We have used the highly sensitive technique of Photothermal Deflection
Spectroscopy (PDS) to measure changes in the infra-red absorption spectra of
MEHPPV, P3HT and Polydiacetylene-4BCMU induced by pumping these polymers
with light above the π - π* transition energy. In contrast to previous
chopped light transmission measurements of these effects, the PDS technique
can 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 that
above-gap light causes the appearance of a broad infra-red peak in α, which
continues to grow-in hours after the pump light is first applied. For this
polymer the general shape of the absorption spectra in the unpumped state
mimics the photo-induced changes, suggesting that remnant photo-induced
states determine the maximum transparency observed under normal experimental
conditions. For P3HT and to a lesser extent, MEHPPV, we also observe
irreversible photo-induced absorption components which we tentatively
identify with photo-induced oxidation of the polymer matrix.