formed in three‐body collisions and larger ones in two‐body collisions.