Abstractive chemisorption in the initial oxidation of Al(111) has been experimentally verified using variable incident energy ${\mathrm{O}}_2$. Scanning tunneling microscopy images show a transition between single O-adatom reaction products to more pairs of O-adatom reaction products as the ${\mathrm{O}}_2$ incident energy is raised from 0.025 to 0.8 eV. The ejected O atoms have been detected in the gas phase with resonant enhanced multiphoton ionization. The observations that both abstractive and dissociative chemisorption are activated processes are in contrast to current adiabatic models of the absorption process.

• Received 4 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.246103