Exciton-driven highly hyperthermal O-atom desorption from nanostructured CaO

We report qualitatively new highly-hyperthermal (HHT) oxygen atom emission from nanostructured CaO excited by 6.4 eV nanosecond laser pulses. The kinetic energy distribution of emitted O-atoms peaks at 0.7 eV, which is over four times greater than previously observed. Excitation of MgO and CaO nanostructures with UV laser pulses is known to result in thermal and hyperthermal emission of oxygen atoms when photons with energies below and above the band gap, respectively, are used. The highly energetic atomic desorption we observe, following bulk excitation, challenges the conventional view that bulk excitation can only induce thermal desorption. Using density functional theory and an embedded cluster method, we propose a mechanism for this HHT feature based on the interaction of surface holes with bulk excitons. These experimental and theoretical results suggest that specific atomic desorption mechanisms in wide-bandgap materials can be controlled by selective electronic excitation of not only the surface but also the bulk of these materials