Effect of multiple group orientations on sum frequency generation spectra

Abstract

Sum Frequency Generation (SFG) spectra of monolayer films on metal surfaces have been simulated where the monolayer contains methyl groups in two different orientations. The two methyl group tilt angles were taken into account by including a second resonant term in the expression for the second-order non-linear susceptibility of the film, χR, and allowing the two resonant terms to vary independently with tilt angle. The metal surface gives rise to a constant non-resonant susceptibility, χNR. The phases of the SFG signals are determined by the interference between the two resonant susceptibilities and the non-resonant susceptibility of the metal surface. Whether the features corresponding to the methyl symmetric (r+) and asymmetric (r−) stretching modes appeared as peaks or dips in the simulated spectra showed a strong dependence on the two tilt angles, and a weaker dependence on R, the ratio of the two hyperpolarisabilities β aca and β ccc. From the simulated spectra it was determined that there are combinations of the two methyl group tilt angles that result in the symmetric and asymmetric methyl resonances having different spectral profiles with one a peak and the other a dip. A general method has been developed from which boundary conditions for the two methyl group tilt angles can be formulated based on the experimentally determined resonant phases of the methyl symmetric and asymmetric stretching modes.