Photo-induced structural change in liquid sulphur

The structure of liquid sulphur at 400 K, below the polymerization temperat ure T-p = 432 K, is investigated by first-principles molecular-dynamics sim ulations. The calculated structure factor is in excellent agreement with th e experimental data. To investigate the microscopic mechanism of the photo- induced structural change in liquid sulphur below T-p, simulations are perf ormed by exciting electrons in the valence band to the conduction band. It is found that a bond in one of the S-8 rings is easily broken and consequen tly the S-8 ring turns out to be the S-8 chain after an electron excitation , in the same way as was seen for an isolated S-8 ring. After the electron excitation is stopped, the S-8 chain does not reconstruct the S-8 ring; ins tead a 'tadpole' structure is formed. Since the tadpole structure persists for a long time, it is considered that the tadpole is one of possible struc tures for the experimentally observed long-lived product in the relaxation process. We have shown that photo-induced polymerization occurs in liquid s ulphur when the S-8 chains or the S-8 tadpoles are close to each other at t heir ends.