Ultrafast Spectroscopy of Even-Parity States inπ-Conjugated Polymers

Relaxation dynamics of even parity ( $A_g$) states in poly( $p$-phenylene vinylene) derivatives are studied using a novel fsec transient spectroscopy, in which two different excitation pulses successively generate odd parity ( $1{}_{}{}^1{}_{}{}^{}B_u^{}{}_{}{}^{}$) excitons at 2.2 eV and then reexcite them to higher $A_g$ states. For reexcitation energies $ħ\omega <1.1\mathrm{}\mathrm{eV}$ ultrafast internal conversion back to $1{}_{}{}^1{}_{}{}^{}B_u^{}{}_{}{}^{}$ takes place in accordance with Vavilov-Kasha's rule. However, for $ħ\omega >1.1\mathrm{}\mathrm{eV}$ the decay occurs in a nonemissive state identified as a polaron pair, showing that the $A_g$ states above 3.3 eV mediate charge transfer.