Molecule vibrations at ångström-scale resolution
Image: Professor V. Ara Apkarian (right) and Joonhee Lee with the femtosecond titanium sapphire laser used in their tip-enhanced Raman spectromicroscopy experiments; the STM sits in the background [Steve Zylius/UCI]
Using tip-enhanced Raman spectromicroscopy in a high vacuum, low temperature scanning tunnelling microscopy, researchers from the University of California, Irvine, have imaged molecular vibrations at ångström-scale resolution, observing modes only previously seen in computational models.
By positioning an atomically-sharp silver STM tip above a molecule and using laser light to amplify the molecule's signal, Professor V. Ara Apkarian from the Center for Chemistry at the Space-Time Limit (CaSTL) and colleagues, recorded the vibrational spectra and studied how the charges and currents bonding the atoms governed the molecular vibrations.
“From structural changes in chemistry to molecular signalling, all dynamical processes in life have to do with molecular vibrations, without which all would be frozen,” says Apkarian, Director of CaSTL. “We’ve long been aware of these vibrations... and have been measuring their frequencies through spectroscopy, but only now have we been able to see what is moving and how.”
As part of their experiment, the researchers fixed a cobalt-based porphyrin molecule to a copper substrate; porphyrins play a critical role in respiration and photosynthesis.
They then interrogated the molecule with the nanoscopically smooth STM tip, while focusing the electromagnetic field of laser light onto the tip to enhance Raman scattering, recording frequency differences within the molecule.
Light was focused to the size of an atom at the silver tip of a STM and, aided by a laser beam, used to visualize the inner workings of a molecule. [CaSTL/UCI]
“To date, molecular vibrations have been pictorially explained using wiggling balls and connecting springs to represent atoms and bonds,” says CaSTL researcher, Joonhee Lee. “Now we can directly visualise how individual atoms vibrate within a molecule.”
“The images we provide will appear in textbooks to help students better understand the concept of vibrational normal modes, which until now had been a theoretical concept,” he adds.
The researchers will now refine measurements of electrical fields within molecules in order to detect where atoms are missing from molecular structures, and also use quantum interference principles to characterise even finer details.
“This National Science Foundation-supported team reached a major milestone by overcoming impossible barriers to develop a new instrument to ‘see’ the individual atoms of a molecule in real time and space,” highlights Kelsey Cook, NSF chemistry program director. “This invention will lead to unprecedented, transformational understanding of how molecules react and cells function.”
Research is published in Nature.