SPECTROSCOPIC INVESTIGATION AND NORMAL-MODE ANALYSIS OF AS4S4 POLYMORPHS

Tetra-arsenic tetrasulfide polymorphs have been investigated by IR, Ra man and UV-visible spectroscopies. The mineral realgar (alpha-As4S4) a nd the high-temperature phase beta-As4S4 transform to pararealgar by e xposure to monochromatic laser light with a reaction rate depending on the wavelength of the exciting radiation. The photoinduced alteration process has been monitored by Raman spectroscopy, following in real t ime the spectral changes of the samples undergoing irradiation. A sati sfactory assignment of realgar, beta-As4S4 and pararealgar was obtaine d by employing a simplified valence force field which provided a compl ete set of normal coordinates suitable for describing the vibrational spectra of these As4S4 polymorphs.