Theories of ledge dynamics for crystals evaporating by the stepwise mechanism predict a change from crystal edge control to dislocation control of the evaporation kinetics as the gas phase saturation is reduced. This effect was studied by measuring the rate of evaporation from the (100) face of high purity KCl crystals vs gas phase saturation with a specially designed spherical effusion cell of variable orifice area. The experiments minimized or eliminated variation in sample temperature, area, dislocation density, and purity and gave results of sufficient precision to confirm the theoretical predictions. Ledges arising from dislocations do not influence the evaporation rate of KCl if the incident to equilibrium flux ratio (Vi/Veq) exceeds 0.93plusmn;0.02, at 460thinsp;deg; C. This conclusion is based on an observed change in slope of evaporation rateversusincident flux at a value ofVi/Veqconsistent with the theory of stepwise evaporation and independent estimates or measurements of surface energies, surface reaction activation energies, and ledge spacings on evaporated crystals.
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