New Method Provides a More Comprehensive Molecular Vibrational Fingerprint

When molecules vibrate, they make sounds, like most other objects. In fact, each molecule has its own unique tone, a vibrational “fingerprint” that reflects not only its chemical structure but also the nanoscale environment around it. Unfortunately, the frequencies are far beyond human hearing and traditional infrared spectroscopy captures only the aggregate signal from millions or billions of molecules, since single-molecule voices prove too faint.

Now, University of California San Diego researchers, led by Shaowei Li, have developed infrared-integrated STM (IRiSTM) to detect a single molecule's vibrations. This technique combines infrared excitation with scanning tunneling microscopy, a technique best known for imaging individual atoms and molecules by measuring the quantum tunneling of electrons between a sharp metal tip and a surface.

Chemists aspire to control reactions by depositing energy into a single bond, steering molecules along desired pathways, and single-molecule infrared spectroscopy advances this vision.

“Infrared spectroscopy is one of our most powerful tools, but until now it has always been an ensemble technique. This gives us a way to see, at the most fundamental level, how vibrational energy couples to molecular motion," said Shaowei Li, senior author of the paper published in Science.