The Diffraction of X-Rays by Liquid Argon

The x-ray diffraction pattern of liquid argon has been obtained with crystal-reflected $\mathrm{Mo}K\alpha$ radiation. Argon gas under a gauge pressure of 50 lb. per sq. in. was cooled to 90°K in a thin-walled Pyrex glass capillary of 1.7 mm diameter and under these conditions, the argon became liquid. An evacuated camera of 9.53 cm radius was so constructed that the film could be placed outside the vacuum, with entrance and exit windows of aluminum. Exposures of 140 hours were required to give patterns of satisfactory density. Microphotometer records of the films showed three peaks; a very strong peak at $\frac{sin\theta }{\lambda }=0.154\mathrm{}$, a medium peak at $\frac{sin\theta }{\lambda }=0.280\mathrm{}$ and a weak peak at $\frac{sin\theta }{\lambda }=0.415\mathrm{}$. No convincing evidence of another peak could be found beyond this and up to $\frac{sin\theta }{\lambda }=1.20\mathrm{}$. A Fourier analysis of our diffraction pattern results in an atomic distribution curve [$4\pi r^2\rho \mathrm{}\left(r\right)\mathrm{} \mathrm{vs}.\mathrm{} r$] showing approximately 7 atoms at 3.90A and 5 atoms at about 5.05A. A third broad peak occurs at over 7A. This indicates that the distribution of atoms in liquid argon does not follow closely that in crystalline argon, since in the crystal there are 12 nearest neighbors.