FEMTOSECOND 2-PHOTON ABSORPTION AND RAMAN RESONANCES IN POLYDIACETYLENE YELLOW SOLUTIONS

We describe a femtosecond pump and white probe experiment in Kerr gate configuration. We use the Kerr ellipsometry technique, where the analysis of the transmitted probe polarization provides a separate determination of the real and of the imaginary parts of anisotropic index changes. In solutions of polydiacetylene (4BCMU, yellow form) excited at 620 nm, the largest contribution to the Kerr signal is a quasi-resonant stimulated anti-Stokes Raman absorption. A flat two-photon transition spectrum is observed close to 4 eV. The two-photon excited states have a lifetime larger than 5 ps and they present a broad absorption band in the 2 eV range.