  We present ultrafast transient absorption spectra of two oligofluorene
derivatives in dilute solution. These spectra display clear vibronic structure,
which we analyze rigorously using a time-dependent formalism of absorption to
extract the principal excited-state vibrational normal-mode frequencies that
couple to the electronic transition, the configurational displacement of the
higher-lying excited state, and the reorganization energies. We can model the
excited-state absorption spectrum using two totally symmetric vibrational modes
with frequencies 450 (dimer) or 400 cm$^{-1}$ (trimer), and 1666 cm$^{-1}$. The
reorganization energy of the ground-state absorption is rather insensitive to
the oligomer length at 230 meV. However, that of the excited-state absorption
evolves from 58 to 166 meV between the oligofluorene dimer and trimer. Based on
previous theoretical work [Shukla et al., Phys. Rev. B \textbf{67}, 245203
(2003)], we assign the absorption spectra to a transition from the $1B_u$
excited state to a higher-lying $mA_g$ state, and find that the energy of the
excited-state transition with respect to the ground-state transition energy is
in excellent agreement with the theoretical predictions for both oligomers
studied here. These results and analysis permit profound understanding of the
nature of excited-state absorption in $\pi$-conjugated polymers, which are the
subject of general interest as organic semiconductors in the solid state.
