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=S2= S2, known as diatomicsulfur; disulfur; sulfur; sulfur diatomicmolecule; sulfur dimer; sulfur molecule, is the simplest allotrope of sulfur. It is analogous to the dioxygen molecule but does not occur naturally at room temperature. It is commonly generated as a violet gas at above 720˚C and comprises 99% of vapor species at low pressure (1 mmHg) at 530°C. S2 is one of minor compounds at Io’s atmosphere which is dominantly composed of SO2. It was detected in the Pele plume against Jupiter’s continuum at the terminator.

Production
When COS is photosensitized by Hg and irradiated with UV or when CS2, H2S2, S2Cl2 or C2H4S are photolyzed. Singlet S2 is also formed when sulfur compounds such as H2S, PSF3 or COS are photolyzed.

Properties
S2 exists in the triplet ground state like diradical O2. It has S-S double bond length of 189pm while S-S single bonds of S8 are all 206pm. S-S vibrational band from Raman spectra is 715cm-1 in a gas phase. Otherwise, vibrational band of O-O from Raman spectra is 1122cm-1. S-S bond energy is 265kJ/mol and O-O is 498kJ/mol.

Sulfur has a large number of allotropes, and their specific properties are investigated by many types of molecular spectroscopy such as vibrational spectroscopy and Raman spectroscopy. About 30 of the allotropes are well known. The only stable form of sulfur allotrope at normal condition is S8. S2 can be generated by heating organosulfur precursor.