Phase measurements of surface IR-visible sum frequency generation: Determination of polar orientation of molecules at surfaces

Infrared-visible sum frequency generation (SFG) can be used to obtain the vibrational spectrum of molecular monolayers at any interface accessible by light. However, because the polar orientation of the molecular group depends on the sign of the surface susceptibility, the phase of the SFG field must be measured to determine if the group is pointing up or down. Two techniques for accomplishing this are described. The first technique is based on the interference of the SFG field from the monolayer with the signal generated by the bulk of the substrate on which the monolayer is adsorbed. The second technique is applicable at any interface. It is based on the interference of the surface signal with the signal from a remote nonlinear crystal. If the polar orientation of the molecular group is known, the above techniques determine the phase of the molecular SFG polarizability for a particular normal mode. This phase gives the relative sign of the normal mode's IR and Raman polarizabilities.