Temporary Negative-Ion Formation in Chemisorbed Species: The Energy and Angular Dependence of Vibrational Losses for Electron Scattering Studies of HCOO/Ni(110)

Abstract

We have studied both the energy and angular dependence of vibrational losses for the formate species chemisorbed on Ni(llO), using high-resolution electron-energy-loss spectroscopy. Temporary negativeion formation, at an incident electron energy of -14 eV, leads to a dramatic enhancement in the intensity of the symmetric O-C-O stretch. Electron emission shows a strong angular dependence which peaks at high emission angles away from the surface normal. This effect is compared to previous photoemission studies of adsorbed CO, where the scattering in the cr-shape resonance is directed along the C-O bond.PACS numbers: 68.35.Ja, 34.80.Gs, 68.35.Bs, 79.20.Kz Although it has long been recognized that the resonant capture of an incident electron into a quasibound state of gaseous molecules, giving rise to a temporary negative ion, may result in a strong enhancement of the intensities of vibrational excitations, l it is only comparatively recent that similar resonant scattering behavior has been observed for molecules adsorbed on surfaces. 2 " 6 The first conclusive evidence for such an effect arose from inelastic electron scattering studies of weakly bound systems, such as O2 and N2 physisorbed on silver surfaces. 2,3 For more strongly bound adsorption systems, Kesmodel has recently reported an enhancement in the intensity of the symmetric C-H stretching mode for benzene chemisorbed on Pd(100), which was attributed to a resonance centered at an incident electron beam energy of £,--2.7 eV. 6 Essentially all of these studies have been concerned with probing the energy dependence of the resonance and the mode selectivity. 6,7 An extension of these observations has recently been reported for physisorbed O2 on graphite by Palmer et al.,4 in which a negative-ion resonance was observed at £,-9 eV. Palmer et al also investigated the angular distribution of the inelastically scattered electrons which revealed a peak at 15° ±5° relative to the surface normal, independent of the incident electron angle. From this study, information regarding the orientation of the O2 molecules at the graphite surface was obtained. 4 In this Letter, we demonstrate strong resonant enhancement of the symmetric O-C-O stretching mode, v 5 (COO), centered at E{~-14 eV, for the surface formate species, HCOO, chemisorbed on Ni(llO). We relate this to the so-called a resonance in near-edge x-rayabsorption spectroscopy (NEXAFS) at -15 eV for HCOO on Cu(l 10) and Cu(lOO), involving transitions from the O-ls and C-\s core levels to a quasibound state, which is localized in the C-O bonds of the adsorbed species. 8,9 In addition, we find that there exists a strong angular dependence in the inelastically scattered electron distribution which is peaked at high emission angles away from the surface normal and along the (110) azimuth, corresponding to an enhanced emission in directions correlated with the C-O bonds of the formate species. This represents the first energy and angular study of resonant scattering from a chemisorbed system. Experiments were carried ...

Resonances in electron scattering by molecules on surfaces

“…A resonance is evident in the excitation function obtained 10 degrees away from the specular direction (solid circles). Jones and Richardson (1988). (Fig.…”

Resonant Electron Scattering of 11-Mercaptoundecanoic Acid Self-Assembled Monolayer Adsorbed on Au (111)

“…Indeed, no charging effect can be evidenced due to the limited and well-controlled thickness of the organic layer and the structural organization of such layer contributes to improve the resolution. , Moreover, due to self-organization, the extreme surface is essentially composed of the −COOH groups. This work will thus allow us to make a comparison with former studies of resonances in CO in the gas phase, in physisorbed ,− or chemisorbed ,, layers on different substrates, in physisorbed CO 2 , , and in adsorbed HCOOH overlayers. ,, Other works dedicated to HREELS studies of the thermal stability of alkanethiol SAMs should also be mentioned.…”

“…This work will thus allow us to make a comparison with former studies of resonances in CO in the gas phase, 25 in physisorbed 12,29-32 or chemisorbed 28,33,34 layers on different substrates, in physisorbed CO 2 , 28,35 and in adsorbed HCOOH overlayers. 8,36,37 Other works 38 dedicated to HREELS studies of the thermal stability of alkanethiol SAMs should also be mentioned.…”

Resonant and Nonresonant Vibrational and Electronic Excitations in Trimethylaluminum Adsorbed on GaAs

“…Early HREELS studies of molecules chemisorbed on metal surfaces identified dipole and impact scattering as the dominant vibrational excitation processes, with the two mechanisms distinguished experimentally by changing the electron scattering geometry (specular and off-specular, respectively) . More recent studies, however, have shown that resonant vibrational excitation (eq 1) is a more common scattering process than initially thought. − The incident electrons are captured in unoccupied molecular orbitals of the adsorbate (AB (ads) ), leading to the formation of a temporary negative ion ([AB (ads) ] - ). Since capture generally takes place in an orbital dominated by antibonding character, the equilibrium nuclear coordinates may differ from those of the ground-state neutral molecule.…”

“…5 More recent studies, however, have shown that resonant vibrational excitation (eq 1) is a more common scattering process than initially thought. [6][7][8][9][10][11][12][13][14][15] The incident electrons are captured in unoccupied molecular orbitals of the adsorbate (AB (ads) ), leading to the formation of a temporary negative ion ([AB (ads) ] -). Since capture generally takes place in an orbital dominated by antibonding character, the equilibrium nuclear coordinates may differ from those of the ground-state neutral molecule.…”