High-resolution electron energy-loss spectroscopy has been used to study the vibrational properties of trimethylamine [(CH3)(3)N] chemisorbed on GaAs(100) at room temperature. A number of vibrational modes of the adsorbate, involving both the C-H and C-N bonds, are preferentially excited via a resonance scattering mechanism involving the formation of a temporary negative ion due to capture of the incident electrons. The resonance, which is centered around 6 eV, is characterized by a large enhancement in the intensity of several fundamental vibrations and the appearance of a number of combination and overtone bands. Detailed energy-dependent studies and comparison with previous electron scattering studies of related gas phase molecules suggest that electron capture can occur in three molecular orbitals of the adsorbed molecule. By considering the electron density distributions obtained from semiempirical calculations of the isolated molecule, an assignment of the vibrational modes which are selectively excited due to occupation of a specific molecular orbital has been attempted.