Recent advancements in femtosecond laser technology and measurement have brought about new and exciting opportunities in exploring photochemical processes including photoinduced phase transitions (PIPTs) in molecular crystals. One such PIPT is the Spin-Peierls (SP) phase transition in one dimensional molecular crystals, in which the interaction between lattice distortions with the electron spin of a monomer drives charge transfer upon photoexcitation. Characterization of SP phase transitions in molecular crystals has been the subject of many previous transient reflectivity measurements that have shown coherent oscillations in the excited state time-resolved reflectivity. While these studies do suggest a coupling between low frequency optical phonon modes and the exciton, a detailed description of the vibrational wavepacket dynamics associated with the charge transfer state potential energy surface has not been completely fleshed out. Specifically, a description of the exciton transfer dynamics with respect to both the low frequency optical phonons and localized intramolecular vibrations has not been explicitly measured. Uncovering these details requires a vibrational spectroscopy approach which can probe coherent oscillations in the electronic excited state associated with both electronic and nuclear phenomena. To this end, we have conducted a study in which we combine femtosecond stimulated Raman spectroscopy (FSRS) and pulse shaping to characterize and modulate the coherent nuclear wavepacket phenomena in the charge transfer state associated with SP phase melting in potassium tetracyanoquinodimethane (KTCNQ) molecular crystals. Our FSRS spectra of these crystals reveal quantum beats in the time resolved spectra, indicative of coupling between low frequency phonon modes and high frequency molecular vibrational modes that describe the potential energy surface associated with the photoinduced melting of the SP phase. Further, we show the ability to control this phase transition through precise phase shaping of our photoexcitation pulses that can amplify and diminish the relevant optical phonon oscillations and thus influence charge transfer. The results of this study demonstrate our ability to both characterize and manipulate SP phase transitions in KTCNQ crystals and provide a method for investigating exciton dynamics and exciton-vibrational interactions involved with relevant photochemical phenomena in molecular crystals and assemblies.

Characterization and manipulation of vibrational wavepacket motion on the potential energy surface of spin-Peierls melting using femtosecond stimulated Raman spectroscopy

256th National Meeting (2018)

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