2D IR Spectroscopy

Two-dimensional infrared spectroscopy, or 2D IR spectroscopy, is an exciting technique with many advantages over standard linear infrared spectroscopies like infrared absorption spectroscopy and FTIR spectroscopy. In 2D IR, infrared spectra are spread into a second dimension, providing information on vibrational couplings and separating the effects of homogeneous and inhomogeneous dynamics. This provides a powerful tool for studying molecular structures, environmental dynamics, and structural kinetics.

2D IR utilizes three excitation pulses to create a nonlinear polarization in the sample. Two pump pulses are used to excite the sample. The time delay between these pulses (τ) is scanned to obtain information about the dynamics of the system. At some time T after the pump pulses, a third pulse probes the sample, and the emitted field provides information about how the system has evolved. We detect the emitted field by heterodyning with a local oscillator to amplify the signal and obtain information about its phase.

Because overtone and combination bands are measured along with the fundamental transitions, 2D IR spectra contain information about vibrational mode anharmonicities. Spectra also exhibit cross peaks between coupled vibrational modes, which provide additional information useful for deducing structure (for example, whether two modes are in physical proximity to each other or what the relative angle between them is). 2D peak shapes also report on the frequency fluctuations of the vibrational modes, which are related to the dynamics of the environment.

Generating 2D IR pulse sequences and aligning a 2D IR spectrometer can be quite challenging, especially because mid-IR laser light is invisible to the naked eye. We have invented a mid-IR pulse shaper which simplifies alignment and allows pulse trains to be generated programmatically on a shot-to-shot basis. Femtosecond pulse shaping has led to dramatic advances in the control of physical phenomena. Based on work by Warren and coworkers who pioneered TeO2 AOM pulse shaping in the visible, we have built a pulse shaper from a germanium acousto-optic modulator (Ge AOM) that works directly in the mid-IR. The Ge AOM has a resolution and transmission efficiency comparable to commercially available visible shapers, but is capable of programmable phase and amplitude modulation between 2 and 18 μm. Our Ge AOM can generate shaped pulses with 50% throughput efficiency and with a resolution equivalent to 500 pixels with high phase stability. With our shaper the pulse sequence time-delays can be updated with every laser shot, leading to very fast data acquisition (0.2 s for a full 2D scan). This makes possible the acquisition of 2D IR spectra of rapidly evolving systems. The high phase stability of the shaper also affords us to implement a wide variety of different types of pulse sequences to take 2D IR spectra and phase cycling schemes to eliminate scatter from the spectra. Using the pulse shaper to acquire 2D spectra dramatically shortens data acquisition times for 2D spectra and allows us to follow irreversible kinetics (such as folding of amylid fibrils) and other fast processes in real-time.

Selected Publications
Petti, M. K.; Lomont, J. P.; Maj, M.; Zanni, M. T. Two-Dimensional Spectroscopy Is Being Used to Address Core Scientific Questions in Biology and Materials Science. J. Phys. Chem. B 2018, 122, 1771–1780.
Ghosh, A.; Ostrander, J. S.; Zanni, M. T. Watching Proteins Wiggle: Mapping Structures with Two-Dimensional Infrared Spectroscopy. Chem. Rev. 2017 , 117, 10726–10759.
Moran, S. D.; Zanni, M. T. How to Get Insight into Amyloid Structure and Formation from Infrared Spectroscopy. J. Phys. Chem. Lett. 2014, 5, 1984–1993.
Middleton, C. T.; Woys, A. M.; Mukherjee, S. S.; Zanni, M. T. Residue-Specific Structural Kinetics of Proteins through the Union of Isotope Labeling, Mid-IR Pulse Shaping, and Coherent 2D IR Spectroscopy. Methods 2010, 52, 12–22.