Kinetics of Vaporization of Molten Selenium

Vaporization of molten selenium was studied under equilibrium and non-equilibrium conditions. A quadrupole mass spectrometer was used to measure the vapor composition, and a vacuum microbalance was used to measure the total rate-of-weight-loss. Both measurements were made as a function of temperature near 500 K. Experimental results indicate that the vaporization of molten selenium is a retarded reaction having vaporization coefficients of about 0.1 for all species Se _n (g) (n = 2–8). Results also show that the activation free energy of vaporization for all species is approximately equal to the free energy change of depolymerization reactions of liquid molecules, as obtained by other authors. These molecules consist of long chains in thermodynamic equilibrium with octacyclic rings. Mecha­nisms for the vaporization of molten selenium are postulated that involve the formation of short chains or small, nearly closed loops on liquid surface as the rate-limiting step. These activated complexes have the same number of atoms as the gaseous ring molecules to be formed. A critical test for this model is proposed which, if proven correct, may lead to a novel technique for growing pure single crystals of trigonal selenium.