Spin-polarized hydrogen-helium film system: A surface "polaron"

Experimental investigations of spin-polarized hydrogen H↓ are carried out in a sample cell lined with a helium film whose purpose is to shield the H↓ atoms from magnetic impurities in the walls. The purpose of the He buffer is to minimize spin flips which cause recombination into the ${\mathrm{H}}_2$ molecular ground state and degradation of the sample. In this paper, we focus our attention on the H↓-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ film system and investigate the consequences of allowing the adsorbed H↓ atom to interact with a dynamic He surface. The coupling between the H↓ atom and the modes of the film raises the possibility that the atom will find it energetically favorable to be localized in the plane parallel to the surface of the film, i.e., to form a H↓ polaron. Using a simple model for the atom-film interaction, we show that polaron production can be important. We estimate the order of magnitude for the parallel binding energy and find that it can be comparable to the experimentally measured binding energy. We call attention to the important characteristics of the particle-film system that influence the size and nature of the polaron; e.g., the effect of evolution of the film from ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ to ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and the consequences of the mobility of the surface modes. Where polaron effects are important the parallel binding problem and the perpendicular binding problem are strongly interdependent and cannot be factored. We then use a simple perturbation theory argument to calculate the polaron effective mass. We finally calculate the ripplon-mediated H↓-H↓ interaction and show that it is short ranged and attractive.