Detecting Centrosymmetric Molecular Ions at an Interface with Vibrational Sum Frequency Generation Spectroscopy

Under the electric dipole approximation, second-order nonlinear optical phenomena, such as vibrational sum frequency generation (vSFG) and second harmonic generation (SHG), are forbidden in centrosymmetric environments, making them surface-specific processes where the centrosymmetry is inherently broken and suitable for studying interfacial chemistry. Although the underlying theory of vSFG/SHG pertaining to centrosymmetry has been extensively studied, fundamental questions remain to be addressed. For example, what leads to the breaking of centrosymmetry in a centrosymmetric molecule at interfaces? Is the interface capable of perturbing the environment of a centrosymmetric molecule, rendering it vSFG active? At what point does the centrosymmetric molecule lose its centrosymmetry to become vSFG active? In this work, we address such basic questions by studying the vSFG response from the azido stretch of N₃^-, a centrosymmetric molecule, at the α-Al₂O₃(0001)/H₂O interface. We observed the azido asymmetric stretch vSFG response at the aqueous alumina interface. There are several possible mechanisms that render this centrosymmetric molecule vSFG active, including dipole and quadrupole mechanisms. Therefore, we analyzed the possible vSFG mechanisms of the azide signal via electronic structure calculations and AIMD simulations. We show that a uniaxial electric field acting on centrosymmetric azide, whose modes are either IR or Raman active but never both, can make modes simultaneously IR and Raman active, but this turns out not to be enough to explain the sum frequency activity that we observe. The electronic structure calculations revealed that azide has sufficiently large quadrupolar susceptibility, which provides the dominant contribution in the azide vSFG response.