Number 95 (Story #3), September 22, 1992 by Phillip F. Schewe and Ben Stein TIME-RESOLVED REFLECTION HIGH ENERGY ELECTRON DIFFRACTION (RHEED) permits the detailed study of melting. Hani Elasayed-Ali and John Herman, scientists at the University of Rochester, use laser light to simultaneously heat a crystalline sample and also unleash an electron gun which shoots probing electrons at the crystal. The distinctive pattern of electrons diffracting from the crystal surface changes when the top layers begin to melt. Elasayed-Ali and Herman used RHEED to study the melting of lead crystals with a time resolution of 190 psec. They got different results for different angles of attack: looking at the so-called (110) crystal surface, their laser beam produced a sort of premelting---the formation of a liquid-like film---at a temperature some 40 K below the normal melting point (660.7 K). For the more densely packed (111) surface, the Rochester scientists found evidence for superheating; heated at that angle, the lead atoms resisted melting up to a point 120 K above the normal melting point. (Science News, 12 Sept. 1992.)