Abstract

Upon electronic excitation, linear π- conjugated chains (both oligomers and polymers) show substantial alterations in the geometrical configuration of the carbon skeleton. Re-organization into a stable excited state may result into variations of the C-C bond length up to 5 pm. Bond-order alternation may be reduced to zero or switched, and in long chains it has been suggested that topological defects can be generated.¹ Two important consequences of the strong electron-phonon coupling in such systems are the presence of a Peierls distortion in the ground state and the possibly large vibronic coupling, especially in case of closely lying excited states.² For these reasons, vibrational dynamics and vibronic coupling have been the subject of extensive studies, theoretical as well as experimental. Impulsive coherent vibrational spectroscopy (ICVS)³ is a recent approach to this challenge, made possible by the advent of new ultrashort laser sources (below 10 fs) which allow to monitor in real time linear chain normal modes with frequency up to 2000 cm⁻¹

© 1999 Optical Society of America