Physics Colloquium, March 30, 2004

Femtosecond electron diffraction: making ultrafast movie of atomic motions

Jim Cao

Physics Department and National High Magnetic Field Laboratory,
Florida State University, Tallahassee, FL 32310

Atomic motions on the fundamental time scale of single vibrational period (100 femtoseconds to ~1 picosecond) determine the genesis and evolution of new phases in solids, the kinetic pathways of chemical reactions, and the biological functioning processes. The study of structural dynamics on this fundamental level requires directly monitoring the atomic motions on the milli-ångström length scale and on the relevant time scale of 10^-12 s or less. Previously, only time-resolved optical measurements can provide adequate temporal resolution, albeit with indirect sensitivity of structural changes. Recent developments in time-resolved diffraction, using both pulsed X-ray and electron beams, have led to the capability of directly observing the laser-induced lost of long-range order. However, a clear picture of nuclear motions during the phase transition remains obscure. The recent development of femtosecond electron diffraction in our lab has provided, for the first time, the capability of real-time monitoring the lattice dynamics with the unprecedented 400 fs temporal resolution and structural sensitivity of less than one thousands of ångström. With this new technique, we are able to measure both coherent and thermal atomic motions, which provides a clear atomic level view of laser-induced lattice dynamics. In this talk, I will first review some background knowledge about the field of ultrafast diffraction, then present some results of our very recent studies of laser-induced structural dynamics in thin-film aluminum. The implication of these observations on the mechanism of laser-induced melting will also be discussed.

3:30 p.m., Smith Laboratory, Room 1005

Refreshments served in Smith 1094 at 3:00 p.m.