CAREER: From Quantum to Classical and Back: Bringing 2D Spectroscopy Insights into Focus

Purdue University, West Lafayette IN

Investigators

Abstract

Mike Reppert of Purdue University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to explore the interplay between quantum and classical coherence in two-dimensional (2D) spectroscopy. Two-dimensional infrared (2DIR) spectroscopy and two-dimensional electronic spectroscopy (2DES) use ultrafast laser pulses to monitor the motion of atoms and electrons on timescales faster than one trillionth of a second. Such experiments have the potential to provide unprecedented insight into fundamental biological processes such as protein folding and photosynthesis. However, the data they produce is often difficult to interpret, limiting their usefulness in real-world applications. Dr. Reppert and his research group are developing new theoretical tools to better understand the physical origins of 2DIR and 2DES signals (especially the distinct roles played by quantum and classical effects), paving the way for more transparent and efficient interpretation of 2D data. Newly developed 2DIR simulation methods will be made available to other researchers by incorporating them into an online application for protein 2DIR simulations. In addition, the Reppert group will work with K-12 educators to develop 2D acoustic experiments that demonstrate the working principles of 2DES and 2DIR measurements using audible signals. Together, these efforts are expand expand both accessibility to these spectroscopic tools and the application of these 2D methods to a broad class of problems in the chemical sciences. Despite the impressive array of quantum and semiclassical dynamics methods available for simulating two-dimensional (2D) optical spectra, a surprising gap remains in the 2D theory arsenal: no classical theory for 2D spectroscopy or systematic framework for classifying quantum effects has been thoroughly developed. On the one hand, this limitation leaves largely unexplored the potential numerical advantages of fully classical simulations; on the other hand, the inability to systematically classify quantum and classical effects obscures the physical origins of coherence in coupled molecular systems. To explore these issues, the Reppert group is working to develop a systematic, diagrammatic framework for disentangling quantum and classical contributions to coherent vibrational dynamics, beginning with the relatively simple case of 2D infrared (2DIR) spectroscopy and working toward more complex vibronic dynamics and the quantitative interpretation of Amide I (protein C=O stretch) 2DIR experiments. To test the limits of classical models for coherent nonlinear processes, a new class of 2D measurements investigating acoustic nonlinear response will be designed and implemented experimentally. The broader impact of this work will be maximized by developing educational materials (K-12 through graduate level) that illustrate nonlinear processes through conceptually approachable “ball and spring” models. In addition, the Reppert group will incorporate newly developed Amide I simulation methods into the AmideSpec app, offering a research-grade online tool for interpreting protein 2DIR data to researchers around the globe. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.