Comparison of chemical selectivity and kinetic energy release in Si(s)+ICl(g) and H(g)+ICl(g)

Abstract

ICl chemisorbs onto Si(111)–7×7 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)–7×7. 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. The ICl and Si(111)–7×7 reaction can be compared to the ICl and atomic hydrogen (deuterium) reaction which has been studied extensively by others. The chemical selectivity of ICl+Si(111) is greater than the chemical selectivity of the gas phase reaction of H+ICl where the ratio of formation of HI to HCl is only 4:1. In both reactions, the iodine atom of ICl molecules is more reactive than the chlorine atom because the πx,y* antibonding orbital (the orbital that covalently reacts with other species) consists primarily of atomic iodine orbitals. 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. The median translational energies of ejected 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 energies of ejected atoms is due to the fact that the iodine–chlorine bond of ICl lengthens as the Si–I bond contracts; thus, there is little repulsion energy attributed to the Si–I–Cl transition state.