Quantum Tunneling of Carbon Monoxide in Molecule Cascades

Abstract

We combine atom-manipulation, in-situ isotope selection, and molecule cascades to study the quantum tunneling of molecules bound to a surface. The hopping rate of individual CO molecules was measured with scanning tunneling microscopy between 0.5K and 10K.We find hopping rates that are independent of temperature below 6K and exhibit a pronounced isotope effect, hallmarks of quan- tum tunneling. This hopping rate can be tuned over several orders of magnitude by tailoring interactions with neighboring molecules. At higher temperatures we observe thermally-activated hopping with an anomalously low Arrhenius prefactor that we interpret as tunneling from excited vibrational states.