1. G. Brusdeylins, H: D. Meyer, J. P. Toennies, K. Winkelmann: In Rarefield Gas Dy- namics, Vol. 51, Part II, ed. by J. L. Potter, AIAA, New York (1977) p. 1047
2. R. Campargue, A. Lebéhot, J. C. Lemonnier: ibid., p. 1033
3. For a discussion of the early history of inelastic helium scattering see J.P.Toennies: In Dynamics of Gas-Surface Interaction, ed. by G.Benedek, U.Valbusa, ( Springer, Berlin, Heidelberg 1982 ), p. 208
4. G. Brusdeylins, R. B. Doak, J. P. Toennies: Phys. Rev. Lett. 46, 437 (1981)
5. G. Brusdeylins, R. B. Doak, J. P. Toennies: Phys. Rev. B 27, 3662 (1983)