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 vapori zation kinetics of cesium iodide single crystal. In electron-impact-ionizat ion 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+, Cs 2I+, and Cs-2(+) originating from CsI and Cs2I2 molecular precursors were d etected in the temperature range 656-838 K. The temperature dependence of i on 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 exh ibited a departure from the linearity. From a comparison between the equili brium and nonequilibrium vaporization rates, it was concluded that the valu e of the vaporization coefficient passes through a maximum. A dimer-to-mono mer ratio was found to increase with temperature at a continually increasin g rate in case A, and at a rate passing through a maximum in case B. The el ectron-impact-fragmentation pattern of CsI molecules exhibited a roughly li near dependence on temperature in case A, but exhibited a minimum in case B . In the thermionic emission mass spectrum, the positive ions Cs+, Cs2I+, C s2I2+, and Cs-2(+) were identified in the temperature range 600-890 K. The emission of I- and CsI2- negative ions was detected only above approximatel y 850 K. It was found that in contrast to the molecular sublimation, the co mpositions of ion beams from the Knudsen cell and from the free crystal sur face differ significantly. The temperature dependence In I(Cs+) - 1/T, meas ured in the range 640-820 K, revealed a change in slope. The results are di scussed 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.