Femtosecond mid-infrared spectroscopy of hydrogen-bonded systems: Advances in experimental and computational approaches

This dissertation describes significant experimental and computational advances in ultrafast mid-infrared (mid-IR) spectroscopy of hydrogen-bonded hydride-stretching vibrations. Sub-65 fs pulses tunable throughout the 2800–4600 nm spectral region are generated using a potassium niobate optical parametric amplifier with pulse compression, constituting durations of approximately four to five cycles of the pulse center frequency. This light source produces the shortest pulses to date with such broad tunability and provides the improvement in time resolution necessary for a comprehensive interrogation of H-bonded hydride stretch dynamics. Measurement of pulse profiles after propagation through solutions of HDO in liquid D₂O, with concentrations used in past nonlinear studies, clearly demonstrates that beat-like distortions of the incident fields occur due to optical density effects. These distortions are further characterized by a new application of the cross-correlation frequency-resolved optical-gating technique to determine the amplitude and phase of the signal electric fields. In order to model the propagation phenomena that these measurements demonstrate and thus include them in analyses of measured signals, a new implementation of the finite-difference time-domain (FDTD) method is developed. This new variant of FDTD incorporates a density matrix representation of the field-matter interaction, the standard model used in the ultrafast community, leading to a correlation function-based, semi-classical full-field simulation technique named the CF-FDTD method. The CF-FDTD algorithms developed in this dissertation allow any sum of correlation functions to be included in the simulation; this not only allows matching of the material with the results of nonlinear spectroscopy, but also permits an examination of the suitability of those functions by comparing simulations with experimental pulse propagation measurements. The propagated fields are found to be sensitive to the early-time behavior of the correlation function, and simulations using proposed correlation functions for the HDO OH-stretching mode yield distinct mid-IR fields with beat-like distortions (i.e., driving fields modified by molecular responses) after propagation. This body of work constitutes a significant advance in over-coming fundamental limitations in time resolution, spectral coverage, and signal analysis that burden the current practice of ultrafast mid-IR spectroscopy. Extensions to nonlinear spectroscopy and CF-FDTD algorithms are anticipated and discussed.