Energy- 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 C 2 H 5 Br on GaAs(110). Temperature-programmed desorption measurements indicate that this species is molecularly adsorbed with a binding energy of 0.43 eV. After photoinduced C−Br bond cleavage two channels for ethyl fragments ejected are observed in the TOF: one resulting from direct ejection along 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, resulting from transient scattering from the surface potential energy well with a kinetic energy of 0.4 eV. Additionally, the kinetic energy of the directly ejected fragments varies with detection angle, a behavior consistent with a fragment−surface potential with normal energy scaling. The TOF measurements also indicate that, at coverages of <1 ML, the C 2 H 5 Br is oriented with the C−Br bond at an angle near 30° from the surface normal in the [01̄] direction. This result is in accord with earlier NEXAFS and TOF measurements and ab initio calculations of CH 3 Br on the same surface, which show that such a polar molecule is individually oriented by electrostatic interaction with the surface GaAs dipoles.