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

The adsorption of a nonconjugated diene, 1,5-cyclooctadiene, on Si(001)−2×1 is studied using valence-band and core-level soft-x-ray photoemission, near-edge x-ray absorption fine structure at the C1s edge, and autoionization spectroscopy. Core-level spectroscopies validate the bonding model for room-temperature adsorption proposed by Hovis and Hamers [J. Phys. Chem. B 101, 9581 (1997)]—based essentially on scanning tunneling microscopy (STM) and infrared spectroscopy experiments—where the diene attaches to the silicon 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 molecule per Si dimer, and not to a coverage of one molecule per two Si dimers, as the STM observation of locally 2 × 2 or c(2 × 4) ordered adsorption domains would suggest. The orientation of the free p orbital is deduced from the angular dependence of the C1s absorption spectra. This orientational order is lost after thermal annealings of the molecular film (610–680 K range), before the molecule decomposes (at 820 K). Valence-band spectra are taken at various photon energies (between 22 and 170 eV) to investigate the nature of the adsorbate-derived molecular levels resulting from the formation of the molecular film at 300 K. Valence-band spectra recorded at photon energies near the C1s edge show that an electron promoted to the lowest unoccupied molecular orbital (π*) of the chemisorbed molecule remains localized to the core–hole site, as both participator and spectator decay channels are observed.