Apparent Size of an Atom in the Scanning Tunneling Microscope as a Function of Bias

In order to determine how the sample density of states is reflected in topographic images obtained in the scanning tunneling microscope, the apparent size of an adsorbed atom is studied as a function of bias voltage for several different atoms. This quantity is found to reflect state-density features that lie well above the Fermi level, but it shows a striking absence of any trace of $d$-state peaks below the Fermi level.