Sputtered cluster mass distributions, thermodynamic equilibrium and critical phenomena

Abstract

The mass distribution of clusters, sputtered by keV atom or ion bombardment of a target surface, is investigated under the assumption that local thermodynamic equilibrium is established in part of the irradiated volume. When cooling down, that region may pass the liquid-gas coexistence curve and undergo a (non-equilibrium) phase transition. The resulting cluster mass distribution is described using Fisher's droplet model: The distribution decays exponentially with cluster mass, if chemical equilibrium between the liquid and the gas phase is not established; otherwise its decay is determined by the cluster surface free energy. Around the critical temperature T c , the mass distribution follows a power law with a power of − 7 3 . Experimentally measured mass distributions of clusters sputtered from condensed gas targets follow the power law valid at T c . Metal and alkali halide cluster mass distributions may be described, if the surface free energy is used as a fit parameter. Its value is small, indicating that the relevant temperature is slightly below T c . The relationship of the present approach to existing models is discussed.