Sequential absorption processes in two-photon-excitation transient absorption spectroscopy in a semiconductor polymer

We investigate two-photon-excited transient absorption (TA) in poly[$9,{9}^{\ensuremath{'}}$-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine]. Using excitation pulses at $1.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and of around $1\text{\ensuremath{-}}\mathrm{ps}$ duration, we measured the pump intensity dependence of the TA in the visible part of the spectrum and were able to resolve multiple sequential excitation and absorption processes. Polymers in the ground state $({S}_{0})$ were first excited by absorbing two pump photons to the band of two-photon excited states $({S}_{2})$. There is then fast relaxation from ${S}_{2}$ to the lowest singlet excited state $({S}_{1})$. Based on this transient modulation of the population on the excited states, the ${S}_{2}$ and ${S}_{1}$ excitons were further excited by absorbing another pump or tunable probe photon to higher-lying excited states of ${S}_{n}$ and ${S}_{m}$, respectively. Moreover, the probe photon was also absorbed by the ground-state molecules with the aid of a pump photon $({\ensuremath{\omega}}_{\text{pump}}+{\ensuremath{\omega}}_{\text{probe}})$ for the transition from ${S}_{0}$ to ${S}_{2}$. A fraction of higher-energy ${S}_{n}$ and ${S}_{m}$ excitons dissociated into charges that provided an additional absorption channel for the probe pulses. Comparing the simulation and the experimental results, we evaluated quantitatively the contributions from the above absorption channels to the total TA of the probe. The photocurrent measurement in a photodiode structure confirmed that dissociation of excitons from the higher-lying excited states is the dominant mechanism for charge generation in two-photon excitation.