Reversible Photoproduction of Stable Charged Defects in Trans-Polyacetylene

Abstract

ABSTRACTPhotoproduction and photodepletion of stable charged defects in trans-polyacetylene were observed, at low temperatures, by monitoring the IR signature of these defects: absorptions at 1368, 1289, 640 cm−1 (1053, 600 cm−1 for (CD)x). These absorptions could be observed at long times (two to several hundred minutes) after photolysis, only when the sample was protected from photolysis by the probe beam, by blocking v > 2,300 cm−1.The location of these absorptions and the temperature dependence of their intensities were similar to those of previously observed photoinduced defects reported to have much shorter lifetimes (τ - ms). The stable defects reported here can be produced at much lower photolysis energies (v > 5,000 cm−1) than transient-photoinduced defects (v > 9,000 cm−1). A mechanism for the photoproduction of stable defects, involving a two-photon process catalyzed by existing neutral defects, is proposed to explain this low onset.The slow decay of these defects observed when the sample is kept in the dark is attributed to photoinduced depletion by blackbody irradiation. This process was slow enough to allow the wavelength dependence of photodepletion (observed at 2,800 < v < 8,000 cm−1) to be studied. The photodepletion was fastest near 4,500 cm−1, suggesting that the defects possess an electronic absorption (with a peak near 4,500 cm−1) similar to that observed directly for the transient-photoinduced defects.