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A new method of broad-band heterodyne-detected sum frequency generation (HD-SFG) spectroscopy is developed and its high sensitivity is demonstrated, allowing measurements of vibrational spectra of submonolayers. The HD-SFG spectroscopy has provided the phase as well the amplitude of the signal and has allowed accurate subtraction of the non-resonant background.

Nonlinear surface-selective vibrational sum frequency generation (SFG) spectroscopy is applied to characterize the conformation of dodecanethiol on gold nanoparticle surfaces. The results have shown the relation between the nanoscale geometry, such as the surface curvature of spherical gold nanoparticles and the molecular conformation of the chemisorbed dodecanethiol ligand, and have illustrated the utility of the second-order nonlinear SFG spectroscopy as a sensitive probe of the molecular conformation, particularly for surfaces of metallic nanostructures.