Resonant Second Harmonic Generation Studies of p-Nitrophenol Adsorption at Condensed-Phase Interfaces

A comprehensive analysis of adsorption strength, average molecular orientation, and absolute molecular direction for p-nitrophenol at solid-air, solid-liquid, and liquid-air interfaces is obtained from the nearresonant optical second harmonic generation (SHG) from the interface. Perturbation theory calculations of the molecular nonlinear polarizability tensor elements using *-electron wave functions are used to identify the dominant tensor elements. The near-resonant SHG from a monolayer of p-nitrophenol is shown to require the use of two molecular nonlinear polarizability tensor elements in the equations relating the experimentally measured SHG to the average molecular orientation in the monolayer and therefore cannot be correctly described with the usual assumption of the complete dominance of a single molecular nonlinear polarizability tensor element. The average molecular orientation within the adsorbed monolayer is then obtained from the polarization dependence of the surface SHG and is described by the orientation parameter, D = (cos3 8 )I (cos e ) , where 8 is the angle between the molecular symmetry axis and the surface normal. A more realistic description of the molecular orientation is obtained by allowing for a partial Gaussian distribution in the angle 8. Additional measurements of the phase of the surface SHG are used in conjunction with the theoretical calculations of the molecular nonlinear polarizability to ascertain the direction of the transition dipole moment relative to the surface.