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High-resolution 2D spectroscopy shows molecules in a new light

20 October 2022

Complicated gaseous samples can now be analyzed in detail.

A gas-phase IR spectrum is a treasure trove of information. The IR region is home to molecular vibrational transitions, which couple to lower-energy quantized molecular rotations. From the resulting rovibrational peaks, researchers can learn a lot about molecular bonds and structures. And because the peaks from a gas-phase sample aren’t broadened by intermolecular interactions, as they would be in a liquid or a solid, they can be crisply resolved.

But even simple molecules can have overwhelmingly complicated rovibrational spectra. Methane, with just five atoms, has a whopping 90 000 known rovibrational peaks. Researchers can often simplify a spectrum by cooling the sample to reduce the number of thermally populated starting states. If that’s not possible—or if it doesn’t simplify the spectrum enough—another option is to spread the spectrum out over two frequency dimensions.

The tools of 2D spectroscopy were developed decades ago for NMR and adapted more recently for other spectral regions (see the article by Steven Cundiff and Shaul Mukamel, Physics Today, July 2013, page 44). In general, a peak’s two frequency coordinates correspond to transitions that are connected in some way; in 2D IR spectroscopy, they represent molecular vibrations that are coupled. By uncovering those connections, researchers can more easily determine which peaks correspond to which transitions.

Most 2D spectroscopy techniques are tailored to condensed-phase samples, whose broadened peaks don’t need to be finely resolved. Now DeAunna Daniels, Thresa Wells, and Peter Chen, of Spelman College in Atlanta, Georgia, have developed a method for 2D IR spectroscopy that’s capable of resolving the myriad needle-thin lines of a gas-phase sample.

This IR spectrograph with two wavelength coordinates has a sharp peak of intensity at nearly 461 nm across the range from 3206 nm to 3474 nm.
Credit: Adapted from D. A. Daniels, T. Wells, P. C. Chen, J. Chem. Phys. (2022), doi:10.1063/5.0109084

Two factors contributed to the improved resolution. First, instead of the usual time-domain methods for generating a 2D spectrum—in which light pulses hit the sample sequentially and the resolution is limited by the precision of the successive time delays—the researchers used a frequency-domain method, whose resolution is determined by laser linewidths and monochromator resolution.

Second, one of the wavelengths they used—λ4, plotted on the horizontal axis in the spectrum above—lay not in the IR but in the visible regime. Visible light can be detected with extremely high sensitivity and with subpicometer spectral resolution. Because of the nonlinear optical process that the researchers excited in the sample, even though they were measuring blue light with wavelengths around 460 nm, they were probing rovibrational transitions with frequencies around 6000 cm−1, or wavelengths around 1667 nm.

For their proof-of-concept demonstration, the researchers used methane, whose structure and rovibrational spectrum are well known. But they anticipate that the same method can be readily applied to other gases—including mixtures, floppy molecules, and chemical reactions in progress—that have previously been beyond the reach of IR spectroscopy. (D. A. Daniels, T. Wells, P. C. Chen, J. Chem. Phys., 2022, doi:10.1063/5.0109084.)

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