We investigate the adsorption of pyridine on Cu(110) in ultra-high vacuum with a combination of work function measurements and femtosecond infrared-visible sum and difference frequency generation (SFG/DFG). A monolayer of pyridine substantially reduces the work function by 2.9 eV due to the large pyridine dipole. We perform density functional theory (DFT) calculations that provide us with a dipole moment change upon adsorption in very good agreement with the experimental results. The pyridine dipole strongly enhances the sum frequency response of the surface electrons, but surprisingly reduces the surface difference frequency signal. We propose a model based on the static electric field-induced nonlinear optical response generated by the collective electric field of the adsorbate layer. The pyridine dipole switches direction from the ground to the excited electronic state, as charge moves from nitrogen to the ring. SFG can then be enhanced by the electric field of adsorbed pyridine in its ground electronic state, while the 2.33 eV incident photon in DFG excites electrons into the pyridine LUMO, which reverses the electric field in the adsorbate layer and reduces the nonlinear optical response. The model is verified by 2.33 eV pump – SFG probe spectroscopy, where the pump pulse is found to reduce the surface electron response on a subpicosecond timescale. This demonstrates the potential to manipulate the work function in organic electronic devices by photon-induced dipole moment reversal.
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