Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 11 / c

Nitrogen-down adsorption places the positive hydrogen end farther from the metal, producing an outward molecular electric dipole moment and the observed decrease in work function. Its maximum normal component, however, is only $1.42$ D. Tilting alone can only reduce that component; it cannot explain an effective $2.20$ D.

The adsorbed molecule and metal therefore undergo appreciable electron density. Lone-pair donation into the metal, polarization of the substrate, image response and distortion of the adsorbate can change the effective interface dipole. The inferred increase relative to an upright isolated molecule is about $0.78$ D. For scale, transferring about $0.16$ elementary charges over $1$ Å would produce that dipole increment:This scale illustrates a plausible interfacial polarization; the work-function measurement alone does not uniquely determine a transferred charge or adsorption geometry.