Excitation, Ionization, and Desorption: How Sub-Band Gap Photons Modify the Structure of Oxide Nanoparticles

Nanoparticles of wide band gap materials MgO and CaO, subjected to low-intensity ultraviolet irradiation with 266 nm (4.66 eV) photons, emit hyperthermal oxygen atoms with kinetic energies of up to ∼0.4 eV. We use an ab initio embedded cluster method to study theoretically a variety of elementary photoinduced processes at both ideal and defect-containing surfaces of these nanoparticles and to develop a mechanism for the desorption process. The proposed mechanism includes multiple local photoexcitations resulting in sequential formation of localized excitons, their ionization, and further excitations. It is suggested that judicious choice of sub-band gap photon energies can be used to selectively modify surfaces of nanomaterials.