Researchers have developed a method for 2D IR spectroscopy capable of resolving a gas-phase sample’s myriad needle-thin lines. The improved resolution was due to two factors.
First, the researchers used a frequency-domain rather than standard time-domain methods for generating a 2D spectrum. In standard time-domain methods, light pulses hit the sample sequentially, and the precision of the successive time delays limits the resolution. In a frequency-domain method, the resolution is determined by laser line widths and monochromator resolution.
Second, λ4 – one of the wavelengths they used, plotted on the horizontal axis in the spectrum above – was in the visible regime rather than the IR. Visible light can be detected with sub-picometer spectral resolution and extremely high sensitivity. Because of the nonlinear optical process that the researchers excited in the sample, they probed rovibrational transitions (frequencies around 6000 cm-1 or wavelengths around 1667 nm) while measuring blue light with wavelengths around 460 nm.
The researchers used methane for their proof-of-concept demonstration (2D IR spectroscopy) because its structure and rovibrational spectrum are well understood. However, they believe that the same method can be easily applied to other gases, such as mixtures, floppy molecules, and ongoing chemical reactions, that were previously out of reach of IR spectroscopy.
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