Liquid
Experimental methods

The absence of long-range order in liquids is mirrored by the absence of Bragg peaks in X-ray and neutron diffraction. Under normal conditions, the diffraction pattern has circular symmetry, expressing the isotropy of the liquid. Radially, the diffraction intensity smoothly oscillates. This can be described by the static structure factor S (q) {\displaystyle S(q)}, with wavenumber q = (4 π / λ) sin θ {\displaystyle q=(4\pi /\lambda)\sin \theta } given by the wavelength λ {\displaystyle \lambda } of the probe (photon or neutron) and the Bragg angle θ {\displaystyle \theta }. The oscillations of S (q) {\displaystyle S(q)} express the short-range order of the liquid, i.e., the correlations between a molecule and "shells" of nearest neighbors, next-nearest neighbors, and so on.

An equivalent representation of these correlations is the radial distribution function g (r) {\displaystyle g(r)}, which is related to the Fourier transform of S (q) {\displaystyle S(q)}. It represents a spatial average of a temporal snapshot of pair correlations in the liquid.
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