AVS 46th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS2-WeA

Paper SS2-WeA2
Chemical Selectivity and Remote Dissociation- ICl/Al(111)

Wednesday, October 27, 1999, 2:20 pm, Room 607

Session: Gas-Surface Dynamics
Presenter: K.A. Pettus, University of California, San Diego
Authors: K.A. Pettus, University of California, San Diego
A.C. Kummel, University of California, San Diego
Correspondent: Click to Email
We have investigated the chemical selectivity of ICl adsorption onto Si(111)-7x7, a high work function surface, and Al(111), a low work function surface. ICl chemisorbs onto these surfaces by two mechanisms, abstractive chemisorption and dissociative chemisorption. Abstractive chemisorption, in which one halogen atom of ICl bonds to the surface while the other is ejected into the gas phase, is the dominant mechanism for both the ICl/Si(111)-7x7 and the ICl/Al(111) reactions. Multiphoton ionization (205 nm MPI) spectroscopy and time-of-flight (TOF) mass spectrometry were used to determine that the ratio of iodine-selective abstraction to chlorine-selective abstraction is at least 10:1 for both the ICl/ Si(111)-7x7 and ICl/Al(111) reactions. However, the dynamics leading to the chemical selectivity are quite distinct. For the ICl and Si(111)-7x7 reaction, a transition state is formed by the donation of electrons from ICl to an empty dangling bond on the Si(111)-7x7 surface. Since the highest molecular orbital (HOMO) of ICl consists of atomic iodine orbitals, the iodine atom is the more reactive atom and chemical selectivity occurs. The ICl/Al(111) reaction occurs by a very different mechanism, remote chemisorption. In this process, an electron is harpooned from the aluminum metal surface to the lowest unoccupied molecular orbital (LUMO) of ICl. The adsorption of the electron transforms ICl into an excited repulsive state of the negative ion, ICl-. The LUMO primarily consists of atomic iodine orbitals; therefore, iodine again is the more reactive atom and ICl/ Al(111) abstraction favors the adsorption of iodine.