Atomic Motions in Water by Scattering of Cold Neutrons

Abstract

The inelastic scattering of cold neutrons, of energy about 4×${10}^{-3\mathrm{}}$ ev, has been used to study the atomic motions in water, mainly in the liquid phase. As a result of the incoherent nature of scattering by protons, the interpretation of the energy changes in terms of atomic motions is particularly simple. The water samples used were extremely thin in order to avoid multiple scattering effects. Instead of the smooth distribution in energy of the scattered neutrons expected for a classical liquid, the experimental results exhibit a number of distinct energy changes. The observed transition energies are (in units of ${10}^{-3\mathrm{}}$ ev), 61, 21, 8, 5, and 0.5; the first three of these agree with Raman spectroscopy results. The highest energy transition increases in intensity up to the boiling point, but does not shift in energy. For water vapor at 20 atmospheres, a smooth energy distribution is obtained, which agrees well with the theory of Krieger and Nelkin. The sharp energy levels observed for liquid water indicate that the molecules do not act as free gas atoms of mass 18; in the vapor the effective mass is about 4. An elastic peak is observed that exhibits no spread in energy of the type expected from the theory of classical diffusive motions. Instead, two small but sharp peaks are found to correspond to the gain and loss of 0.5×${10}^3$ ev energy; the nature of the transition corresponding to this energy change is unknown. The absence of diffusive broadening shows that the water molecules remain in one location for a relatively long time, about ${10}^{-12\mathrm{}}$ sec, before undergoing diffusive "jumps." In general, the present experiments show that the atomic motions in water are similar to those in a solid rather than a gas.