Surface molecular structure of self-assembled alkanethiols evidenced by UPS and photoemission with synchrotron radiation

With the aim of investigating the configurational and conformational characteristics of monolayers of alkanethiols on gold by means of a photoemission experiment, we compare the valence data obtained from these layers using synchrotron radiation and ultraviolet photoemission spectroscopy (UPS). The incident energy used for synchrotron radiation has been chosen very close to the Cooper minimum of the 5d levels of gold, in order to limit as much as possible interference from the substrate. The recorded spectra differ markedly from one sample to another, notwithstanding their closely related chemical nature. These variations are compared consistently with semiquantitative simulations on model clusters and found thereby to reflect specifically the influence of the connectivity and of specific interactions on the electronic structure of long hydrocarbon chains.