Three types of nonlinear optical spectroscopies, ultrafast two-dimensional infrared (2DIR) spectroscopy, transient infrared (IR) absorption/pump-probe spectroscopy, and sum-frequency generation (SFG) vibrational spectroscopy, are used to investigate molecular structure and dynamics in two distinct classes of materials. First, 2DIR and pump-probe spectroscopies are used to study ultrafast rotational and vibrational energy relaxation in dense gaseous and supercritical fluid solutions, special solvation effects near the critical point, and the evolution of cooperative, liquid-phase dynamics as a function of density for two different solvent systems. 2DIR’s demonstrated capabilities offer a unique tool for identifying co-existing free rotor and liquid-like populations within the same fluid sample, evaluating the adequacy of isolated binary collision (IBC) relaxation descriptions in dense gas and near-liquid density fluids, and learning about how solute-solvent intermolecular properties separately influence rotational and vibrational relaxation in these dynamic and heterogeneous environments. Analysis of the density-dependent 2DIR and pump-probe spectra of a quasi-free rotor (asymmetric stretch rovibrational band of N2O) in SF6 and Xe provides timescales for rotational energy relaxation rates (1 – 3 collisions), but much slower vibrational energy relaxation rates. A critical slowing effect on the rate of rotational relaxation is identified, and liquid-like solvation is observed in dense gaseous solutions at state points lower than the critical density. Solvent-dependent differences in energy relaxation and IBC model breakdown, as well as applications of this 2DIR methodology to other high density and supercritical solution dynamics and descriptions are discussed. In a second nonlinear spectroscopy project, SFG is used to study the role of substrate type, gold or silver, and surface roughness on the parity odd-even effect in n-alkanethiolate (n = 10 – 16) self-assembled monolayers (SAMs), materials of potential importance to molecular scale electronics. SFG methyl vibrational transition intensities, frequencies, and linewidths display parity and metal dependence attributable to the orientational differences of the interfacial ethyl group, which inverts for SAMs on Ag substrates relative to SAMs on Au. Substrate roughness, an often-underreported experimental parameter, is shown here to affect the extent of odd-even methyl orientation anisotropy, and this SFG analysis establishes a new roughness limit for the appearance of odd-even effects on Ag substrates.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/45314 |
| Date | 04 November 2022 |
| Creators | Rotondaro, Matthew C. |
| Contributors | Ziegler, Lawrence D. |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Rights | Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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