Structure and dynamics of ammonia absorbed on graphitized carbon black. Part 3.—Neutron quasielastic and inelastic spectra

Abstract

Incoherent neutron quasielastic and inelastic scattering experiments on the system ammonia on Graphon are described. The range of temperatures and coverages used for the experiments were similar to those for the isotherm measurements and diffraction experiments described in two previous papers.

The freezing of ammonia on the surface was followed by very high resolution quasielastic experiments. In agreement with the neutron diffraction results, ammonia does not solidify until well below the freezing point of bulk ammonia. The freezing point depends on the coverage and freezing takes place over a range of temperature. There are qualitative differences between the freezing curves obtained by the two techniques because the diffraction experiment only observes the formation of crystalline ammonia while the quasielastic experiment does not distinguish amorphous and crystalline solid.

Quantitative analysis of the quasielastic spectra shows that the solid ammonia is in equilibrium with an adsorbed fluid. The character of the adsorbed fluid is slightly different above and below the melting point of bulk ammonia (195 K). Above the melting point, it is an extremely mobile anisotropic fluid, the quasielastic spectra being best explained by a model of diffusion in two dimensions obeying Fick's law. The diffusion coefficient parallel to the surface is three times larger than for bulk liquid ammonia. The difference results from changes in both the activation energy and frequency factor for diffusion. Some diffusion may be occurring perpendicular to the surface but the resolution of the instrument does not allow it to be well characterized. Rotational diffusion is rapid in the adsorbed fluid indicating that the surface does not orient the ammonia molecules.

Below 195 K the fluid starts to form clusters in which the ammonia molecules diffuse much less freely, having a diffusion coefficient comparable to that calculated by extrapolation of the properties of the bulk liquid. The clusters are probably the immediate precursors of the solid that is formed as the temperature is lowered. The formation of such clusters may therefore be an important step in the process of heterogeneous nucleation.

The difference in the dynamical properties of the adsorbed ammonia above and below 195 K accounts qualitatively for differences in the heats and entropies of adsorption in the two temperature ranges.

Inelastic spectra of the ammonia confirm much of the picture given above. At very low temperatures the spectrum is different from that of bulk ammonia. This is consistent with the presence of some amorphous solid and possibly also with molecules vibrating independently of each other in the surface field.