Strongly coverage-dependent excitations of adsorbed molecular hydrogen

We have observed prominent nonlinearities in differential conductance spectra of H 2 on copper surfaces using a low-temperature scanning tunneling microscope. These nonlinearities result from transitions between states of H 2 with distinct conductances. Tunneling electrons drive these transitions by giving up energy to highly coverage-dependent excitations that do not correspond to known vibrational or rotational modes of H 2. The nonlinear conductance features can be modeled by extending the conventional framework for inelastic electron tunneling spectroscopy to include saturation effects. ©2005 The American Physical Society.