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43390-AC6
A Time Correlation Function (TCF) Theory of 2DIR Spectroscopy with Applications to Chemical Systems and Peptides
The major thrust of our efforts was developing TCF theories of nonlinear spectroscopies. However, constructing a TCF theory of 2D Raman or 2DIR spectroscopies is challenging because the relevant response function cannot be written exactly in terms of a single TCF. The response functions that are probed in these spectroscopies can be written as sums and differences of quantum mechanical multitime TCF's that are all equivalent classically. Thus, the response functions consist of higher order hbar contributions from the quantum TCF's that would appear to not be amenable to classical simulation. However, based on the striking success of our semiclassical (quantum corrected classical) TCF approach to SFVS and linear spectroscopy, it was desirable to develop a semiclassical MD/TCF theory of 2D Raman and 2DIR spectroscopies that can benefit from the many body molecularly detailed description inherent in such an approach. Also, alternative approaches, while often effective in their own way, usually rely on reduced descriptions of the system (and bath) that lack a detailed molecular interpretation. Our TCF approaches have been proven quite effective.
We also developed a distributed hyperpolarizabilty model for use in calculating third order effects at charged interfaces. This serves as input to our novel TCF theory of such effects. To estimate these contributions, it is convenient to consider systems that do not have interfacial charged species, but rather have an externally applied static field in Electric Field Induced SFG/SHG/DFG studies because these are easier to quantify. This static field is generally on the order of 50kV/cm. In such cases, third order effects typically are responsible for up to 20% of the observed signal. For silica/water systems, the local static field strength near the interface is on the order of up to 10,000 kV/cm and silica/solid interfaces have been shown to produce relatively significant third order Electric Field Induced SFG/SHG/DFG effects.
Collectively, this underscores the importance of assessing third order contributions to the SFVS spectra for systems such as silica/water where there is ordering and/or distinct charged interfacial species, and thus a competition between enhanced second order effects and genuine third order contributions. The longer range induced liquid anisotropy raises the question of the origin of the SFVS signal, and we can directly assess the relative contributions using our methods. Thus, we are pursuing studies of charged interfaces in collaboration with experimentalists to quantify the magnitude and information content of these separate phenomena. Silica / water is an ideal choice because of its inherent importance and tractability from both an experimental and theoretical perspective.
