Cs + reactive scattering from a Si(111) surface adsorbed with water

Reactive scattering of hyperthermal Cs ion is examined from a Si(111) surface adsorbed with water. Collision of Cs+ beams with the Si surface at the energy of 10–100 eV produces Cs+-bound cluster ions as scattering products, including CsOH+, CsOH2+, CsSi+, CsSiHn+(n=1,2), and CsSiO+. The yields for these clusters are examined as a function of Cs+ beam energy and water exposure. Kinetic energy distributions for the clusters are measured. The reactive scattering process is explained in terms of collision-induced desorption of adsorbate, followed by ion–molecule association between the scattered Cs+ and the desorbed molecule. The probability that Cs+ undergoes reactive scattering is 5×10−4–2×10−3 for 50 eV collision energy. The corresponding probability for the Cs+–molecule association reaction is in the order of 5×10−3–2×10−2 or slightly lower. It is proposed that CsOH+ and CsOH2+ are formed from OH and H2O adsorbates, respectively, via direct collisional desorption. CsSiO+ formation can be related to desorption of surface oxide species or, at high energy, to collisional dissociation of adsorbates. Several aspects of using Cs+ reactive scattering for surface adsorbate detection are discussed.