LOW-ENERGY HELIUM DIFFRACTION STUDIES OF CH3BR OVERLAYERS PHYSISORBED ON C(0001), NACL(001), AND LIF(001) SURFACES

Low energy helium diffraction has been used to determine the unit mesh parameters of overlayers of CH, Br physisorbed on C(0001), NaCl(001), and LiF(001) at almost-equal-to 35 K. CH3Br forms a uniaxially commensurate overlayer on C(0001) with unit mesh parameters 4.26 angstromX6.75 angstrom. On NaCl(001), CH3Br forms a high coverage and a low coverage phase. The high coverage phase is incommensurate and has unit mesh parameters 4.54 angstromx6.73 angstrom, whereas the low coverage phase is commensurate with a (square-root 2X3 square-root 2)R 45-degrees unit mesh. The structure of CH3Br/LiF (001) is essentially the same as that of the high coverage phase of CH3Br/NaCl(001) with unit mesh parameters 4.52 angstromx6.71 angstrom. The unit mesh parameters (with the exception of low coverage CH3Br/NaCl) are very similar to the lattice parameters of the a-b [or (001)] plane of bulk crystalline CH3Br at almost-equal-to 153 K. By analogy with the bulk crystal, it is likely that there are two molecules per unit mesh and that the CH3Br dipoles are nearly perpendicular to the surface and antiferroelectrically ordered. Our results suggest that the unit mesh for the low coverage phase of CH3Br/NaCl contains four molecules and that the molecular axes are parallel to the substrate surface. The implications of these results for photodissociation studies of physisorbed CH3Br are briefly discussed.