Raman Spectrometric Study of the Composition of Sulfur Vapor at Temperatures up to 700°C

Abstract

Abstract Sulfur vapor has been investigated by Raman spectroscopy in a wide temperature and pressure range (300 to 900°C, 0.05 to 0.5 MPa for overheated vapor and 0.007 to 1.65 MPa for saturated vapor). At least eight different species have been observed through their characteristic Raman lines. Well separated Raman lines have been selected for the observation of the change in vapor composition with temperature: symmetrical bending modes for the cyclic species S 8 , S 7 , and S 6 and stretching modes for the chain‐like species S 3 and S 2 . There are indications for the presence of two conformational isomers of S 4 , and in addition a new species has been discovered by its resonance Raman line at 635 cm −1 which is tentatively assigned to chain‐like molecules S n ( n > 4) of helical conformation. Several relationships between the thermodynamic properties of the vapor and the Raman measurements are proposed, e.g., the determination of the temperature at which the molar fraction x i is maximum for the overheated vapor at constant pressure; estimation of the x i / x 8 ratios using solution measurements of the Raman cross section for the molecules S 8 , S 7 , S 6 . A careful comparison of these results with the values deduced from Rau's model results in good agreement for the behavior of most of the species apart from S 7 which is underestimated in Rau's model. S 7 is the dominating species in saturated sulfur vapor above 600 K until S 2 takes over at ca. 1000 K.