Spin dynamics in liquid and rotationally disordered solid oxygen

The spin and nuclear dynamics of the liquid and plastic-solid phases of molecular oxygen are studied by means of inelastic neutron scattering. A detailed spectral analysis over wide energy- (0–18 meV) and momentum- $\left(0.2–4.5{\text{Å}}^{-1}\right)$ transfer ranges permits a robust separation of magnetic and nuclear contributions to the dynamic structure factor. Reliable spin-exchange constants are obtained for both liquid and plastic phases, which are found to be remarkably similar to each other, with no evidence for long-range antiferromagnetic spin correlations beyond next-nearest neighbors. Analysis of the time scales involved in spin-flip processes suggests that spin diffusion in these structurally disordered phases takes place within a lapse of time comparable to the lifetime of an ${\text{O}}_4$ unit in the liquid.