Detecting Centrosymmetric\nMolecular Ions at an Interface\nwith Vibrational Sum Frequency Generation Spectroscopy

Under the electric dipole approximation, second-order\nnonlinear\noptical phenomena, such as vibrational sum frequency generation (vSFG)\nand second harmonic generation (SHG), are forbidden in centrosymmetric\nenvironments, making them surface-specific processes where the centrosymmetry\nis inherently broken and suitable for studying interfacial chemistry.\nAlthough the underlying theory of vSFG/SHG pertaining to centrosymmetry\nhas been extensively studied, fundamental questions remain to be addressed.\nFor example, what leads to the breaking of centrosymmetry in a centrosymmetric\nmolecule at interfaces? Is the interface capable of perturbing the\nenvironment of a centrosymmetric molecule, rendering it vSFG active?\nAt what point does the centrosymmetric molecule lose its centrosymmetry\nto become vSFG active? In this work, we address such basic questions\nby studying the vSFG response from the azido stretch of N<sub>3</sub><sup>–</sup>, a centrosymmetric molecule, at the α-Al<sub>2</sub>O<sub>3</sub>(0001)/H<sub>2</sub>O interface. We observed\nthe azido asymmetric stretch vSFG response at the aqueous alumina\ninterface. There are several possible mechanisms that render this\ncentrosymmetric molecule vSFG active, including dipole and quadrupole\nmechanisms. Therefore, we analyzed the possible vSFG mechanisms of\nthe azide signal via electronic structure calculations and AIMD simulations.\nWe show that a uniaxial electric field acting on centrosymmetric azide,\nwhose modes are either IR or Raman active but never both, can make\nmodes simultaneously IR and Raman active, but this turns out not to\nbe enough to explain the sum frequency activity that we observe. The\nelectronic structure calculations revealed that azide has sufficiently\nlarge quadrupolar susceptibility, which provides the dominant contribution\nin the azide vSFG response.