Nanoscale interference effect in sum frequency generation from Langmuir–Blodgett fatty acid films on hydrophobic gold

Langmuir–Blodgett (LB) multilayer films of the cadmium salts of per-deuterated arachidic acid (CD3(CD2)18COOH) with a top layer of per-protonated cadmium arachidate have been investigated in the C–H stretching region by sum frequency generation (SFG) vibrational spectroscopy. The per-deuterated film was used to spatially separate the SFG active per-protonated top layer from the underlying gold surface. The phase of the symmetric methyl stretching mode (r+) was constant with film thickness, while that of the asymmetric stretching mode (r−) varied linearly with increasing film thickness. A periodic phase/film thickness relationship was observed with a periodicity of 164 ± 7 nm, in satisfactory agreement with the recently developed theoretical model.