Diffusive Motions in Water and Cold Neutron Scattering

Using a model of liquid water in which a molecule, in its equilibrium position, performs an oscillatory motion for a mean time ${\tau }_0$, and then diffuses by continuous motion for a mean time ${\tau }_1$, and repeats this sort of motion, the differential scattering cross section for cold neutrons has been calculated. It is found that the shape of the "quasi-elastic" scattering is, in general, not Lorentzian. The formula for the broadening of the quasi-elastic peak assumes a simple form in two limiting cases: In case (i) ${\tau }_1\gg {\tau }_0$, it reduces to the formula derived on the simple diffusion theory; and in case (ii) ${\tau }_1\ll {\tau }_0$, the broadening is the same as in case (i) if ${\kappa }^{2}D{\tau }_0\ll 1$, and it approaches the asymptotic value $\frac{2\hslash }{{\tau }_0}$, if ${\kappa }^{2}D{\tau }_0\gg 1$, where $\hslash \kappa$ is the momentum transferred to the system and $D$ is the diffusion coefficient of water. The observed value of the broadening can be explained for a value of ${\tau }_0=4\mathrm{}\times {10}^{-12\mathrm{}}$ sec. Besides, the theoretical quasi-elastic scattering in case (ii) has certain interesting features which are in general agreement with experiment. In part II of this paper, inelastic scattering (hindered translations only) of cold neutrons has been calculated using two different models of water: (a) a gas model and (b) a Debye model; and the results have been compared with experiment.