Soft-x-ray photoelectron, x-ray absorption, and autoionization spectroscopy of 1,5-cyclooctadiene on Si(001)-2 x 1

The adsorption of a nonconjugated diene, 1,5-cyclooctadiene, on Si(001)-2 X 1 is studied using valence-band and core-level soft-x-ray photoemission, n ear-edge x-ray absorption fine structure at the C Is edge, and autoionizati on spectroscopy. Core-level spectroscopies validate the bonding model for r oom-temperature adsorption proposed by Hovis and Hamers [J. Phys. Chem. B 1 01, 9581 (1997)]-based essentially on scanning tunneling microscopy (STM) a nd infrared spectroscopy experiments-where the diene attaches to the silico n surface via only one C-C bond. On the other hand, the complete quenching of the silicon surface states at saturation points to a coverage of one mol ecule per Si dimer, and not to a coverage of one molecule per two Si dimers , as the STM observation of locally 2 x 2 or c(2 x 4) ordered adsorption do mains would suggest. The orientation of the free pi orbital is deduced from the angular dependence of the C Is absorption spectra. This orientational order is lost after thermal annealings of the molecular film (610-680 K ran ge), before the molecule decomposes (at similar to 820 K). Valence-band spe ctra are taken at various photon energies (between 22 and 170 eV) to invest igate the nature of the adsorbate-derived molecular levels resulting from t he formation of the molecular film at 300 K. Valence-band spectra recorded at photon energies near the C Is edge show that an electron promoted to the lowest unoccupied molecular orbital (pi*) of the chemisorbed molecule rema ins localized to the core-hole site, as both participator and spectator dec ay channels are observed. [S0163-1829(99)12027-7].