Molecular oxygen chemisorbed at room temperature on Si(111)-(7 × 7) exhibits a molecular adsorption site which is unexpectedly long lived. Photofragmentation from this specific ${\mathrm{O}}_2$ adsorption site is studied by correlating the time decay of the corresponding molecular surface state observed in photoemission with the time decay of ${\mathrm{O}}^{+}$ desorbed photofragments. Photofragmentation occurs via direct nonresonant $\frac{{\mathrm{O}}_2}{\mathrm{Si}}\left(2\mathrm{}{\sigma }_g^{-1\mathrm{}}\right)$ and resonant $\frac{{\mathrm{O}}_2}{\mathrm{Si}}(2p\to 3{\sigma }_u)$ electronic transitions. State selectivity of the photofragmentation is demonstrated by translational energy measurements of the ${\mathrm{O}}^{+}$ desorbed fragments.

• Received 18 April 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.1727