Dipole Moment Reversal in a Polar Organic Monolayer Probed by Sum and Difference Frequency Spectroscopy

We have investigated the adsorption of pyridine on Cu(110) in ultrahigh vacuum through 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 because of the large pyridine dipole. We performed density functional theory (DFT) calculations that provided the change in dipole moment 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, it 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...