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

Abstract

Vaporization studies on elemental tellurium and selenium were conducted by Knudsen effusion mass spectrometry in the temperature range of 590–690K and 380–480K, respectively.The ionic species Tei+ (i=1–7) and Sei+(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 Te2(g) (with all other gaseous species Te3–Te7 put together constituting less than 5%), the major species over elemental selenium was found to be Se6(g), closely followed by Se5(g) (with other gaseous species Se2–Se4 and Se7–Se9 put together also moderately constituting ∼25%). From the partial pressures, the thermodynamic data for the sublimation reactions i Te(s)=Tei(g) and i Se(s)=Sei(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 Te2(g) and Se6(g), but still some differences to be resolved with regard to minor species in both systems.