AVS 49th International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeA

Paper SS1-WeA3
Hyperthermal Ejection of Atomic Cl from the Reaction of Cl@sub 2@ on the Al(111) Surface: Evidence of a Nonadiabatic Electron Harpooning Mechanism

Wednesday, November 6, 2002, 2:40 pm, Room C-108

Session: Gas-Surface Dynamics
Presenter: G.C. Poon, University of California, San Diego
Authors: G.C. Poon, University of California, San Diego
T.J. Grassman, University of California, San Diego
A.C. Kummel, University of California, San Diego
Correspondent: Click to Email
Resonantly Enhanced Multiphoton Ionization (REMPI) and Time-of-Flight Mass Spectroscopy (TOF-MS) have been used to demonstrate that the reaction of Cl@sub 2@ on the low work function Al(111) surface proceeds via a prompt nonadiabatic electron harpooning process. As Cl@sub 2@ approaches the Al(111) surface, an electron harpoons from the surface, suddenly converting Cl@sub 2@ to Cl@sub 2@@super -@. This places the molecule high on the repulsive portion of the Cl@sub 2@@super -@ potential curve leading to rapid dissociation into Cl@super -@ and Cl fragments. The Cl@super -@ proceeds toward the surface and sticks while Cl is ejected into the gas phase. An experimentally observable signature of this harpooning process would be a hyperthermal translational energy of the ejected fragment, whose energy is determined by the vertical transition between Cl@sub 2@ and Cl@sub 2@@super -@ and should be nearly independent of incident translational energy. Three beams of Cl@sub 2@ were prepared by seeding with translation energies ranging from 0.989 eV to 0.105 eV. Each of the three Cl@sub 2@ beams was directed at the surface at three incident angles: 0°, 20°, and 40°. The translational energy of the ejected Cl was shown to be a weak function of the incident translational energy. For 0.989 eV normal incidence Cl@sub 2@ the ejected Cl had a translational energy of 0.147 eV, while 0.105 eV normal incidence Cl@sub 2@ produced ejected Cl of 0.100 eV. Therefore, for incident Cl@sub 2@ with a velocity of 535 m/sec, the ejected Cl atoms were accelerated to a velocity of 740 m/sec. This acceleration of ejected Cl compared to incident Cl@sub 2@ is consistent with a nonadiabatic process that converts electronic to kinetic energy.