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106 Ralf Steudel ed very thin samples of liquid sulfur with a pulsed laser (355 nm) and ob- served the time dependence of the transient absorption spectra of the photo- generated species. Irradiation at sulfur temperatures of between 130 and 150 C produced two types of species with relaxation times of 60 s and 40 min at 130 C, respectively. Both species showed absorption maxima near 3.5 eV (354 nm). The short-lived product was formed by repeated illumina- tion with 1.0 mJ pulses and was interpreted as polymeric chains of sulfur atoms. The long lived product was originally assigned to the short chains which were assumed to bind to S8 rings as charge-transfer complexes [115]. As a seemingly more convincing species the branched S7=S structure was later proposed as the long-lived product which had been found by molecular dynamics simulations of the photochemistry in liquid sulfur [116] (see be- low). However, as has been demonstrated in the previous section, the most likely candidate(s) for the long lived product absorbing near 360 nm is the p-sulfur in the sulfur melt, especially the medium-sized rings Sx(x>8) which form from polymeric sulfur by depolymerization. This follows from the rates of formation and decay of the long-lived product when it was generated thermally [115]; these rates agree very well with the rates of formation and decay of p-sulfur of which the Japanese authors were however not aware. 3.5 Electrical Conductivity of Liquid Sulfur Under ambient conditions elemental sulfur is one of the best electrical insu- lators known. In fact, sulfur is the prototype of a non-metal defined as a ma- terial of zero electrical conductivity at 0 K. However, this statement applies to ambient pressures only. At very high pressures sulfur—like other typical non-metals—becomes an electrical conductor and, at very low tempera- tures, even a superconductor [117]. Because of the very low conductivity at standard conditions (e.g., 107 W1 cm1 at 550 C) impurities play a major role and those studies which reported the lowest conductivity must be con- sidered the most trustworthy. The electrical conductivity s of liquid sulfur increases with temperature except near the viscosity maximum of ca. 170 C where a minimum of the conductivity is observed. Above 200 C the plot of log s vs 1/T was found by several authors to be linear but the slopes of these linear relationships as well as the absolute conductivities vary considerably [118–122]. On the as- sumption that the conductivity at these temperatures is intrinsic, values of about 1.6 eV were derived for the activation energy at high temperatures (up to 900 C) [121, 122], an energy which is much higher than the activation energy for the formation of free spins by homolytic bond dissociation (see above). In a more recent investigation the conductivity of highly purified sulfur was measured in the range of 300–900 C using gold electrodes and a quartz cell. It was found that the slope of the linear function log s vs 1/T changes relatively sharply near 550 C (see Fig. 5). Activation energies of 1.9 eV be- low this temperature and of 1.05 eV above 550 C were obtained for the gen-PDF Image | Topics in Current Chemistry
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