The mechanism of photoinduced expansion/contraction in glassy As 2 S 3 has a photochemical nature. Photostructural transformation is connected with rearrangement of As 4 S 4 molecules and similar non-stoichiometric compounds, As 4 S 4+x . These molecules are believed to exist in clusters which correspond to the second glass-forming region of As-S systems, including the As 56 S 44 eutectic.The chalcogenide systems As-S(Se) have wide ranges of glass formation and a broader spectrum of modifications with third elements into tertiary glassy systems [1]. Almost all photo-induced effects are found in these, which are linked to chalcogenide semiconducting glasses, which are of considerable interest to industry and from the theoretical point of view. Most of the effects discussed appear to a large extent in these systems: photodarkening (red shift of the absorption edge) [2], giant photo-expansion in As 2 S 3 [3, 4], giant mass transfer and photoplastic effect in As 20 Se 80 [5,6], and reversible nanoexpansion/contraction (the so-called optico-mechanical effect) in As 50 Se 50 [7]. At the present time no single view on the mechanism of these phenomena has been formulated. In the present work we propose to study the mechanism of photo-induced processes in chalcogenide glasses in the search for a single factor present in each of the processes. In our view, this factor is a photochemical reaction.Although in this report attention will be concentrated on photo-induced expansion/contraction (PEC) in As 2 S 3 , use of this approach may be useful in investigation of other processes and other chalcogenide glasses. With this objective in mind we have discussed other non-stoichiometric compositions As x S 100-x (x is the at.% of As in the glass-forming region). It should be noted that in glasses the stoichiometry has a purely nominal nature and the usual formula As 2 S 3 is purely traditional and the formula "As 40 S 60 " will be used here.Photo-induced effects are observed in bulk samples of chalcogenide glasses and in thin-film condensates, but the extent to which these effects appear is considerably larger in films. Naturally, the degree of photostructural rearrangement is greater in the less equilibrated system. Hence films are excellent model systems, in which it is possible to compel non-equilibrium. Measurements of certain effects, including PEC, are associated with particular difficulties in bulk glasses. Consequently we have studied films (thickness d » 2.5 mm) obtained by condensation in vacuum on a non-heated base (polished Si, sapphire, and freshly prepared KCl monocrystals) of gaseous thermally evaporated As 2 S 3 glass. 0040-5760/09/4501-0069

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