Improved infrared spectroscopy technique to resolve rotational peaks in gas phase spectra

Infrared spectroscopy is widely used for identifying molecules and determining their structure, but the conventional one-dimensional technique does not work well with mixtures since peaks may overlap and become indistinguishable.

Two-dimensional infrared (2D IR) spectroscopy was developed to overcome this limitation. Peaks that overlap in one dimension can sometimes be resolved when spread out in the orthogonal direction. However, the resolution has been insufficient for resolving rotational peaks in gas phase spectra.

Daniels et al. described a method for rotationally resolved 2D IR spectra in the gas phase using a combination of narrowband and broadband nanosecond laser pulses and a method known as Doubly Vibrationally Enhanced (DOVE) spectroscopy. The output beam is easily detected since it is in the visible region.

“This method allows us to scan over regions of the infrared that contain large numbers of peaks while recording a 2D spectrum containing a smaller subset of coupled peaks in just a few tens of minutes,” said author Peter Chen.

The investigators tested their approach on a sample of methane, which produced a characteristic asterisk-shaped pattern.

“Within each asterisk, all the peaks have the same vibrational quantum number but different rotational quantum numbers,” Chen said. “And the technique automatically sorts those peaks by rotational quantum number. We expect to see additional kinds of patterns from other molecules with different geometries.”

The method can be used at any temperature, which will allow its use in applications such as analyzing chemical reactions or spectra from distant planetary atmospheres.

Source: “High resolution two-dimensional infrared (HR-2DIR) spectroscopy of gas phase molecules,” by DeAunna A. Daniels, Thresa A. Wells, and Peter C. Chen. Journal of Chemical Physics (2022) The article can be accessed at https://doi.org/10.1063/5.0109084 .

This paper is part of the Celebrating 25 Years of Two-dimensional Infrared (2D IR) Spectroscopy Collection, learn more here .