Direct observation of the quantum tunneling of single hydrogen atoms with a scanning tunneling microscope

Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunnel ing microscopy (STM). The vibrations of individual H and D atoms against th e surface were excited and detected by inelastic electron tunneling spectro scopy (STM-IETS). Variable temperature measurements of H atom diffusion sho wed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling we re observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4 x 10(-7) s(-1).