Mass spectrometric study of the molecular and ionic sublimation of cesium iodide single crystals

A mass spectrometric method was used to study the thermodynamics and vaporization kinetics of cesium iodide single crystal. In electron-impact-ionization mass spectra of molecular fluxes effusing from Knudsen cell (case A) or vaporized from an open crystal surface (case B), ions of Cs+, CsI+, I+, Cs2I+, and Cs-2(+) originating from CsI and Cs2I2 molecular precursors were detected in the temperature range 656-838 K. The temperature dependence of ion currents, 1n(I1T) - 1/T, of the most abundant Cs+, CsI+, and Cs2I+ ions were measured in both cases. From the results of case A (Knudsen cell), the enthalpies of sublimation to monomers and dimers were determined as, Delta H-s(0)(CsI, 736 K) = 170.9 +/- 1.7 and Delta H-s(0)(Cs2I2, 749 K) = 209.8 +- 3.2 kJ mol(-1), respectively. For case B, the temperature dependence exhibited 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 passes through a maximum. A dimer-to-monomer ratio was found to increase with temperature at a continually increasing rate in case A, and at a rate passing through a maximum in case B. The electron-impact-fragmentation pattern of CsI molecules exhibited a roughly linear dependence on temperature in case A, but exhibited a minimum in case B. In the thermionic emission mass spectrum, the positive ions Cs+, Cs2I+, Cs2I2+, and Cs-2(+) were identified in the temperature range 600-890 K. The emission of I- and CsI2- negative ions was detected only above approximately 850 K. It was found that in contrast to the molecular sublimation, the compositions of ion beams from the Knudsen cell and from the free crystal surface differ significantly. The temperature dependence In I(Cs+) - 1/T, measured in the range 640-820 K, revealed a change in slope. The results are discussed in light of the terrace-ledge-kink and surface charge models. (Int J Mass Spectrom 202 (2000) 121-137) (C) 2000 Elsevier Science B.V.