Theoretical Model of Femtosecond Coherence Spectroscopy of Vibronic Excitons in Molecular Aggregates
Document Type
Article
Publication Date
4-28-2024
Abstract
When used as pump pulses in transient absorption spectroscopy measurements, femtosecond laser pulses can produce oscillatory signals known as quantum beats. The quantum beats arise from coherent superpositions of the states of the sample and are best studied in the Fourier domain using Femtosecond Coherence Spectroscopy (FCS), which consists of one-dimensional amplitude and phase plots of a specified oscillation frequency as a function of the detection frequency. Prior works have shown ubiquitous amplitude nodes and π phase shifts in FCS from excited-state vibrational wavepackets in monomer samples. However, the FCS arising from vibronic-exciton states in molecular aggregates have not been studied theoretically. Here, we use a model of vibronic-exciton states in molecular dimers based on displaced harmonic oscillators to simulate FCS for dimers in two important cases. Simulations reveal distinct spectral signatures of excited-state vibronic-exciton coherences in molecular dimers that may be used to distinguish them from monomer vibrational coherences. A salient result is that, for certain relative orientations of the transition dipoles, the key resonance condition between the electronic coupling and the frequency of the vibrational mode may yield strong enhancement of the quantum-beat amplitude and, perhaps, also cause a significant decrease of the oscillation frequency to a value far lower than the vibrational frequency. Future studies using these results will lead to new insights into the excited-state coherences generated in photosynthetic pigment–protein complexes.
Copyright Statement
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in:
Rode, A.J., Arpin, P.C., & Turner, D.B. (2024). Theoretical Model of Femtosecond Coherence Spectroscopy of Vibronic Excitons in Molecular Aggregates. The Journal of Chemical Physics, 160(16), 164101,
and may be found at https://doi.org/10.1063/5.0200570
Publication Information
Rode, Alexander J.; Arpin, Paul C.; and Turner, Daniel B.. (2024). "Theoretical Model of Femtosecond Coherence Spectroscopy of Vibronic Excitons in Molecular Aggregates". The Journal of Chemical Physics, 160(16), 164101. https://doi.org/10.1063/5.0200570