| The properties of the minimum energy W of a low-energy free electron in an inert gas are discussed. This quantity is one of the basic characteristics of the electron in problems of the formation of various charged clusters (bubbles or compact aggregates) in low-dimensional electronic states along interfaces. A relation between the energy W and a similar energy W_0 determined in the so-called optical approximation, where the energy W_0 is proportional to the gas density n_g has been obtained. Comparison allows to determine the n_g dependence of the scattering length a_0 introduced in the optical description of the gaseous medium and to reveal the conditions under which a_0(n_g) may change sign, which behavior, by definition, is beyond the framework of the optical approximation and is observed experimentally. |
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