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In the past decades,sum-frequency vibrational spectroscopy(SFVS)has been well established as a powerful technique for surface studies.It is based on the idea that under the electric dipole(ED)approximation,SF generation originates only from the surface or interface where the inversion symmetry is broken.There is,however,always a lingering doubt that the electric-quadrupole(EQ)bulk contribution to surface SF generation may not be negligible.Despite many theoretical and experimental studies in the past,a comprehensive picture supported by experiment is still missing.