Molecular Vibrations Imaged for First Time

Scientists at the University of California, Irvine (UCI), focused light down to the size of an atom to produce the first images of a molecule’s normal modes of vibration. The images could provide a better understanding of the concept of vibrational normal modes, which until now has been a theoretical concept.

“We’ve long been aware of these vibrations,” said professor V. Ara Apkarian. “For ages, we have been measuring their frequencies through spectroscopy — but only now have we been able to see what is moving and how.”

Professor V. Ara Apkarian (r), director of UCI’s Center for Chemistry at the Space-Time Limit (CaSTL), and research scientist Joonhee Lee stand over the femtosecond titanium:sapphire laser used in their experiments. The machine in the background is CaSTL’s ultrahigh-vacuum cryogenic scanning tunneling microscope. Courtesy of Steve Zylius/UCI.

The researchers set up their experiment in an extremely high-vacuum, low-temperature (6 K) environment to eliminate all external motions. They positioned the atomically terminated silver tip of a scanning tunneling microscope just angstroms from a cobalt-based porphyrin molecule affixed to a copper platform. They used tip-enhanced Raman spectromicroscopy (TER-SM) to show that angstrom-scale resolution could be attained at subatomic separation between the tip atom and a molecule. By prodding the molecule with the light confined on the silver atom, they were able to record vibrational spectra and observe how the charges and currents that bond atoms are influenced by molecular vibrations.

To ensure that the laser beam did not agitate the target molecule, the researchers froze the sample onto a copper substrate, binding the molecule to the copper. This caused the molecule to flatten, increasing its exposure to the probe.

The researchers moved the silver tip of the probe in relation to the sample to maintain a distance of about 2 Å. At this proximity, the researchers were able to record differences in frequencies at various positions within the molecule. The researchers said that the quality of the resolution they achieved came from quantum mechanical tunneling of plasmons.

“We have a microscope now that can resolve atoms, and we’re using it to look inside molecules, which was unthinkable only a few years ago,” Apkarian said. “The spatial resolution of optical microscopy has been advanced by another notch, and what we’re seeing at this scale is truly amazing.”

In addition to recording vibrational spectra within a single molecule, the researchers obtained images of normal modes and atomically parsed the intramolecular charges and currents driven by molecular vibrations.

The team plans to further refine its measurements of electrical fields within molecules, work to detect where atoms are missing from molecular structures, and use quantum interference principles to characterize 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,” said Kelsey Cook, NSF chemistry program director. “This invention will lead to unprecedented, transformational understanding of how molecules react and cells function.”

The research was published in Nature (https://doi.org/10.1038/s41586-019-1059-9).