Direct Observation of Wave-Function Variation Outside a Solid Surface

In an extension of its capabilities, ion-neutralization spectroscopy is used to determine transition density functions corresponding to ion neutralization at different ion positions. This provides the first direct measurement of the relative variation in the local density of states outside a solid surface. It is shown that the $d$ orbitals of Hg adsorbed on Si are a minor component of the total wave function at the ion position and decay more rapidly with distance outside the solid than does the $\mathrm{Si}p$ component.