Study of He flow properties to test He dimer potentials

Abstract

The parameters which characterize the energy distribution of a supersonic helium beam are measured at different source parameters. The data are compared with the results of calculations based on three different He–He interatomic potentials in order to test their ability to describe the helium free jet expansion. This article follows a previous paper where calculations were performed at source temperatures between 20 and 80 K using the the Lennard-Jones and the Tang–Toennies–Yiu potential [K. T. Tang, J. P. Toennies, and C. L. Yiu, Phys. Rev. Lett. 74, 1546 (1995)] but no satisfactory agreement was found. Here, calculations are presented also for a potential curve recently proposed by Hurly and Moldover [J. J. Hurly and M. R. Moldover, J. Res. Natl. Inst. Stand. Technol. 105, 667 (2000)]. The source temperature range where calculations are performed is extended between 6 and 300 K in order to compare the predictions of the three potentials with other measurements present in literature. Possible experimental limitations to the beam performances are discussed and in particular the skimmer interference is taken into account. As the considered potentials only partially describe the experiments, a phenomenological viscosity cross section is proposed which represents in a satisfactory way the He flow properties over the whole range of source temperatures.