The vibrational and electronic excitations of trimethyaluminum (Me3Al) adsorbed on GaAs(100) have been studied using high-resolution electron energy loss spectroscopy (HREELS). The C-H vibrational modes of the adsorbed Me groups are excited by a resonance scattering mechanism involving the formation of a temporary negative ion. Strong vibrational mode enhancement occurs at 4, 8, and 14 eV with several overtone and combination bands observed at these energies. Reflectivity measurements of both the clean and adsorbate-covered surface are consistent with resonance scattering at these energies. The molecular orbitals involved in the resonant process are identifed through calculation of electron affinities and charge distributions for the free Me3Al molecule. HREEL spectra recorded at electron beam energies greater than 10 eV reveal new vibrational modes not associated with CH3 groups but characteristic of adsorbed CH2, a species also formed after annealing the Me3Al exposed surface to temperatures greater than 350 degrees C. Electron beam induced dissociation of the Me groups occurs with a threshold energy of 10 eV, and the dissociation cross section for CH2 production is very large, varying from 0.7 x 10(-16) cm(2) at 20 eV to 2.0 x 10(-16) cm2 at 30 eV. The results are consistent with a nonresonant dissociation mechanism, and an electron impact ionization mechanism is proposed. The high cross sections reflect the relatively long lifetime of the dissociative excited states, which are not quenched efficiently for adsorption on semiconductor surfaces.