Photo-induced structural change in liquid sulphur

Abstract

The structure of liquid sulphur at 400 K, below the polymerization temperature Tp = 432 K, is investigated by first-principles molecular-dynamics simulations. The calculated structure factor is in excellent agreement with the experimental data. To investigate the microscopic mechanism of the photo-induced structural change in liquid sulphur below Tp, simulations are performed by 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 and consequently the S8 ring turns out to be the S8 chain after an electron excitation, in the same way as was seen for an isolated S8 ring. After the electron excitation is stopped, the S8 chain does not reconstruct the S8 ring; instead a `tadpole' structure is formed. Since the tadpole structure persists for a long time, it is considered that the tadpole is one of possible structures for the experimentally observed long-lived product in the relaxation process. We have shown that photo-induced polymerization occurs in liquid sulphur when the S8 chains or the S8 tadpoles are close to each other at their ends.