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 As--S--As 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 As4Ss 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.