UV-induced desorption of CH3X(X=I and Br)/TiO2(110)

The UV photodesorption of methyl halides from TiO2(110) has been investigated using quadrupole mass spectrometry. For low fluence irradiation (<800 mu J/cm(2)) and less than or equal to 1 ML coverages of methyl iodide, the photodesorption yield decreases exponentially with photon dose, indicating a nonthermal, single-photon process. The wavelength and coverage dependence of the photodesorption yield and the effect of spacer layers support the conclusion that photodesorption of CH3X (X=I and Br) is initiated by substrate excitation, i.e., photogenerated electrons in the conduction band of TiO2. CH3I and CH3Br desorb from TiO2(110) on 257 nm irradiation while CH3Cl does not photodesorb in the absence of a photoactive coadsorbate, such as CH3I. This indicates that desorption results from a resonant interaction of subvacuum-level electrons with an adsorbate electronic state. Momentum transfer from energetic photofragments to adsorbates can account for only a minor fraction of the total photodesorption yield for coverages of 1 ML or less. For high fluence irradiation (greater than or equal to 7 mJ/cm(2)), the nonlinearity of the desorption yield, the calculated transient surface temperature rise, and the desorption of an inert adsorbate (CH3OH at greater than or equal to 12 mJ/cm(2)) indicate that a laser-induced thermal desorption mechanism is dominant. (C) 1998 American Institute of Physics.