The origin of STM contrast differences for inequivalent S atoms on a Mo(100) surface

The theoretical electron scattering quantum chemistry (ESQC) method is used to explain the origin of contrast in scanning tunneling microscopy (STM) images of the S-c(2 x 4) structure on Mo(100). We show that the calculated images are very sensitive to the presence of surface reconstructions, and the theory can predict their effect on the STM images. The unit cell of the S-c(2 x 4) structure contains three S atoms in four-fold coordination sites. However, the STM images of this surface show only one maximum per unit cell at the position of one of the S atoms, while over the other two atoms the tunneling probability is a minimum. This result is surprising in view of the small 0.22 Angstrom height difference between these S atoms found in a recent LEED study. Our theoretical analysis revealed that: (i) the experimental STM images can be reproduced using the atomic coordinates of the reconstructed Mo surface determined by LEED, but they cannot when the bulk termination coordinates are used instead; (ii) the reduction of tunneling probability over two of the S atoms is due to destructive interference between channels involving orbitals at sulfur atoms in adjacent sites, and (iii) these channels involve states that include a bonding combination of d Mo orbitals and S 3p(z) orbitals. The anti-phase arrangement of the 3p(z) orbitals in adjacent S atoms explains the destructive interference effect and is related to their repulsive interaction.