Time-resolved study of ultrafast dephasing processes in solution

Electronic dephasing of solutions of all-trans-bis-(dimethylamino)-heptamethinium chloride, its photoisomer, and of 1,$1^{|\mathrm{I}\mathrm{H}}$,3,3,$3^{|\mathrm{I}\mathrm{H}}$,$3^{|\mathrm{I}\mathrm{H}}$-hexamethylindotricarbocyanine iodide (HITCI) is studied using forced light scattering by broad-bandwidth incoherent pump lasers in the wavelength region from 440 to 770 nm. The dephasing slows down as the pump laser wavelength is tuned over the 0-0 transition. Photoisomer and parent heptamethine molecule show comparable behavior in dephasing experiments. Absorption line-shape analysis including resonance Raman experiments indicates a considerable spectral cross relaxation in the absorption band of the parent molecule. The results for HITCI are close to those reported for two-pulse femtosecond photon echoes.