Ga(CH3)3 (TMGa), Ga(C2H53 (TEGa) and Ga(C2H5)2Cl (DE-GaCl) are used in combination with arsine (AsH3) in atomic layer epitaxy (ALE) of GaAs. We stimulated this process in a UHV system by dosing a c(2×8) or (1×6) reconstructed GaAs(001) surface with AsH3 and organometallics at different temperatures. HREEL spectra were taken after annealing the sample to successively higher temperatures. Vibrations of the adsorbate and its fragments induce changes of the dipole moment at the surface. The orientation of the dipole moment is different for molecularly and dissociatively adsorbed organometallics. By considering the change of the dipole moment perpendicular to the surface we determined how the adsorbates and their fragments are adsorbed on the surface. By following the relative intensities of the energy-loss peaks of the corresponding vibrations through an annealing series, we concluded how the pyrolysis proceeds. We found that most of the TMGa and TEGa adsorbs molecularly on an As-rich cooled surface but a large fraction decomposes upon adsorption on a Ga-stabilized surface. Complete desorption occurs at a lower temperature on the (1×6) compared to the c(2×8) surface. For DEGaCl our spectra indicate that it is adsorbed dissociatively on both surfaces. © 1992.