Combining Raman and Infrared Spectroscopy

Researchers have built a new tool to study molecules using a laser, a crystal and light detectors. This new tech­nology will reveal nature’s smallest sculptures – the structures of molecules – with increased detail and specificity. “We live in the molecular world where most things around us are made of molecules: air, foods, drinks, clothes, cells and more. Studying molecules with our new technique could be used in medicine, pharmacy, chemistry, or other fields,” said Takuro Ideguchi from the Uni­versity of Tokyo Institute for Photon Science and Tech­nology.

Illustration of the complementary vibrational spectroscopy which relies on improvements in ultrashort pulsed laser technology. (Source: T. Ideguchi, CC BY-ND-4.0)

The new technique combines two current tech­nologies into a unique system – the complementary vibrational spectroscopy. All molecules have very small, distinctive vibra­tions caused by the movement of the atoms’ nuclei. Spectro­meters detect how those vibrations cause molecules to absorb or scatter light waves. Current spectro­scopy techniques are limited by the type of light that they can measure. The new comple­mentary vibra­tional spectro­meter can measure a wider spectrum of light, combining the more limited spectra of infrared absorption and Raman scattering spectro­meters. Combining the two spectro­scopy techniques gives researchers different and comple­mentary information about molecular vibrations.

“We questioned the common sense of this field and developed something new. Raman and infrared spectra can now be measured simul­taneously,” said Ideguchi. Previous spectro­meters could only detect light waves with lengths from 0.4 to 1 micrometer (Raman spectro­scopy) or from 2.5 to 25 micrometers (infrared spectro­scopy). The gap between them meant that Raman and infrared spectro­scopy had to be performed separately. The limi­tation is like trying to enjoy a duet, but being forced to listen to the two parts separately.

Complementary vibrational spectro­scopy can detect light waves around the visible to near-infrared and mid-infrared spectra. Advance­ments in ultrashort pulsed laser technology have made comple­mentary vibrational spectro­scopy possible. Inside the comple­mentary vibrational spectro­meter, a titanium-sapphire laser sends pulses of near-infrared light with the width of 10 femto­seconds towards the chemical sample. Before hitting the sample, the light is focused onto a crystal of gallium selenide. The crystal generates mid-infrared light pulses. The near- and mid-infrared light pulses are then focused onto the sample, and the absorbed and scattered light waves are detected by photo­detectors and converted simul­taneously into Raman and infrared spectra.

So far, researchers have tested their new technique on samples of pure chemicals commonly found in science labs. They hope that the technique will one day be used to understand how molecules change shape in real time. “Especially for biology, we use the term label-free for molecular vibrational spectro­scopy because it is noninvasive and we can identify molecules without attaching arti­ficial fluorescent tags. We believe that comple­mentary vibrational spectro­scopy can be a unique and useful technique for molecular measure­ments,” said Ideguchi. (Source: U. Tokyo)

Reference: K. Hashimoto et al.: Complementary vibrational spectroscopy, Nat. Commun. 10, 4411 (2019); DOI: 10.1038/s41467-019-12442-9

Link: Optical Science Lab, Dept. of Physics, University of Tokyo, Tokyo, Japan

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