Photodesorption from powdered zinc oxide

Mass spectrometric measurements of photodesorption from powdered ZnO under controlled radiation are reported. Neutral carbon dioxide and atomic oxygen prevail as the desorbing species. From photodesorption rate decay curves we obtain cross sections for photodesorption of ~ 2 × 10^?17 cm² for O and ~4 × 10^?19 cm² for CO₂. Both species desorb only when the incoming photon energy exceeds the ZnO band gap energy (3.2 eV). The desorption rate is linear with incoming photon flux. All the data seem to support a substrate dependent process of photodesorption by neutralization of chemisorbed species by photogenerated holes. The temperature dependence of the photodesorption rate for CO₂ yields a value of 0.26 eV for the apparent binding energy of the physisorbed CO₂ molecule.