The application of step-scan interferometry to two-dimensional infrared (2D IR) spectroscopy is described. In this 2D FT-IR experiment, a step-scan interferometer is used to study a system undergoing dynamic changes induced by an external perturbation. Because step-scanning removes the spectral multiplexing from the temporal domain, the time dependence of the sample response to the perturbation can be retrieved more conveniently, in comparison to conventional rapid-scan techniques. Time-resolved IR data are then converted to 2D IR correlation spectra. Peaks located on a 2D spectral plane provide information about interactions among various functional groups associated with the IR bands. In the step-scan mode, the FT-IR multiplex advantage is retained; thus, spectral regions far removed from each other can be correlated with the use of 2D analysis from a single scan. 2D FT-IR spectra for a composite film of isotactic polypropylene and poly(γ-benzyl-L-glutamate) subjected to a small-amplitude sinusoidal strain are presented. The 2D FT-IR spectra clearly differentiate bands arising from the polyolefin and polypeptide. Overlapped bands are deconvoluted into individual components on the 2D spectral plane due to their different dynamic behavior. The applicability of step-scan 2D FT-IR to a variety of dynamic experiments is discussed.
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