Understanding the Hopping Mechanism of Molecule Cascades at Very Low Temperatures

We use a novel low‐temperature scanning tunneling microscope to study the motion of individual CO molecules in molecule cascades on a copper (111) surface. The microscope has a base temperature of 1K in continuous circulation of 3He and 0.5K in a single shot mode with a hold time of about 10 hours. The initial liquefaction of 3He is achieved by utilizing Joule‐Thomson expansion of 3He gas which eliminates the need for a pumped 4He reservoir. Samples can be prepared and transferred into the microscope in ultra high vacuum. We found a strong dependence of the hopping rate on the carbon isotope, which leads us to the conclusion that CO molecules tunnel from initial to final state at temperatures below 6K. At higher temperatures a thermally activated process contributes to the hopping rate and several models for this process are discussed.