The possibility and limits of superheating solids are studied. A superheated solid is one in which long-range order remains present at temperatures higher than the bulk melting point, Tm. Time-resolved transmission electron diffraction (TED), with up to 1 ps temporal resolution, will be used to probe the structure of laser-heated thin films. The laser provides an ultrafast heating pulse, while ultrafast electron diffraction probes the structural integrity of the crystal to detect any metastable superheated state. While supercooling of the molten state is a common phenomenon, observations of superheating of solids are rare. Surfaces and extended defects, such as grain boundaries, provide vast nucleation sites for disorder below Tm and, thus, inhibit superheating of solids. Some single-crystal surfaces, however, remain ordered up to Tm allowing a surface to be superheated. The present work focuses on transmission electron diffraction studies to probe up to several hundred A thin films, rather than the first few atomic layers probed in reflection high-energy electron diffraction (RHEED) used in previous work. Time-resolved TED (1-ps or less) will be conducted on single-crystal and polycrystalline thin metal and semiconductor films and on microcrystallites bounded by close-packed surfaces. The structural integrity, atomic mean vibrational amplitude, and lattice spacing of thin films subjected to heating and melting with laser pulses of 80-fs and 100-ps pulse durations will be probed.. Graduate and undergraduate students will participate in this research. %%% Melting is one of the most common phase transformation in nature. While our understanding of melting from a thermodynamic point of view is well established, microscopic understanding of this phase transformation is lacking. Supercooling of the molten state is a common phenomenon; however, observations of superheating of solids are rare. A supercooled liquid is one that remains in a liquid state for some time at temperature below the bulk melting point. A superheated solid is one in which the structural integrity remains present at temperatures higher than the bulk melting point. Melting is initiated at surfaces and defects and propagates into the bulk of the crystal. The main objective of this study is to investigate the possibility and limits of superheating solids, thus, providing an answer to the question of how far and for how long can a solid exists above its melting point. Because of the fast nature of melting, this study requires the use of a fast heating source and a probe capable of monitoring the atomic arrangement during melting. We use laser pulses of ultra-short duration, one trillionth of one second or less, as a fast heating source. Diffraction of a subsequent electron pulse in the material allows for monitoring the structure of the heated material. These high time resolution studies will be conducted on single-crystal and polycrystalline thin metal and semiconductor films and on microcrystallites. Graduate and undergraduate students will participate in this research. They will receive training that prepares them for entry into the industrial, government, or academic job market during the coming decades of the century. ***
