Determining Surface-Adsorbate Binding Energies from Collision-Induced Desorption Experiments.

The work in this dissertation details the determination of surface-adsorbate bond energies from collision-induced desorption experiments. The collision-induced desorption experiment utilizes an intense, nearly monoenergetic beam of neutral, noble gas atoms to desorb surface bound molecules into the gas phase. A general equation derived from classical mechanics is used to calculate a physisorption or chemisorption bond energy from the threshold energy required to desorb an adsorbate from the surface. The first system studied in this dissertation is the collision-induced desorption of CH_4 physisorbed to Ni{111 } by energetic Ar atoms. A theory is presented to extract the bond energy for this system using a one -dimensional square well to model the Ni-CH_4 interaction. The threshold for desorption scales with the fraction of energy transferred from Ar to CH _4 along the direction of the surface normal. This model calculates the CH_4/Ni {111} bond energy to be 120 meV which is close to experimental value of 126 meV. The second investigation involves the collision -induced desorption of NH_3 chemisorbed on Pt{111}. The absolute cross sections for NH_3 desorption at 0.25 ML were measured by x-ray photoelectron spectroscopy as a function of Ar kinetic energy and incident angle. The threshold desorption energy, 1.95 eV, was independent of the Ar angle of incidence and suggests a strong lateral corrugation in the NH_3/Pt {111} interaction potential. The collision model calculates the NH_3/Pt {111} binding energy to be 1.45 eV. The third system probed by collision-induced desorption was the di-sigma configuration of rm C_2H_4 chemisorbed to Pt{111} at 90 K. An energetic Xe beam was used to desorb rm C_2H_4 and the threshold energy for desorption, 5.2 eV, was independent of the Xe angle of incidence. The model calculates the di-sigma binding energy to be 2.1 eV and predicts that rm C_2H_4 desorbs from the surface in a planar configuration. From this result a thermal desorption mechanism involving a radical intermediate for ethylene was proposed. This mechanism may provide new insight into the hydrogenation of olefins on metals surfaces.