Development of Advanced Heterodyne-Detected 2D IR Spectroscopy for the Study of Various Types of Samples

Abstract

Two-dimensional infrared (2D IR) echo spectroscopy, with high sensitivity and femtosecond time resolution, enables us to understand the structure and ultrafast dynamics of molecular systems. However, a broad application of this experimental technique on a variety of samples has been hampered by several technical problems. For example, the difficulty in the accurate phase determination of the echo signals of weakly absorbing samples prevents us from obtaining reliable spectra of the samples. For heterogeneous samples such as perovskite films or MOF powder, severe light scattering prevents us from obtaining high-quality 2D IR spectra of these samples. Here, I introduce a new experimental scheme that can overcome the two technical problems. With phase-resolved heterodyne-detected transient grating (PR-HDTG), the phase ambiguity issue is almost resolved. By using two choppers and two shutters, unwanted scattered light can be eliminated. As an example of the use of PR-HDTG, a study on the dynamics of azide ions (N3‒) in H2O is presented. The advantages of using two choppers and two shutters are demonstrated by a 2D IR study on a perovskite film having a very rough surface, containing a small amount of residual DMF, and having a maximum absorbance of ~0.0004 in the C=O stretch region.