Mass Spectrometric Study of the Vaporization Kinetics of Potassium Bromide Single Crystals

Abstract

A mass-spectrometric method was used to investigate the kinetics of the vaporization of potassium bromide single crystals. In electron-impact-ionization mass spectra of molecular fluxes vaporized from a free-surface of KBr, ions of K⁺, KBr⁺, Br⁺, and K₂Br⁺ originating from KBr and K₂Br₂ molecular precursors were detected in the temperature range 724–918 K. The temperature dependencies of ion currents, ln I_iT − 1/ T, of the most abundant ions (K⁺, KBr⁺, and K₂Br⁺) revealed a departure from the linearity. From a comparison between the equilibrium and nonequilibrium vaporization rates, it was concluded that the value of the vaporization coefficient for monomers and dimers passes through a maximum at about 800 and 825 K, respectively. An electron-impact-fragmentation pattern of KBr molecules, I(K⁺)/I(KBr⁺), was observed to pass reproducibly through a minimum at the temperatures of about 800 to 830 K. In the fluxes from a free surface and from one inside a Knudsen cell, the dimer-to-monomer ratios were found to vary with temperature in different ways. The difference in the equilibrium and nonequilibrium rates of vaporization, the temperature dependence of the fragmentation pattern, and the mechanism of association reactions are discussed in light of the terrace-ledge-kink and surface-charge models.