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Electric-dipole sum-frequency generation spectroscopy is a coherent second-order optical probe. It is sensitive to appropriately oriented, symmetry-allowed molecular responses at the interface; randomly oriented populations can cancel in the orientational average, and centrosymmetric arrangements give no net electric-dipole second-order response.

Thus an ordered fraction of surface-bound carboxylate or hydrocarbon groups may dominate the measured spectrum, while disordered, differently oriented or weakly active adsorbates contribute little. The observed vibrational intensity is not a direct count of every molecule retained in an adsorption measurement. Selection rules, polarization and interference between contributions also matter.