The role of molecular dipole moment, charge transfer, and Pauli repulsion in determining the work-function change ($\Delta \Phi$) at organic-metal interfaces has been elucidated by a combined experimental and theoretical study of $({\mathrm{CH}}_3\mathrm{S}{)}_2/\mathrm{Au}(111)$ and ${\mathrm{CH}}_3\mathrm{S}/\mathrm{Au}(111)$. Comparison between experiment and theory allows us to determine the origin of the interface dipole layer for both phases. For ${\mathrm{CH}}_3\mathrm{S}/\mathrm{Au}(111)$, $\Delta \Phi$ can be ascribed almost entirely to the dipole moment of the ${\mathrm{CH}}_3\mathrm{S}$ layer. For $({\mathrm{CH}}_3\mathrm{S}{)}_2/\mathrm{Au}(111)$, a Pauli repulsion mechanism occurs. The implications of these results on the interpretation of $\Delta \Phi$ in the presence of strongly and weakly adsorbed molecules is discussed.

• Received 2 February 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.046804