Proton stopping in dense molecular hydrogen: A molecular-confinement model

A molecular-confinement model is proposed for the calculation of density effects on the electronic stopping cross section (S-e) in a condensed medium. In this model, the collective intermolecular interactions in the medium are represented by a mean field in which a particular molecule is embedded including the spatial constrictions imposed by the surrounding molecules. A molecule is thus viewed as a caged-in system within a spherical boundary with finite potential barrier height V-B,. Changes in the molecular electronic properties and molecular conformation as a function of medium density are self-consistently treated. As a first example of a general treatment for more complicated target structures, the model is explicitly applied to the case of proton stopping in dense molecular hydrogen. The lowest barrier height (V-B= 0) was selected for the stopping calculations since it provides a more realistic pressure-density relation at T = 0 K than higher barrier values. Our results for dense molecular hydrogen predict a very small to moderate reduction in S, relative to the gas phase in going from atmospheric pressure (0.036 mol/cm(3), Delta S-e approximate to 0.5%) up to 136 GPa (0.380 mol/cm3 Delta S-e approximate to 24%) for either the liquid or solid phase as determined by the phase diagram for this medium. [S1050-2947(99)08609-6].