High-resolution vibrational spectrocopy

    Sum Frequency Generation Spectroscopy (SFG),  in which two photons hv₁ and hv₂ of frequencies w₁ and w₂ generate one photon of sum frequency (w₃=w₁+w₂), is one of second order nonlinear optical processes, which do not occur in media with inversion symmetry but take place only at the interface between those media where the inversion symmetry is necessarily broken. Briefly the SFG technique relies on the spatial and temporal overlap of two laser pulses. One of these is tunable in the IR, inducing coherence between vibrational levels, and the other one is in the VIS, inducing Raman transitions of the excited vibrational level to a virtual excited level that then relaxes to the fundamental state, releasing photons at the sum frequency. This technique allows the identification of the surface molecules, the determination of their molecular conformation and understands dynamics of energy transfer between: substrate-adsorbate, adsorbate –substrate or adsorbate –adsorbate at real time and can be used in different media such as solid/liquid, liquid/liquid and solid/gas interfaces (see Figure).

    Indeed we use this technique to understand the structure and bonding at water –oxide (FeO, Fe₃O₄, and SiO₂) interface. Since SFG is sensitive to molecular order of water at the interface, many parameters can play a role to monitor the structural response of water molecules at oxide-liquid interface, such as pH, electrical potential and chemical identity of adsorbates. Pump probe will be applied to understand the vibrational relaxation of adsorbates, and monitor vibrational energy flow between these species.