AVS 45th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM8
Comparison of Chemical Selectivity and Kinetic Energy Release in Si@sub (s)@ + ICl@sub (g)@ and H@sub (g)@ + ICl@sub (g)@

Wednesday, November 4, 1998, 10:40 am, Room 309

Session: Gas-Surface Dynamics
Presenter: K.A. Pettus, University of California, San Diego
Authors: K.A. Pettus, University of California, San Diego
T. Ahmadi, University of California, San Diego
E. Lanzendorf, University of California, San Diego
A.C. Kummel, University of California, San Diego
Correspondent: Click to Email
The chemisorption of ICl on the Si(111)-7x7 surface is comparable to the gas reaction of ICl and hydrogen. ICl chemisorbs onto Si(111)-7x7 by two mechanisms: dissociative chemisorption and abstractive chemisorption. Abstractive chemisorption, in which one halogen atom of ICl bonds to the silicon surface while the other is ejected into the gas phase, is the dominant chemisorption mechanism for ICl/Si(111)-7x7. 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 34 +/- 4 :1. Furthermore, the chemical selectivity of ICl + Si(111)-7x7 is greater than the chemical selectivity of the gas phase reaction of H +ICl where the ratio of formation of HI to HCl is 4:1.@footnote 1@ In both reactions, the iodine atom of ICl molecules is more reactive than the chlorine atom because the pi @sub x,y@ @super *@ antibonding orbital (the orbital that reacts with any other species) consists primarily of atomic iodine orbitals.@footnote 2@ The difference in the chemical selectivities of the silicon surface and gaseous hydrogen reactions with ICl is due to the ability of the silicon surface to rotationally steer ICl molecules and the inability of silicon to migrate between the iodine and chlorine atoms in ICl . The median translational energies of the abstracted halogen atoms were determined to be 0.18 +/- 0.04 eV for chlorine atoms and 0.53 +/-0.27 eV for iodine atoms which are a small fraction (14% for ejected iodine atoms and 9% for ejected chlorine atoms) of the total reaction exothermicities. The low translational energy of ejected atoms is due to the fact that the iodine-chlorine bond of ICl lengthens as the Si-I bond contracts and there is little repulsion energy attributed to the Si-I-Cl transition state. @FootnoteText@ @footnote 1@J.C. Polanyi, Chemica Scripta, 27, 229 (1986) @footnote 2@J.D. McDonald, P.R. LeBreton, Y.T. Lee, and D.R. Herschbach, J. Chem. Phys., 56, 769 (1972)