Photo-induced Structural Change and the Relaxation in Liquid Sulfur

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We have studied what happens in the structural and electronic properties of liquid chalcogens such as S and Se when we remove LP electrons by the photo-excitation.It is well known that liquid S forms a light yellow liquid consisting of S_8 molecules, which has the shape of puckered ring, between the melting point (113ﾟC) and the polymerization temperature Tp (159ﾟC). We have found that liquid S is very photo-sensitive and a distinct photo-induced structural change occurs when the pulsed laser with a wave length of 355 nm illuminates below Tp. With increasing intensity of laser we have observed three different structural changes : (1) A charge transfer complex is produced when the intensity is less than 10 mJ/pulse. (2) A polymer is produced when the intensity is more than 10 mJ/pulse. (3) When the intensity is more than 20 mJ/pulse we have observed another interesting phenomena, namely, the illuminated region appears in the visible light like rainbow. Studies using a microscope revealed an inhomogeneous region where a giant-molecule seems to be generated. It was found, furthermore, that the two-dimensional restriction of the optical cell less than 0.1 micron in which liquid S is contained plays an important role when the giant-molecules are generated. The geometrical restriction may force the giant-molecules to have an ordered structure.We have also found a strong photo-darkening for liquid Se, that is, an enormous reduction exceeding 50% in the transmittance when the pulsed laser with band gap energy illuminates. The results of the transient absorption spectra suggest the possibility that the illumination of laser induces the structural instability of liquid Se eventually causing the semiconductor-metal transition.