Real-Time Spectroscopy of Molecular Vibrations with Sub-5-Fs Visible Pulses

Abstract

In this review article, research on ultrafast processes in a molecular aggregate system is described to show the usefulness of sub-5-fs real-time spectroscopy in studies of vibrational dynamic processes in molecules. We focus here on the J-aggregates of porphyrin, which have Frenkel excitons as elementary excitations, but the method can be applied to many other molecular systems including dyes, charge-transfer complexes, polymers, biomolecules, and organo-metallic compounds. Using such short pulses, many interesting phenomena can be discovered. Dynamic intensity borrowing is one such phenomena clarifying the mechanism of the dynamic mixing of the electronic states via molecular vibration. Photo-excitation of J-aggregate systems using extremely broadband visible—near-IR pulses with sub-5-fs duration was performed. By the vibronic coupling mechanism, a coherent molecular vibration is expected to be observed in real-time pump-probe spectra at various probe wavelengths. The vibration has a frequency of 244 cm-1 corresponding to an oscillation period of 135 ± 4 fs. This vibration was analyzed to obtain the time-frequency two-dimensional difference transmission spectrum. An analysis of the phase and amplitude of the oscillation has revealed that both the negative (bleaching and photo-induced emission) and positive (photo-induced absorption) signals are modulated synchronously. This experimental result is well explained in terms of a transition dipole moment modulated by dynamic intensity borrowing from an intense B-transition (or Soret transition) to a weak Q-transition through vibronic interaction. The wave packet observed in the present study can be classified as a Herzberg-Teller type, which is a different mechanism from the frequently studied Franck-Condon type.