Vibrational excitation of adsorbed molecules by photoelectrons of very low energy: Acrylonitrile on Cu (100)

In this study, we report on a powerful method of primary photoelectron scattering by adsorbed species. Specifically, threshold-energy (E kin,max < 0.5 eV) two-photon photoelectrons (2PPE) are used to probe acrylonitrile (ACN) molecules chemisorbed onto a Cu(100) substrate, held at room temperature. This has proven to constitute a perfect tool to reveal the ACN vibrational modes in the chemisorbed state. From the dynamics of the directional (perpendicular to the copper surface) electron energy loss we conclude that only a few fundamental vibrational motions of adsorbed ACN are excited, namely the CC, CN and C-H stretch modes. From the excitation probability spectra threshold energies, Eth, of these modes was extracted: Eth(CC) = 182(15) meV, Eth(CN) = 248(16) meV - which are shifted noticeably from the equivalent gas phase values; and E th(C-H) ∼360-380 meV - which varies only marginally from the gas phase value. The interpretation of the excitation spectra suggests that the di-σ adsorption configuration of the terminal C- and N-atoms dominates, which agrees well with the orientation and bindings predicted in Density Functional Theory (DFT) calculations. Consistent with this is the observation that the contribution to the 2PPE excitation spectra from the C-H stretch motion is by far the largest, which are not directly affected by chemisorption bonding. © 2011 the Owner Societies.This research received financial support from the Ministerio de Ciencia y Tecnología of Spain (grants CSD2009-00038 and CTQ2007-61749) and from the Santander-Complutense University research grant programme. J. T. acknowledges a PhD fellowship from the Ministerio de Educación y Ciencia of Spain.Peer Reviewe

Vibrational excitation of adsorbed molecules by photoelectrons of very low energy: acrylonitrile on Cu (100)

In this study, we report on a powerful method of primary photoelectron scattering by adsorbed species. Specifically, threshold-energy (Ekin,max N and C–H stretch modes. From the excitation probability spectra threshold energies, Eth, of these modes was extracted: Eth(C=C) = 182(15) meV, Eth(CN) = 248(16) meV – which are shifted noticeably from the equivalent gas phase values; and Eth(C–H) ~ 360–380 meV – which varies only marginally from the gas phase value. The interpretation of the excitation spectra suggests that the di-sigma adsorption configuration of the terminal C- and N-atoms dominates, which agrees well with the orientation and bindings predicted in Density Functional Theory (DFT) calculations. Consistent with this is the observation that the contribution to the 2PPE excitation spectra from the C–H stretch motion is by far the largest, which are not directly affected by chemisorption bonding