Modification of electron density in surface states: standing wave observation on Pd overlayers by STM

Our recent attempts for modifying electron density in the surface states of Au(1 1 1) and Cu(1 1 1) surfaces are presented. Using a low-temperature scanning tunneling microscopy electronic standing wave patterns are observed on monolayer-high Pd layer deposited on those surfaces. Longer wave length of the oscillatory structure on the Pd layer than that of the substrates indicates upward shift of an energy level of the surface states and reduced electron density in the states on the overlayer. The observed energy level variation is explained with the phase shift analysis.