Heterodyne-detected electronic sum frequency generation: “Up” versus “down” alignment of interfacial molecules

Heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy is newly developed to obtain complex electronic chi(2) spectra of interfaces for a simultaneous detection bandwidth broader than 100 nm. HD-ESFG provides linear chi(2) spectra that have unambiguous information on the "up" versus "down" alignment of interfacial molecules. It is demonstrated for p-nitroaniline, a prototypical molecule of nonlinear optical materials, that the up versus down alignment at an air/fused silica interface is critically influenced by a fine modification of the molecule.