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. An example of a 2D IR pulse sequence is shown in Figure 1(a). 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.

Figure 1. (a) A typical pulse sequence for a 2D IR experiment. (b-d) Spectra illustrating signatures of vibrational coupling (b, a cross-peak is present, indicated by a box), homogeneous dynamics (c, spectrum is round), and inhomogeneous dynamics (d, spectrum is elongated along the diagonal). Fundamental vibrational transitions (v=0->1) are shown in blue, and overtones (v=1->2) appear in red.

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 (Fig. 1b), 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 (Fig. 1c-d).

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, as shown in Figures 2 and 3 and as described in our page on mid-IR pulse shaping. 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.

Figure 2. "Boxcar" geometry for 2D IR spectroscopy. This geometry requires spatially and temporally overlapping four different mid-IR beams at the sample, and time-delays must be scanned manually.

Figure 3. Shaper-based 2D IR spectrometer. This geometry only requires overlapping two IR beampaths at the sample, and pump pulse time delays and phases can be updated programmatically on a shot-to-shot basis.