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Published online by Cambridge University Press: 21 February 2011
We have used the highly sensitive technique of Photothermal DeflectionSpectroscopy (PDS) to measure changes in the infra-red absorption spectra ofMEHPPV, P3HT and Polydiacetylene-4BCMU induced by pumping these polymerswith light above the π - π* transition energy. In contrast to previouschopped light transmission measurements of these effects, the PDS techniquecan directly measure the buildup or decay of the absorption coefficient, m,on the time scale of seconds to days. In the case of MEHPPV we observe thatabove-gap light causes the appearance of a broad infra-red peak in α, whichcontinues to grow-in hours after the pump light is first applied. For thispolymer the general shape of the absorption spectra in the unpumped statemimics the photo-induced changes, suggesting that remnant photo-inducedstates determine the maximum transparency observed under normal experimentalconditions. For P3HT and to a lesser extent, MEHPPV, we also observeirreversible photo-induced absorption components which we tentativelyidentify with photo-induced oxidation of the polymer matrix.