Core-electron spectroscopy of nonconjugated linear dienes chemisorbed on Si(001)-2X1 with synchrotron radiation

The adsorption of two nonconjugated linear dienes, 1,7-octadiene and 1,4-pe ntadiene, on Si(001)-2X1 is studied using soft x-ray photoemission spectros copy (XPS), C is near-edge x-ray absorption fine structure (NEXAFS) and cor e-decay spectroscopy. It is found that both dienes attach to the silicon su rface 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 th e distance separating two adjacent silicon dimer rows. The angular dependen ce of the C 1s absorption spectra indicates some ordering of the chemisorbe d layer. C K-edge con-decay spectroscopy gives complementary information on the electronic structure, more specifically on the localization (versus de localization) to the C is core hole of the electron promoted to an unoccupi ed level, which can be useful for the understanding of the transport proper ties of the grafted films. Independently of the molecule length, it is show n that the excited electron, promoted to a pi* level, remains bound to the core hole, while it delocalizes when it is lifted to the sigma(CH)* level ( although this level lies below the ionization potential) and to higher-ener gy levels. The energy positions of NEXAFS resonances, in the chemisorbed ca se (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.