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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 Ϫ2 K. In the same temperature range the light He-cluster size is analyzed. ͓S1050-2947͑99͒01004-5͔

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Scattering of thermal He atoms from a low-temperature graphite (0001) surface was carried out. The measured diffraction pattern shows that the repulsive part of the periodic gas-surface potential is rather well represented by the first Fourier component. The "hard wall" peak-to-peak spatial modulation is 0.21 A. Bound-state resonances allow one to determine four distinct energy levels of the laterally averaged potential well. A comparison with available adsorption data is presented.

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