J. Dupont-Roc

Inhomogeneous liquid4He: A density functional approach with a finite-range interaction

The properties of liquid helium-4 are investigated in a mean-field approximation using a finite-range effective interaction. The long-range part (i.e.,r>2.556 Å) coincides with the standard Lennard-Jones potential and the hard core is phenomenologically renormalized. The model reproduces the static polarizability of bulk liquid, as well as the actual surface energy of helium at zero temperature. The surface width is found to be 5.8 Å. A comparison is made with the earlier work of Ebner and Saam.

Wetting properties of liquid helium on rubidium metal

Wetting properties of liquid helium on rubidium metal have been investigated in the temperature range 1.0–1.7 K using a heat conduction method. The rubidium surface is found to be wetted under saturated vapor conditions. Prewetting transitions have been observed for rather weak offsets from saturation, indicating that rubidium is close to the nonwetting limit at T = 0 K. Presumably because of substrate inhomogeneities, the prewetting transitions are experimentally found to be continuous and hysteretic. No evidence for the prewetting critical point has been found up to 1.7 K. The prewetting line verifies a simple linear relation between the liquid-vapour surface tension and the two third power of the chemical potential offset, but does not conform quantitatively to the simple model currently used.

Experimental approaches to the wetting transition with helium films

Alkali metal surfaces have been suggested not to be wetted by helium at zero temperature. Several experiments have been realized to check if these surfaces, as well as other weak binding substrates, indeed exhibits this particularly interesting property at any finite temperature.Although works are still in progress, essential results are (i) substrates non wetted by helium films do exist (ii) prewetting jumps from low to high coverage have been observed for weakly wetted ones (iii) the consequences of substrate roughness and chemical impurities are suspected to be important and remain to be assessed. These results are reviewed and discussed. Applications of those substrates for both3He and4He isotopes are considered.

A new type of resonances in saturated absorption spectroscopy of 3-level systems

Abstract We report the experimental observation of new resonances in saturated absorption spectra of a J = 1 to J = 0 transition of Ne atoms in a static magnetic field. These resonances, which are distinct from the well-known Zeeman and cross-over resonances, result from the modification of stimulated Raman processes by the simultaneous resonant saturation of an optical transition. The light-shifts of the various resonances are also studied.

A more accurate model of wetting transitions with liquid helium

Up to now the analysis of the liquid helium prewetting line on alkali metal substrates have been made using the simple model proposed by Saam et al.1 Some improvements on this model are considered within a mean field, sharp kink model. The temperature variations of the substrate-liquid interface energy and that of the liquid density are considered, as well as a more realistic effective potential for the film-substrate interaction. A comparison is made with the experimental data on rubidium and cesium.

Investigation of the effect of oxygen impurities on the wetting properties of alkali metal surfaces by liquid helium

The weak binding properties of heavy alkali metal surfaces for helium have been related to their metallic nature and their particularly small work function. A natural consequence would be that oxidized surfaces should loose their non wetting properties for liquid helium. It is pointed out that the special oxidization process of these metals could unexpectedly lead to an opposite result at some intermediate stage. First experimental results on rubidium shows evidences for such an effect.

Compensation of Doppler braodening by light shifts in two photon absorption

Abstract We show that the residual Doppler broadening of a two photon absorption line — the two photons having different frequencies — can be compensated by velocity dependent light shifts. We report the results of an experiment performed on 20 Ne atoms and giving evidence for this compensation effect.

Helium prewetting transition on a disordered cesium metal surface

Abstract The prewetting transition of liquid helium on a thick cesium layer surface has been investigated using an optical method to image the coverage of the substrate. Starting below the transition temperature with a saturated film pinned on the lower half of the surface, the temperature is raised. On this surface, probably a highly disordered one, the film edge is found not to move. Instead the coverage difference between the two sides of the edge disappears gradually from 1.85 to 2 K, suggesting that a non-saturated helium film, presumably lacunar, grows on the initially dry area. The advancing contact angles have been measured on the same area for various temperatures, suggesting a wetting temperature around 1.88 K, consistent with current values.

Alkali suboxides as weaker binding substrates for helium

Suboxides of heavy alkali metals are known for their particularly low work function. This property should make the surface of those materials less binding for helium than that of pure metals. A semi-quantitative assessment of their wetting properties for liquid helium is proposed. Preparation of such surfaces has been undertaken for rubidium suboxides. The wetting properties obtained for liquid helium will be reported.

Wetting transitions of liquid helium on oxidized rubidium metal surfaces

Prewetting transitions of liquid helium on rubidium surfaces exposed to increasing quantities of oxygen have been observed. They reveal a decrease of the wettability of the surface. A state has been reached where the rubidium surface appears non wetted for temperatures below 1.45 K. The dewetting transition has been observed.