Selective modification of the surface structure of oxide nano-particles using sub-bandgap photons

Controlled manipulation of the physical and chemical properties of nano-structured materials requires correlating their spectroscopic properties and reactivity with specific surface sites. We have demonstrated experimentally that laser excitation of MgO nano-crystalline films and nanocube samples with 4.66 eV photons desorbs neutral O and Mg atoms with hyper-thermal kinetic energies in the range of 0.1--0.4 eV. Using an ab initio embedded cluster approach we developed the mechanisms for the hyper-thermal emission of atomic species. We demonstrate that the desorption of O atoms from 3-coordinated (3C) surface sites occurs via the following steps: i). an exciton is excited at the 3C site; ii) the exciton is ionized, while the remaining electronic hole is trapped at the 3C O site converting it to an O$^{-}$ radical; iii) absorption of another 4.66 eV photon in the vicinity of the 3C site forms an excited state, which relaxes with desorption of a neutral O atom. Similar process was identified for Mg-terminated 3C site. The proposed general mechanism can be used to control atomic scale modification of insulating surfaces. [1] P.E. Trevisanutto et al., \textit{Surf. Sci}., \textbf{593}, 210 (2005); W.P. Hess et al, \textit{J. Phys. Chem. B}, \textbf{109}, 19563 (2005).