Vibrational characterization of the oxidation products on Si(111)-(7 x 7)

The oxidation products on Si(111)-(7x7) are investigated at 82 K by means of high-resolution electron energy loss spectroscopy. The isotope-labeled vibrational spectra with 16O2, 18O2, and 16O 18O show that, in the initial stage of the oxidation, an O2 molecule dissociates to form a metastable product with an O atom bonding on top of the Si adatom and the other inserted into the backbond. The metastable product is observed as a dark site in the topographic scanning tunneling microscopy (STM) image and can be transformed to a stable product by the STM manipulation. Our results are in good agreement with recent theoretical calculations.