Mechanisms of Sum Frequency Generation from Liquid Benzene: Symmetry Breaking at Interface and Bulk Contribution

Liquid benzene emits a significant sum frequency generation (SFG) signal of C-H stretching vibration, though it is composed of centrosymmetric molecules. The present paper theoretically elucidates the SFG spectrum from liquid benzene by considering two mechanisms, symmetry breaking at the interface and bulk contribution. Molecular dynamics and quantum chemical calculations reproduced the observed SFG spectrum and revealed that the two mechanisms are equally significant in the SFG spectrum of liquid benzene. At the interface, the SFG signal arises from local C-H stretching modes via the mixing of IR active and Raman active modes. The local modes are readily induced by the anisotropic environment at the interface. The observed SFG signal should also involve significant amount of quadrupole contribution, which is attributed to the IR active mode of the bulk liquid.