Localization of Inelastic Tunneling and the Determination of Atomic-Scale Structure with Chemical Specificity

The orientation of individual $\mathrm{C}{}_2\mathrm{HD}$ molecules adsorbed on the Cu(100) surface at 8 K was determined from inelastic tunneling images obtained with a scanning tunneling microscope. The simultaneously recorded constant-current images showed that the deuterium end of the molecule appears $0.006Å$ lower than the hydrogen end, which was further substantiated by quantitative measures of the molecular rotation. Extension of the study to $\mathrm{C}{}_2\mathrm{HD}$ on Ni(100) revealed that the orientation of the molecule relative to the molecule's shape in the constant-current images contrasts sharply between the two surfaces.