Core-electron spectroscopy of nonconjugated linear dienes chemisorbed on Si(001)-2×1 with synchrotron radiation

The adsorption of two nonconjugated linear dienes, 1,7-octadiene and 1,4-pentadiene, on Si(001)-2×1 is studied using soft x-ray photoemission spectroscopy (XPS), C 1s near-edge x-ray absorption fine structure (NEXAFS) and core-decay spectroscopy. It is found that both dienes attach to the silicon surface via only one of their two C=C bonds. This result, expected for the 1,4-pentadiene, is more surprising for 1,7-octadiene, as its length fits the distance separating two adjacent silicon dimer rows. The angular dependence of the C 1s absorption spectra indicates some ordering of the chemisorbed layer. C K-edge core-decay spectroscopy gives complementary information on the electronic structure, more specifically on the localization (versus delocalization) to the C 1s core hole of the electron promoted to an unoccupied level, which can be useful for the understanding of the transport properties of the grafted films. Independently of the molecule length, it is shown that the excited electron, promoted to a π* level, remains bound to the core hole, while it delocalizes when it is lifted to the σ_CH* level (although this level lies below the ionization potential) and to higher-energy levels. The energy positions of NEXAFS resonances, in the chemisorbed case (characterized by the presence of a high-dielectric constant substrate) and in the condensed case (molecular solid) have been compared. We propose that resonance energies could be sensitive to final-state relaxation shifts when the promoted electron delocalizes before core-hole decay.