Sum frequency spectroscopy of Langmuir-Blodgett fatty acid films on hydrophobic gold

Langmuir-Blodgett (LB) multilayer films of the cadmium salts of five long chain fatty acids (CnCOOH, n = 14, 16, 18, 20, and 22) deposited onto hydrophobic gold have been investigated in the C-H stretching region by sum frequency vibrational spectroscopy (SFS). The highly ordered multilayer films were found to be strongly SF active, although the spectra were complicated by the presence of an interference effect dependent upon the number of layers deposited. This interference effect was evident in the phase of the measured asymmetric methyl stretching mode (r(-)). Conversely, the phases of the symmetric stretching modes (r(+) and r(FR)(+)) were largely unaffected. The measured SF spectra were shown to arise from the summation of spectra from the uppermost and lowermost fatty acid layers. The SF spectrum of the uppermost fatty acid layer has a characteristic interference behavior arising from interaction of its resonant signal with the nonresonant signal from the substrate. Quantification of the periodicity of this interference effect gives a value in satisfactory agreement with previous theoretical predictions. The SF spectrum of the lowermost fatty acid is, as expected, independent of the multilayer thickness. Summation of individual SF spectra of the uppermost and lowermost fatty acid layers of multilayer films (achieved via selective deuteration) has been shown to accurately reproduce experimental spectra of fully monoisotopic multilayers. These findings imply that interpretation of SF spectra of LB multilayer films deposited on metal substrates must be performed with care and that deconvolution of spectra in terms of contributing layers is imperative.