Photo-induced structural change in liquid sulphur

Abstract

The structure of liquid sulphur at 400 K, below the polymerizationtemperature Tp = 432 K, is investigated by first-principlesmolecular-dynamics simulations. The calculated structure factor isin excellent agreement with the experimental data. To investigatethe microscopic mechanism of the photo-induced structural change inliquid sulphur below Tp, simulations are performedby exciting electrons in the valence band to the conduction band.It is found that a bond in one of the S8 rings is easily broken andconsequently the S8 ring turns out to be the S8 chain after anelectron excitation, in the same way as was seen for an isolated S8ring. After the electron excitation is stopped, the S8 chain doesnot reconstruct the S8 ring; instead a `tadpole' structure is formed.Since the tadpole structure persists for a long time, it isconsidered that the tadpole is one of possible structures for theexperimentally observed long-lived product in the relaxationprocess. We have shown that photo-induced polymerization occursin liquid sulphur when the S8 chains or the S8 tadpoles areclose to each other at their ends.