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Tracking Ultrafast Charge Dynamics in Energy Materials with Atomic Specificity

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[Technical Report, Final Report]

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In this project, supported by the AFOSR Molecular Dynamics and Theoretical Chemistry Program, we mainly made major achievement. First, we pioneered a new area of ultrafast dynamics of molecular vibrational polaritons, quasiparticles between photons and molecular vibrations through strong coupling 1 we demonstrated the first two-dimensional infrared spectroscopy of molecular vibrational polaritons and developed appropriate theory for the spectra at longer waiting time 2 we showed that molecular vibrational polaritons can have large, tunable nonlinearities for quantum simulation applications and photonic circuitry 3 we showed that by selectively exciting lower polaritons, the molecular vibrations can be directly promoted to higher excited states 4 we enabled intermolecular vibrational energy transfer through molecular vibrational polaritons, as a general way to engineer intermolecular interactions in liquid phase, which was otherwise not available without strong coupling. These findings lay the fundamental understanding of molecular vibrational polariton dynamics and also opens new platform for photonic devices in mid-infrared and quantum simulation using molecular states. Second, we constructed a time-resolved EUV spectrometer, and used it to probe ultrafast polaron formation at surfaces of a-Fe203 and FeOOH, in the reflective geometry. We found that in addition to the resonant absorption having transient changes, the non-resonant imaginary refractive index was also reduced by the polaron formation. The decrease of non-resonant imaginary refractive index was due to the change of XUV photoemission cross-section upon the formation of a trapped charge-polaron. Using this observation, we compared the trapping depth relatively between Fe203 and FeOOH, which was not discussed in previous publications. Our work showed that the non-resonant index could also be changed during charge excitation and reflect important physical phenomena.

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  • Physical Chemistry

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[A, Approved For Public Release]