Ultrafast Continuum Mid-Infrared Spectroscopy of Hydrogen-Bonded Dimers

Book excerpt: Hydrogen-bonded systems are ubiquitous in nature, where they provide structure and pathways for energy dissipation. Cyclic, hydrogen-bonded interfaces are capable of mediating proton transfer, but these structures have broad and complex vibrational spectra. To study these vibrational features, an ultrafast continuum midinfrared (CIR) laser pulse has been incorporated as the probe pulse in several vibrational spectroscopies used to study the vibrational dynamics and proton transfer of cyclic, hydrogen-bonded dimers. Unlike traditional ultrafast vibrational spectroscopy, which is limited to a few hundred cm-1 of bandwidth in a single experiment, ultrafast mid-infrared continuum spectroscopy allows vibrational dynamics and coupling to be observed across the full vibrational spectrum. The vibrational dynamics of the 7-azaindole- acetic acid heterodimer were studied with mid-infrared pump-CIR probe and two dimensional infrared (2D IR) spectroscopy, which revealed strong coupling across the spectrum and very fast energy transfer across the bridging hydrogen bonds. Additionally, photoinduced proton transfer was studied in the 7-azaindole homodimer with preliminary UV pump-CIR probe experiments, which showed the formation of the doubly proton-transferred tautomer and spectral signatures of proton transfer. Further development of ultrafast mid-IR spectroscopy was explored with the generation of high energy continuum mid-IR pulses in bulk chalcogenide glass.