Vaporization studies on elemental tellurium and selenium by Knudsen effusion mass spectrometry

Vaporization studies on elemental tellurium and selenium were conducted by Knudsen effusion mass spectrometry in the temperature range of 590-690 K and 380-480 K, respectively. The ionic species Te-i(+) (i = 1-7) and Se-i(+) (g) (i = 1-9) were detected in the mass spectra over these two condensed phases. Measurement of ion intensities were performed as a function of electron impact energy and as a function of temperature (at different electron impact energies) for identifying the gaseous precursor species as well as for determining the partial pressure-temperature relations and sublimation enthalpies for these species. While the major species over elemental tellurium was confirmed to be Te-2(g) (with all other gaseous species Te-3-Te-7 put together constituting less than 5%), the major species over elemental selenium was found to be Se-6(g), closely followed by Se-5(g) (with other gaseous species Se-2-Se-4 and Se-7-Se-9 put together also moderately constituting similar to 25%). From the partial pressures, the thermodynamic data for the sublimation reactions i Te(s) = Te-i(g) and i Se(s) = Se-i(g) were deduced by second- and third-law methods. The atomization enthalpies of tellurium and selenium clusters were also deduced by using the recommended enthalpies of formation of monomeric species. Comparison of the findings obtained in the present study with those in previous studies revealed general agreement with regard to major species Te-2(g) and Se-6(g), but still some differences to be resolved with regard to minor species in both systems. (C) 2014 Elsevier B. V. All rights reserved.