XPS study of oriented organic molecules. The single crystal of an azobenzene-containing alkyl ammonium amphiphile

We have observed the angle-dependent XPS (X-ray photoelectron spectroscopy) N 1s spectral change of a single crystal of azobenzene-containing ammonium amphiphile C₁₂AzoC₅N⁺Br^-. By comparing the calculated intensity ratio of the azo-group nitrogens against the ammonium nitrogen with that experimentally observed, the N 1s spectral change has successfully been correlated with the stacked bilayer model in the outermost surface of the crystal. With the assumption that the molecular axis inclines about 30 ° to the layer surface as determined by X-ray diffraction measurement, the N 1s photoelectron (about 850 eV in kinetic energy) mean-free-path was estimated to be about 24 Å. The value agreed well with theoretical values. We have successfully confirmed that XPS has potentiality for analyzing molecular orientation besides conventional elemental and functional group analyses.