RESONANCE MECHANISMS IN ELECTRON-ENERGY-LOSS SPECTRA OF STEARIC-ACID LANGMUIR-BLODGETT MONOLAYERS REVEALED BY ISOTOPIC EFFECTS

In theoretical study, a distinction is made between the direct and the resonant collisions which govern electron energy loss spectroscopy of vibrational excitations in molecular media. Direct scattering takes p lace via long-range Coulomb and short-range vibronic interactions; the corresponding cross sections depend upon the reduced mass of the vibr ational mode as M-1/2. On the other hand, the resonant collisions, inv olving the formation of a transient negative ion, have cross sections that scale as M-1/4. Notable isotope effects have been found for the C -H and C-D stretching vibrations in organic compounds and were investi gated by considering the high resolution electron energy loss spectra obtained with Langmuir-Blodgett films of mixtures of hydrogenated and deuterated stearic acid. From the ratio of the excitation cross sectio ns for the C-D and C-H stretching vibrations, measured as a function o f incident electron energies up to 10 eV, the relative importance of t he direct and resonance scattering processes was estimated. For incide nt energies below a critical value between 4 and 5 eV, only direct sca ttering is operative; at the critical value, a resonance mechanism set s in, which rapidly contributes at the higher energies, at least equal ly with the direct collisions. Arguments are given indicating that the lowest-energy resonance has the lowest sigma (H-C-H) configuration. (C) 1998 Elsevier Science B.V.