Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / Section B / 38 / e

The large resonance at the smallest field-to-normal angle identifies an unoccupied orbital whose transition dipole is mainly normal to the surface. An N 1s initial state is approximately spherical; the dipole matrix element for a $1s\to {\pi }^{\ast }$ transition is directed along the normal to the adsorbate's aromatic plane. Consequently the adsorbate rings lie predominantly parallel to the copper surface. For perfectly flat molecules the resonance strength scales as $|\mathbf{E}\cdot \hat{\mathbf{n}}{|}^2\propto {\cos }^2\theta$, which decreases strongly from $17^{\circ }$ to $67^{\circ }$ as observed.

For azimuthally disordered molecular normals tilted by $\alpha$ from the surface normal, averaging gives $I(\theta )\propto {\cos }^2\theta {\cos }^2\alpha +\frac{1}{2}{\sin }^2\theta {\sin }^2\alpha$. Strong dichroism therefore implies a large orientational order parameter, but the plotted qualitative curves alone do not determine a unique tilt angle or distribution. Background subtraction, incident polarization and intensity normalization would be needed for such a fit. This experiment identifies the orientation of the pi system; it does not by itself identify a particular nitrogen–copper bonding topology.