When things vibrate, they make sounds. Molecules do too, but at frequencies far beyond human hearing. Chemical bonds stretch, bend and twist at characteristic rates that fall in the infrared region of the electromagnetic spectrum. Infrared spectroscopy, which measures how light excites these vibrations, is often likened to listening to a molecule's voice.
Each molecule has its own unmistakable tone – a vibrational "fingerprint" that reflects not only its chemical structure but also the nanoscale environment around it. But the voices of individual molecules are so faint that traditional infrared spectroscopy can only detect the collective chorus of millions or billions of molecules at once.
Now researchers at University of California San Diego, led by Shaowei Li, have found a way to hear a single molecule sing, using an approach they call infrared-integrated STM, or IRiSTM. It 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 have long dreamed of controlling reactions by depositing energy into a single bond, steering molecules along desired pathways, and single-molecule infrared spectroscopy brings that dream one step closer to reality.
"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 Assistant Professor of Chemistry Shaowei Li.
