Femtosecond coherence spectroscopy: A Fourier-domain view of molecular wavepacket dynamics in transient–absorption spectroscopy

The advent of femtosecond laser pulses in the 1980s quickly revealed their ability to generate coherent wavepackets, observable as oscillatory signals in pulsed spectroscopy techniques such as transient–absorption spectroscopy. Measurements of molecular samples in the condensed phase with pump pulses shorter than vibrational period have yielded time-domain oscillations that have been analyzed by Fourier transformation into the frequency domain, which produce amplitude and phase profiles of each oscillatory signal as a function of detection wavelength. Many such measurements of monomers have produced a sharp amplitude node and a π phase shift, which both appeared at or near the detection wavelength corresponding to the maximum of the steady-state fluorescence signal. These curious and commonplace spectral signatures were initially described theoretically using Gaussian wavepacket dynamics, which yielded an interpretation that was difficult for nonspecialists to conceptualize. Recently, a complementary view of the wavepacket dynamics has emerged that uses a restricted basis set to produce an interpretation that is readily conceptualized by nonspecialists. This insight has since expanded to become a general theoretical framework—referred to as femtosecond coherence spectroscopy (FCS)—that can incorporate a wide variety of theoretical models of intramolecular vibrations and even the vibronic-exciton states of molecular aggregates. This review summarizes many of the microscopic models used to describe FCS features in different molecular systems, where multiple groups have contributed to and applied the FCS framework. Researchers have studied numerous conjugated organic dye molecules, biological pigment–protein complexes, and inorganic molecular clusters. The FCS methodology will aid researchers as they seek to understand nonadiabatic wavepacket dynamics at or near conical intersections, interpret singlet-fission dynamics, examine energy transfer mechanisms, and study many other contemporary research topics in chemical physics.