Tip-sample distance dependence in the STM-based orbital-mediated tunneling spectrum of nickel(II) tetraphenylporphyrin deposited on Au(111)

Orbital-mediated tunneling spectra obtained in the STM environment are reported for nickel(II) tetraphenylporphyrin, NiTPP, as a function of molecule-tip separation. Spectra were acquired over a range of tip motion of 0.42 nm. Spectra did not show the variation in band splitting with tip distance predicted by several models. It appears for molecules such as NiTPP that the average potential at the molecule is essentially the same as at the metal substrate, at least for gap resistance values greater than 500 MOmega. Thus, at least for molecules of the height of NiTPP, the STM-OMT spectra should give reliable occupied and unoccupied orbital energies over a wide range of tip-molecule distances. An unexpected small shift of all orbital energies of NiTPP relative to the vacuum level is observed as a function of tip-molecule separation.