Desorption of water adsorbed on iron oxide by laser irradiation

Desorption of water adsorbed on iron oxide by laser irradiation was studied by means of a time-of-flight (TOF) technique. The wavelength of the laser for desorption was varied from 355 to 600 nm. The energy threshold of the water desorption ranged around 2.0–2.3 eV. Based on the fact that this energy threshold approximately corresponds to the bandgap of Fe2O3, the initial process of water desorption is considered to be the electronic excitation of the iron oxide from the valence band to the conduction band. Analysis of the velocity distribution of the desorbed water suggests that following the electronic excitation of the iron oxide the desorption is caused by both thermal and nonthermal processes. The thermal process is caused by the rise of the surface temperature that occurs after the scattering and de-excitation of the excited electron in the iron oxide. In the case of the laser at λ = 355 nm, the desorption was mainly caused by the thermal process. On the other hand, in the case of the laser at λ = 430 nm, the desorption was mainly caused by the nonthermal process. The desorption caused by the nonthermal process is attributed to the transfer of the electron excited in the iron oxide to the adsorbed hydroxyl.