Picosecond-milliangstrom lattice dynamics measured by ultrafast X-ray diffraction

Fundamental processes on the molecular level, such as vibrations and rotations in single molecules, liquids or crystal lattices and the breaking and formation of chemical bonds, occur on timescales of femtoseconds to picoseconds. The electronic changes associated with such processes can be monitored in a time-resolved manner by ultrafast optical spectroscopic techniques(1), but the accompanying structural rearrangements have proved more difficult to observe. Time-resolved X-ray diffraction has the potential to probe fast, atomic-scale motions(2-5). This is made possible by the generation of ultrashort X-ray pulses(6-10), and several X-ray studies of fast dynamics have been reported(6-8,11-15) Here we report the direct observation of coherent acoustic phonon propagation in crystalline gallium arsenide using a non-thermal, ultrafast-laser-driven plasma-a high-brightness, laboratory-scale source of subpicosecond X-ray pulses(16-19). We are able to follow a 100-ps coherent acoustic pulse, generated through optical excitation of the crystal surface, as it propagates through the X-ray penetration depth. The time-resolved diffraction data are in excellent agreement with theoretical predictions for coherent phonon excitation(20) in solids, demonstrating that it is possible to obtain quantitative information on atomic motions in bulk media during picosecond-scale lattice dynamics.