Number 127 (Story #4), May 5, 1993 by Phillip F. Schewe and Ben Stein HELIUM ATOM SCATTERING (HAS) is developing into a high-resolution technique for studying the structure and dynamical properties of crystal surfaces. One reason for this is that a beam of low-energy (20 meV, equivalent to a wavelength of 1 angstrom) helium atoms will bounce off the very surface of the sample without sticking, making detection of the scattered atoms and analysis of the sample properties relatively simple. By comparison, in low energy electron diffraction (LEED) the probing electrons (with energies of 100 eV) penetrate as far as several atomic layers into the solid, making analysis somewhat more complicated. With HAS, Peter Toennies of the Max Planck Institute for Fluid Dynamics in Gottingen, Germany has been able to work out for the first time the surface structure for a close-packed gold crystal. He discovered that the top layer of atoms is more densely packed than the bulk layers beneath. According to Toennies, HAS is also well suited to measuring vibrational modes (phonons) of the crystal surface down to energies below 30 meV, with a resolution as small as 0.08 meV. Toennies hopes to apply his techniques in making a helium microscope. (Physics World, April 1993.)