Real-time spectroscopy of pseudoisocyanine J-aggregates with sub-5Â fs lasers

The oscillations of the absorbance change due to molecular vibrations in pseudoisocyanine bromide J-aggregates were real-time resolved with the sub-5-fs 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 induced by the break down of the Condon approximation. The non-Condon-type exciton-phonon coupling was invoked to explain the oscillatory signals and the modulation of transition-dipole moment were evaluated. Together with the intramolecular molecular vibrational modes, two lower frequency modes than 100 cm−1 were observed. The band at 38 cm−1 was assigned to an optical phonon mode. A band at 152 cm−1 considered to be low intramolecular vibrational mode was also observed. Because of low frequency this mode had not been previously detected in Raman scattering.