RAPID SUPPRESSION AND MODULATION OF THE DIFFRACTED BEAM IN SINGLE CRYSTAL EXCITED BY ULTRASOUND. E.M.Iolin, Institute of Physical Energetic, Riga, LV-1006, Latvia

The influence of the acoustic waves (AW) on the diffraction of X-rays and Synchrotron radiation was considered by many authors. We report the results of a theoretical analysis of the high-frequency AW influence on the intensity I(q) of a diffracted beam in a perfect crystal for the Laue case (q is the impulse along the surface of a crystal). It is known that I(q) contains many phonon satellites. Kohler, Mohling and Peibst (1974) observed the intensity I(q) decrease of the zeroth order satellite (q<<1) due to AW. Their analysis is limited by the case of moving AW when I(q) shows no time dependence. We are considering I(q) decrease of the zeroth order satellite for the case of a standing AW. A strong time dependence of the I(q) has been found in this case. For example, suppose that at the moment t=t0 the standing AW amplitude W=W0 and I(0)=0. At the next moment t=t0 +dt, W=W0+dW, |dW|<<|W0|. A coherent addition of amplitudes of scattering between the layers of the crystal will be realized. Therefore, for the case of a thick crystal (n>>1) the intensity will be rapidly increasing, I(q)~|dW| (the thickness of the crystal T=n*Ls, Ls - the wave length of the AW). For the case of a thin crystal (n=1) the intensity will be slowly increasing, I(q)~(dW)^2. I(q)~q^4 when q<<1 and t=t0. Therefore a rapid suppression of I(q) may be observed at the central part (q<1) of the main Laue diffraction peak for a very short time dt (for an example, at an f=100 MHz a suppression by a factor 1/50 - 1/100 will exist during dt=50-100 ps). The transfer-matrix method [1] is very useful for the calculation I(q) due to the space periodicity of the AW. It is not necessary to find numerically a solution of the Takagi-Taupin equations for a thick crystal, it is enough to solve TTE for a thin crystal (T=Ls) and then to find the transfer-matrix and analytically calculate I(q) for a thick crystal.

1. E.Iolin. Acta Cryst. (1995), A51, 897-902.