Photo-induced Structural Change and the Relaxation in Liquid Sulfur

我们研究了液态硫族元素S和Se在光激发去除LP电子后的结构和电子性质变化，发现液态S在熔点（113 ℃）和聚合温度Tp（159 ℃）之间形成由S_8分子组成的浅黄色液体，呈皱环状。我们发现液态S具有很强的光敏性，当波长为355 nm的脉冲激光照射到低于Tp的温度时，液态S会发生明显的光致结构变化。随着激光强度的增加，我们观察到三种不同的结构变化：（1）当激光强度小于10 mJ/pulse时，产生电荷转移复合物; (2)当强度大于10 mJ/脉冲时产生聚合物。(3)当光强大于20 mJ/pulse时，我们观察到另一个有趣的现象，即在可见光区出现彩虹状的发光区。使用显微镜进行的研究揭示了一个不均匀的区域，在那里似乎产生了一个巨大的分子。此外，还发现，当巨分子产生时，含有液体S的小于0.1微米的光学盒的二维限制起着重要作用。我们还发现，当带隙能量的脉冲激光照射时，液态Se会发生强烈的光暗化，即透射率大大降低，降低幅度超过50%。瞬态吸收光谱的结果表明，激光照射引起的结构不稳定性的液体硒最终导致的超导体-金属转变的可能性。

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.