= Solution
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 $1s\to\pi^*$ transition is directed along the normal to the <adsorbate>'s aromatic plane. Consequently \b[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 $I(\theta)\propto\cos^2\theta\cos^2\alpha+\tfrac12\sin^2\theta\sin^2\alpha$. 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.
Back to article page
