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Abstract

The wetting behavior of quenched-condensed solid D 2 films has been investigated by means of light scattering. On the substrates used here (graphite and aluminum) molecular deuterium displays triple point wetting and hence dewets in the solid state, provided the temperature is high enough that thermally activated diffusion processes can take place. This manifests itself in a coarse-graining of the D 2 film. In order to avoid this process and to obtain complete wetting of solid D 2 we have modified the adsorbate-substrate interaction potential by preplating the substrates with thin inert layers of Ne, Ar, CH 4 or C 2 H 6 . The predicted change in the wetting properties was not observed, however, suggesting that the existing picture of triple point wetting of van der Waals systems is not complete. The implications of these results for a neutrino mass experiment where quenched-condensed molecular tritium films are used as a source are discussed.

On dewetting dynamics of solid films of hydrogen isotopes and its influence on tritium spectroscopy

“…For none of the substrates tried out, aluminium, carbon or sandwiched ones with some other van der Waals film in between, (noble gases, CH 4 ), the wetted phase was found to be energetically stable in thermal equilibrium, but dewetting was observed always in the temperature region just below desorption. We have reported this finding already in a foregoing paper [8]. Although our hopes to realize a thermodynamically stable T 2 -film for the purpose of precision measurements of the β-spectrum from such a source were not fulfilled, we learned from the present study that the dewetting dynamics is at least sufficiently slow at temperatures T < 2 K to allow long term measurements on quench-condensed T 2 -films without the risk of structural changes.…”

“…The acceleration with temperature follows the Arrhenius law and yields activation energies for the different isotopes. Coverage of the quench-condensed hydrogen films by a few monolayers of more tightly bound van der Waals substances like Ar or CH 4 suppressed dewetting completely and thus proved it to be a surface diffusion process [8,14].…”

“…In all cases investigated, the hydrogen film dewetted. We have reported on these findings in a foregoing paper [8]. In a second attempt we have investigated conditions under which the quench-condensed films could be stabilised at least dynamically; i.e.…”

“…In Section 4 we present a schematic model which treats the process of dewetting and cluster growth in terms of surface diffusion on the residual film between the boundaries of still quench-condensed, disordered 1 adsorbate and those of the energetically favoured crystalline clusters. That surface, not volume diffusion drives the dewetting has been proven unambiguously by covering the hydrogen condensates with some monolayers of other, slightly tighter bound van der Waals systems like Ne, Ar, or CH 4 which definitely prevented the hydrogen film from dewetting [14,8]. The analysis of experimental data in Section 5 yields the decisive activation energies of the process for the different isotopes and proves, moreover, that the schematic model is in satisfactory qualitative agreement with the various features observed.…”

Searching the absolute neutrino mass in tritium β-decay—interplay between nuclear, atomic and molecular physics

“…Later this extra energy loss could be traced as being due to an unexpected phenomenon: At temperatures above 3 K the homogeneously evaporated T 2 -film started to dewet from the substrate, clustering into small crystals which enhanced the average energy loss [23,24]. Cooling the substrate below 2 K in phase 2 of the Mainz experiment fully suppressed this phenomenon and led to stable m 2 ν -results around zero for all evaluation intervals (Fig.…”