Sub-5-fs two-dimensional spectroscopy of pseudoisocyanine J-aggregates

The oscillations of the absorbance change due to molecular vibrations in pseudoisocyanine bromide J-aggregates were real-time resolved with sub-5-fs laser pulses. In spite of weak exciton–phonon coupling in the Frenkel exciton system, it was possible to observe the modulation of difference absorbance because of the enhanced transition moment. The non-Condon-type exciton–phonon coupling was invoked to explain the oscillatory signals and the transition dipole modulations were evaluated. The band at 38 cm−1 was assigned to optical phonon mode, and the band at 152 cm−1, which had never been detected in Raman scattering, was also real-time resolved.