Photo-induced Metastable State of S 8 Clusters in Liquid Sulfur
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0918-H03-02
Photo-induced Metastable State of S8 Clusters in Liquid Sulfur Yoshifumi Sakaguchi, and Kozaburo Tamura Graduate School of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, Kyoto, 606-8501, Japan ABSTRACT It is well known that liquid sulfur undergoes a polymerization transition from S8 rings to the mixture of S8 rings and polymeric chains at Tp =159°C. In addition to the transition, it has been known that there is a transformation from S8 ring to a molecular species other than S8 ring and polymeric chain below Tp since 19th century. Such molecular species can also be obtained by the illumination of light. Several models of the molecular form have been presented. However, the conclusive model which explains all experimental results has not been settled yet. Recently we measured transient photo-induced optical absorption in liquid sulfur and Munejiri et al. performed first-principles molecular-dynamics simulations to clarify the microscopic origin of the experimental results. The results of the simulations presented a plausible molecular form as the molecular species, which is a tadpole-shaped S8 cluster. We show that the molecular form clearly explains the previous experimental results. A possibility of the formation of an intimate valence alternation pair in the small cluster is also discussed. INTRODUCTION Crystalline sulfur consists of molecules which are eight-membered rings of sulfur atoms [1, 2]. When the solid melts at 113°C (for orthorhombic sulfur), it forms a yellow low-viscous liquid. When the yellow liquid sulfur is heated further, it abruptly forms a red-brown gel at Tp =159°C. The transition at Tp is known to be due to the polymerization of the sulfur. At Tp, such properties as the heat capacity and the viscosity show dramatic anomalies [1, 2]. In addition to the polymerization transition, it has been pointed out since 19th century that the transformation from S8 ring to the molecular species other than S8 ring and polymeric chain occurs by either heating or irradiation of light below Tp. It was well known that insoluble sulfur in carbon disulfide is obtained when the melted sulfur above Tp is quenched rapidly [3]. From this fact, it is believed that the insoluble sulfur corresponds to the polymeric sulfur, which is produced in the liquid above Tp. However, a small amount but detectable amount of insoluble sulfur is obtained from the liquid “below” Tp [3, 4]. This result indicates that there is a molecular species other than S8 ring and polymeric chain in liquid sulfur below Tp. The content of the insoluble sulfur increases with increasing temperature of the liquid and with increasing heating time for constant temperature of the liquid. We can expect that there is an equilibrium reaction S8 ring ⇄ S(insoluble) below Tp. The presence of such molecular species other than S8 ring in the liquid results in a freezing point depression. The molecular species can be regarded as a solute in a solvent composed of S8 rings. The freezing point depression is observed by many researchers
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