NEXAFS study of a 17,19-hexatriacontadiyne monolayer on Au(1 1 1)

The structural changes of a 17,19-hexatriacontadiyne (HTDY) monolayer on Au(1 1 1) upon UV irradiation is investigated by polarization dependent near-edge X-ray absorption fine structure spectroscopy. A peak assigned to the σCC∗ resonance and predominantly observed in the normal incidence spectrum of the unirradiated monolayer shows that the carbon zigzag plane of the HTDY molecule is parallel to the surface. Another peak ascribed to the M∗ resonance proves that the σCH/σCH∗ orbitals of the alkyl chains are mixed with metal bands. The peak position of the π∗ resonance for the monolayer is almost the same as that for a multilayer, meaning that the π∗ orbitals of the molecules in the monolayer are little mixed with the metal bands. Upon UV irradiation, the polarization dependence of the spectra is diminished and the π∗ resonance shifts to a lower energy. These results indicate that the molecular orientation becomes random owing to the formation of oligomeric species.