1. TEDA Applied Physics School, Nankai University; The Key Laboratory of Advanced Technique and Fabrication for Weak-Light Nonlinear Photonics Materials, Ministry of Education, Tianjin 300457, China 2. Naval University of Engineerin, Tianjin 300450, China
Abstract：Light-induced degradation in realgar was studied by means of Raman spectroscopy. The result substantiated the following reaction: An additional S atom is inserted between the As atoms in the As4S4 molecule because As—As bonds are weaker than As—S bonds . It is likely that the resulting As4S5 molecule is too unstable to exist. Furthermore, an S atom in the As4S5 molecule is released from one of the equivalent As—S—As linkages in As4S5 which becomes the As4S4 molecular of pararealgar. After the As4S5 molecule is divided into an S atom (radical) and the As4S4 (pararealgar type) molecule, the free S atom is re-attached to another As4S4 (realgar type) molecule, reproducing an As4S5 molecule. The reproduced As4S5 molecule is again divided into an S atom (radical) and an As4S4 (pararealgar type) molecule. This cycle whereby realgar is indirectly transformed into pararealgar via the As4S5 molecule is promoted by light and repeated during light exposure.
