Theory of doubly resonant infrared-visible sum-frequency and difference-frequency generation from adsorbed molecules

We theoretically analyze doubly resonant infrared-visible sum-frequency (DR IVSFG) and difference-frequency generation (DR IVDFG) from a monolayer of adsorbates at an interface. Our calculated results with a model molecule indicate that the resonant amplitude of nonlinear optical susceptibility of DR IVDFG and DR IVSFG processes conveys information about the electron-vibration coupling in adsorbates. Moreover, owing to a dephasing-rephasing procedure involved, DR IVDFG can also be developed into a sensitive probe for investigating the coherent phase relaxation process in adsorbed molecules without the complication of inhomogeneous broadening.