Prephotobleaching Process in Polysilane Films

Abstract

It has been found that the absorption of incident ultraviolet (UV) light by polysilane films with thicknesses comparable to the penetration depth of UV light does not decrease for a while and then decreases rapidly according to a power law under prolonged exposure to UV light, indicating the occurrence of the prephotobleaching process in the period of no change in the absorption. The transition time, when the absorption of UV light begins to decrease significantly, is inversely proportional to the incident photon flux, obeys a power law as a function of the film thickness and is temperature dependent. We explain the prephotobleaching process using the thermally assisted photoscission model where the σ bonds between domainlike σ-conjugated segments of silicon chains have lower activation energy than the σ bonds in the segments contributing to the absorption of UV light and are photoscissored first. The exponential distribution of activation energy is assumed for the two kinds of σ bonds. The minimum activation energy of the intrasegment σ-bonds has been estimated to be 75 meV as a result of the analysis of the experimental results obtained using this model.