RESONANT VIBRATIONAL-EXCITATION IN HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY STUDIES OF TRIMETHYLAMINE CHEMISORBED ON GAAS(100)

High-resolution electron energy-loss spectroscopy has been used to stu dy the vibrational properties of trimethylamine [(CH3)(3)N] chemisorbe d on GaAs(100) at room temperature. A number of vibrational modes of t he 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 la rge enhancement in the intensity of several fundamental vibrations and the appearance of a number of combination and overtone bands. Detaile d energy-dependent studies and comparison with previous electron scatt ering studies of related gas phase molecules suggest that electron cap ture can occur in three molecular orbitals of the adsorbed molecule. B y considering the electron density distributions obtained from semiemp irical calculations of the isolated molecule, an assignment of the vib rational modes which are selectively excited due to occupation of a sp ecific molecular orbital has been attempted. (C) 1996 American Institu te of Physics.