Diffuse Scattering of X-rays from Sylvine at Low Temperature

By use of a modification of the photographic method described by Claus the authors have compared the intensities of x-rays of wave-length 0.43A diffusely scattered from sylvine at angles in the range 25° to 90° at a temperature of 90°K with the intensities at these same angles at a temperature of 300°K. Jauncey and Harvey have shown that the intensity of the diffusely scattered rays should be given by $S=S^{\prime }$ $\frac{-F^2}{Z}$, where $S^{\prime }$ is independent of the temperature and $F$ is the atomic structure factor containing the effect of thermal agitation. With James and Brindley's $F$ values at 90°K and 300°K theoretical values of $S$ for 90°K have been calculated. The experimental $S$ values at 90°K are lower than the theoretical $S$ values. This result is in accord with that found by Claus for rocksalt. Plotting $log \frac{(S^{\prime }-S_1)}{(S^{\prime }-S_2)}$, where the subscripts refer to the two temperatures, against $\frac{\left(\frac{{sin}^2\phi }{2}\right)}{{\lambda }^2}$ a straight line is obtained whose slope agrees with that required by the Waller and not the Debye formula for the temperature effect. It is impossible by means of this experiment to show whether or not there is zero point energy. In order to do this an assumption concerning the electron distribution in the atom must be made.