SCANNING-TUNNELING-MICROSCOPY THEORY FOR AN ADSORBATE - APPLICATION TO ADENINE ADSORBED ON A GRAPHITE SURFACE

An expression is obtained for the current in scanning tunneling micros copy (STM) for a single adsorbate molecule. For this purpose the ''New ns-Anderson'' treatment (a ''discrete state in a Continuum'' treatment ) is used to obtain wave functions and other properties of the adsorba te/substrate system. The current is expressed in terms of the adsorbat e-tip matrix elements, and an effective local density of states of the adsorbate/substrate system, at the adsorbate. As an example, the trea tment is applied to the STM image of adenine adsorbed on a graphite su rface, and the results are compared with experiment. The dependence of the image on the position of adenine with respect to the underlying g raphite is considered. A discussion is given of the type of experiment al STM data needed for suitable comparison of theory and experiment. I n an analysis of the calculations, the role of each atom, its neighbor s, next nearest neighbors, etc., in an adsorbed molecule is considered . The need for using in the present calculation more orbitals than onl y the HOMO and the LUMO of the adsorbate is also noted.