ENERGY-RESOLVED AND ANGLE-RESOLVED FRAGMENTATION OF ETHYL-BROMIDE ON GAAS(110)

Angle- and mass-resolved time-of-flight (TOF) measurements are used to probe the photodynamics and molecular orientation of C2H5Br on GaAs(1 10). Temperature-programmed desorption measurements indicate that this species is molecularly adsorbed with a binding energy of 0.43 eV. Aft er photoinduced C-Br bond cleavage two channels for ethyl fragments ej ected are observed in the TOF: one resulting from direct ejection alon g the C-Br bond with a kinetic energy that varies from 0.6 to 1.1 eV, depending upon incident excitation wavelength, and the other, resultin g from transient scattering from the surface potential energy well wit h a kinetic energy of 0.4 eV. Additionally, the kinetic energy of the directly ejected fragments varies with detection angle, a behavior con sistent with a fragment-surface potential with normal energy scaling. The TOF measurements also indicate that, at coverages of <1 ML, the C2 H5Br is oriented with the C-Br bond at an angle near 30 degrees from t he surface normal in the [0 (1) over bar] direction. This result is in accord with earlier NEXAFS and TOF measurements and ab initio calcula tions of CH3Br on the same surface, which show that such a polar molec ule is individually oriented by electrostatic interaction with the sur face GaAs dipoles.