In situ IR reflectance absorption spectroscopy studies of pyridine adsorption at the Au(110) electrode surface

In situ subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the adsorption of pyridine at the Au(110) electrode surface. The IR spectra provide direct spectroscopic evidence that pyridine molecules adsorb, bonded through the nitrogen atom to the Au(110) surface, in the whole range of electrode potentials. IR spectroscopy was further used to study the effect of the electrode potential on the tilt angle (angle between the C2 axis of the molecule and the direction normal to the surface) of the adsorbed molecules. The IR data consistently show that the tilt angle decreases progressively with increasing electrode potential (charge on the metal). The change of the tilt angle may be described in terms of the electric field–dipole interaction and is caused by the dielectric saturation phenomenon. At low charge density, where the field is small, the film of N-bonded pyridine molecules is less rigid and the molecules exhibit a waving motion. This motion is restrained significantly in the presence of a large electrostatic field (high charge density at the metal). The monolayer of adsorbed molecules can be considered as being effectively frozen at sufficiently high charge densities.