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

Fundamental processes on the molecular level, such as vibrations and rotati ons in single molecules, liquids or crystal lattices and the breaking and f ormation of chemical bonds, occur on timescales of femtoseconds to picoseco nds. 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 fas t, atomic-scale motions(2-5). This is made possible by the generation of ul trashort X-ray pulses(6-10), and several X-ray studies of fast dynamics hav e been reported(6-8,11-15) Here we report the direct observation of coheren t acoustic phonon propagation in crystalline gallium arsenide using a non-t hermal, ultrafast-laser-driven plasma-a high-brightness, laboratory-scale s ource of subpicosecond X-ray pulses(16-19). We are able to follow a 100-ps coherent acoustic pulse, generated through optical excitation of the crysta l surface, as it propagates through the X-ray penetration depth. The time-r esolved diffraction data are in excellent agreement with theoretical predic tions for coherent phonon excitation(20) in solids, demonstrating that it i s possible to obtain quantitative information on atomic motions in bulk med ia during picosecond-scale lattice dynamics.