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 μJ/cm²) and ≤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 (≥7 mJ/cm²), the nonlinearity of the desorption yield, the calculated transient surface temperature rise, and the desorption of an inert adsorbate (CH₃OH at ≥ 12 mJ/cm²) indicate that a laser-induced thermal desorption mechanism is dominant.