EUROPHYSICS LETTERS 15 November 1997 Europhys. Lett., 40 (4), pp. 369-374 (1997) Towards a new (Q, t) regime by time-resolved X-ray diffraction: Ultra-sound excited crystals as an example K. D. Liss 1 ( ∗ ), A. Magerl 2 , R. Hock 3 , A. Remhof 4 and B. Waibel 2,4 1 European Synchrotron Radiation Facility - F-38043 Grenoble Cedex, France 2 Institut Max von Laue Paul Langevin - F-38042 Grenoble Cedex, France 3 Institut f¨ ur Mineralogie der Universit¨ at W¨ urzburg Am Hubland, D-97074 W¨ urzburg, Germany 4 Institut f¨ ur Experimentalphysik 4, Ruhr Universit¨ at Bochum D-44780 Bochum, Germany (received 28 July 1997; accepted 2 October 1997) PACS. 07.85Qe – Synchrotron radiation instrumentation. PACS. 61.20Lc – Time-dependent properties; relaxation. PACS. 43.35+d – Ultrasonics, quantum acoustics, and physical effects of sound. Abstract. – Time-resolved diffraction patterns down to a sub-nanosecond scale obtained with a high-resolution diffractometer for high-energies X-rays on ultrasonically excited samples are presented. The time resolution gives direct insight into the purity of the excited sound waves and reveals the density of states for the lattice parameter at any point of time in a MHz oscillation period. The combination of the time resolution with the momentum transfer Q accessible by the high-energy diffractometer gives access to a unique regime in (Q, t) space. Diffraction in perfect single crystals is described by the theory of dynamical diffraction. This approach considers the complete wave pattern propagating in the periodic potential of an ideal crystal, and a rich variety of phenomena like Pendell¨ osung oscillations, rapid intensity variations within the Borrmann fan, or anomalous transmission of X-rays is explained. All these features are described quantitatively from the interference patterns of the wave fields in the crystal and the appropriate boundary conditions at the crystal surface. One of the most important effects of dynamical diffraction is the strongly limited intensity diffracted by an ideal crystal in Bragg position. The interference effects from the wave fields disappear very quickly if the perfect translational symmetry in a crystal is violated due to any disturbance. Simultaneously, the Bragg-reflected intensity increases rapidly in this case. A pure mode ultrasonic wave is a simple and well-defined disturbance of the crystal lattice and allows to study the transition in diffraction from a perfect crystal reflecting a weak intensity towards an imperfect crystal diffracting an enlarged wavelength band. The degree of crystal imperfection is easily controlled by the sound wave amplitude. On this background several studies have ( * ) E-Mail: liss@esrf.fr c  Les Editions de Physique