The paper reports on calculations of the flow properties in low-temperature He free-jet expansions by means of the Lennard-Jones potential and the recently appearing He-He van der Waals potential based on perturbation theory. The calculated speed ratios are compared with both the values we measured in the 20–80 K source temperature range and data which were already reported. The comparison points out a better agreement of our experimental results with the speed ratio values obtained by the recent potential whenever the parallel temperature of the beam decreases below ${10}^{-2}\hspace{0.5em}\mathrm{K}.$ In the same temperature range the light He-cluster size is analyzed.

• Received 17 July 1998

DOI:https://doi.org/10.1103/PhysRevA.59.3084