Person:

Silvera, Isaac

X-ray-diffraction studies are described for a new phase of carbon called collapsed fullerite (CF) that was produced by application of high pressure to fullerite (C({60})). At (\sim)20 GPa there is an irreversible transition to a phase that has neither the (111) Bragg peak of diamond nor any of the Bragg peaks associated with the fcc phase of C({60}). The spectrum of CF is flat and featureless in the range of study.

X-ray specular reflectivity measurements of a saturated film of helium absorbed onto an atomically flat silicon substrate have been made at several temperatures. The thickness of the film has been determined and some preliminary data on the structure of the (^4He) liquid-vapor interface are reported.

The helium liquid-vapor interfacial density profile has been measured with x-ray specular reflectivity. Measurements were performed on thick films of helium adsorbed onto atomically flat silicon substrates. Both the amplitude and the phase of the complex scattering amplitude of the helium-vapor interface were obtained from measured interference between reflections from the helium liquid-vapor interface and the silicon-helium interface. Films whose thickness varied from 15 Å to 220 Å over a range of temperatures from 1.1 K to 3.0 K were studied. At T=1.13 K the film thickness is 215 Å and the interfacial width is 9.2 (\pm) 1 Å. No significant variation was seen in the interfacial widths measured at temperatures between 1.1 K and 1.8 K. Analysis of these measurements indicates that the interface is asymmetric, with the decay of the density into the vapor having the sharper falloff. The zero-K interfacial width extrapolated from the finite-temperature measurements with a quantized capillary-wave theory is 7.6(\pm^{1}_{2}) Å.

The average liquid-vapor density profiles 〈ρ(z)〉 of thick (^4)He films adsorbed onto a silicon substrate were measured using x-ray reflectivity. The results are well represented by a 90%-10% interfacial width of 9.2(\pm)1 Å at 1.13 K which extrapolates to a T=0 K, 90%-10% interfacial width of 7.6(\pm^{1}_{2}) Å. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.