Visualizing vibrational normal modes of a single molecule with atomically confined light.

Abstract

The internal vibrations of molecules drive the structural transformations that underpin chemistry and cellular function. While vibrational frequencies are measured by spectroscopy, the normal modes of motion are inferred through theory because their visualization would require microscopy with ångström-scale spatial resolution-nearly three orders of magnitude smaller than the diffraction limit in optics¹. Using a metallic tip to focus light and taking advantage of the surface-enhanced Raman effect² to amplify the signal from individual molecules, tip-enhanced Raman spectromicroscopy (TER-SM)^3,4 reaches the requisite sub-molecular spatial resolution⁵, confirming that light can be confined in picocavities^6-10 and anticipating the direct visualization of molecular vibrations^11-13. Here, by using TER-SM at the precisely controllable junction of a cryogenic ultrahigh-vacuum scanning tunnelling microscope^14-16, we show that ångström-scale resolution is attained at subatomic separation between the tip atom and a molecule in the quantum tunnelling regime of plasmons^6,8,9,17. We record vibrational spectra within a single molecule, obtain images of normal modes and atomically parse the intramolecular charges and currents driven by vibrations. Our analysis provides a paradigm for optics in the atomistic near-field.