Vibrational coherence transfer illuminates dark modes in models of the FeFe hydrogenase active site
Within the conceptual framework of Redfield theory, the optical response function arises from the dynamical evolution of the system’s density operator, where nonunitary relaxation is encoded in the Redfield relaxation superoperator. In the conventional approach, the so-called secular approximation neglects terms that induce transitions between distinct coherences and among coherences and populations. The rationale is that these nonsecular terms are small in comparison to the far more dominant population relaxation and coherence dephasing contributions. Since two-dimensional infrared (2D-IR) spectroscopy has significant contributions arising from population relaxation and transfer pathways, it can be challenging to isolate signatures of the nonsecular relaxation. We report om this paper that in three diiron dithiolate hexacarbonyl complexes that serve as small-molecule models of the [FeFe] hydrogenase H-cluster subsite, a fortuitous vibrational energy structure enables direct and clear signatures of vibrational coherence transfer in alkane solution. This finding holds promise towards developing a molecularly detailed understanding of the mechanism of vibrational coherence transfer processes, thanks to the ease of synthesizing derivatives based on the chemical modularity of these well studied diiron compounds. In addition to the fundamental need to characterize coherence transfer in molecular spectroscopy, we find in this set of molecules a practical utility for the nonsecular dynamics: the ability to determine the frequency of an IR-inactive mode. A coherence generated during the waiting time of the 2D-IR measurement transfers to a coherence involving the single dark CO stretching mode, which modulates some peak amplitudes in the 2D spectrum, revealing its transient excitation.
- Research Organization:
- Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE; National Science Foundation (NSF)
- Grant/Contract Number:
- AC05-00OR22725
- OSTI ID:
- 1615780
- Journal Information:
- Journal of Chemical Physics, Vol. 151, Issue 5; ISSN 0021-9606
- Publisher:
- American Institute of Physics (AIP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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Ultrafast molecular sciences by femtosecond photons and electrons
Atomic and molecular spectroscopy
Fourier transform spectroscopy
Infrared spectroscopy
Feynman diagrams
Vibrational spectra
Multidimensional spectroscopy
Transition moment
Density-matrix
Redfield theory