Test of a He-He potential of comparison of calculated and measured nozzle beam speed ratios

Since the weak He-He potential has a bound (E<0) or virtual bound (E>0) state with energy very close to zero the s-wave cross section shows an unusually large zero energy resonance. The resulting resonance enhancement of the viscosity cross section is responsible for the large speed ratios observed in He. Moreover the size of the low energy cross section is strongly sensitive to the energy of the bound state and for this reason calculations show the terminal speed ratio to be very sensitive to the well region of the He-He potential. Since this part of the potential is not sensitively probed in the available scattering experiments it has been suggested that nozzle beams be used to test different He-He potential models. In previous work an ellipsoidal Maxwellian velocity distribution function had to be assumed in order to use the method of moments to solve the Boltzmann equation. Experiments at high speed ratios however, indicate a significant deviation of the observed velocity distribution from Maxwellian indicating the need to carry out a numerical Monte Carlo direct simulation method (MCDSM) calculation of the expansion. In the present work a recently proposed sudden freeze model has been used, which had been calibrated against a MCDSM calculation, to estimate the speed ratios for two different potentials without having to assume ellipsoidal Maxwellian velocity distributions. (Auth.)