Negative Ion Resonances Research Articles

Absolute elastic cross sections for electron scattering from N2O

Absolute elastic differential cross sections are presented for electron scattering from nitrous oxide (N2O) for incident energies between 5 eV and 80 eV. Integral and momentum transfer cross sections have also been determined by extrapolation of the differential cross section data to 0' and 180'. Particular attention has been paid to the measurement of differential cross sections below 10 eV as in this region only one earlier measurement at 5 eV has been reported. Previous works investigating vibrational excitation have shown evidence for a negative ion resonance near 8 eV, however no manifestation of this resonance has been found in the elastic scattering channel in this work. A comparison is also made between the differential cross sections of N2O and CO2 as previously it has been found that the cross sections of isoelectronic molecules (i.e. N2 and CO) exhibit very similar shapes and values.

High-resolution electron energy-loss spectroscopy of adsorbed molecules at low temperature

Transient negative ions in electron-molecule collisions (so-called negative ion resonances, NIR) are known in the gas phase. The same process has been observed for weakly adsorbed molecules more recently. The NIR significantly enhances the cross section for vibrational excitation which can be investigated through high-resolution electron energy-loss spectroscopy (HREELS). Generally, the interaction with a surface reduces the lifetime of the NIR. For weakly adsorbed molecules the lifetime of NIR may still be large enough that NIR can be observed in HREELS. The surface temperature has to be lowered to 10–30 K in order to accumulate some fraction of a monolayer of weakly interacting (physisorbed) molecules at the surface. The characteristics of NIR will be discussed and illustrated by examples: (1) resonance in excitation, (2) peculiar angular distribution of the scattered electrons, (3) high overtone intensity of the intra- and inter-molecular vibrations. Some recent studies on weakly adsorbed O2− will be presented.

The lifetime and energy of negative ion resonances of adsorbed molecules: a layer Korringa-Kohn-Rostoker approach

Abstract We present the first application of a new layer Korringa-Kohn-Rostoker technique for the calculation of the energy and lifetime of negative ion states in adsorbed molecules. This approach allows, for the first time, the proper treatment of elastic multiple scattering by both the surface barrier and the substrate and its effect upon the lifetime and energy of adsorbate negative ions. We present results for two model systems; the 2 Π g negative ion resonance in N2 on Ag(111) and the 4 Σ u resonance in O2 Pt(111) and Pt(100). For the low-lying 2 Π g negative ion resonance in N2 on Ag(111), we demonstrate that the behavior of the lifetime and energy upon adsorption is dominated by the surface barrier; the substrate reflectivity has a negligible influence. In contrast, the lifetime and energy of the higher energy 4 Σ u resonance in O2 on Pt is dominated by the substrate reflectivity which causes the resonance lifetime to oscillate as the molecule approaches the surface. These findings are interpreted in terms of the interaction between the molecular resonant state and surface resonances.

The lifetime and energy of negative ion resonances of adsorbed molecules: a layer Korringa-Kohn-Rostoker approach

We present the first application of a new layer Korringa-Kohn-Rostoker technique for the calculation of the energy and lifetime of negative ion states in adsorbed molecules. This approach allows, for the first time, the proper treatment of elastic multiple scattering by both the surface barrier and the substrate and its effect upon the lifetime and energy of adsorbate negative ions. We present results for two model systems; the 2Π g negative ion resonance in N 2 on Ag(111) and the 4Σ u resonance in O 2 Pt(111) and Pt(100). For the low-lying 2Π g negative ion resonance in N 2 on Ag(111), we demonstrate that the behavior of the lifetime and energy upon adsorption is dominated by the surface barrier; the substrate reflectivity has a negligible influence. In contrast, the lifetime and energy of the higher energy 4Σ u resonance in O 2 on Pt is dominated by the substrate reflectivity which causes the resonance lifetime to oscillate as the molecule approaches the surface. These findings are interpreted in terms of the interaction between the molecular resonant state and surface resonances.

π * negative ion resonance states of (E)- and (Z)-1,3,5-hexatriene. Prediction of electron affinities near threshold

Deviations from the pairing theorem previously noted for the linear polyenes are explained on the basis of 1,3-π,π interactions in the negative ion states. Such interactions also contribute significantly to the stabilization of the π * −2 negative ion state of (Z)-relative to (E)-1,3,5-hexatriene. Koopmans' theorem (KT) does not explain the equal π * −3 electron affinities of (E)- and (Z)-hexatriene but CI-singles calculations closely approximate the relative experimental values. Scaled KT and Δε SCF calculations lead to the predictions that (a) the crossover between unbound and bound first π * negative ion states in the linear polyenes occurs between hexatriene and octatetraene, (b) the first π * negative ion states of fulvene and bicyclo[1.1.0]but-1(3)-ene are bound and (c) that of cyclobutadiene is unbound.

Low-energy electron impact spectroscopy of [1.1.1.]propellane: electron attachment energies and singlet and triplet excited states

Electron transmission spectra and energy and angular dependence of vibrational excitation by electron impact were recorded and used to characterize different states of the short-lived negative ion (resonances) of the title compound. The first attachment energy, corresponding to electron capture into the 3a 2 LUMO, is 2.04 eV, exceptionally low in comparison with a typical value of -6 eV for a saturated hydrocarbon. Observation of (faint) vibrational structure indicates a lifetime broadening of the order of a vibrational spacing for this resonance, much less than is typical for σ * resonances of saturated hydrocarbons (several electron volts)

Electron-molecule dynamics at surfaces

This article discusses the role of short-lived negative ion states in molecule-surface dynamics. Such negative ion resonances may be produced either by electron capture in electron beam impact on adsorbed molecules, e.g. in high resolution electron energy loss spectroscopy or electron stimulated desorption, or by electron capture from the surface itself, e.g. in photodesorption or molecular beam scattering. The prime purpose of the review is thus to highlight the similarities in the fundamental physics underpinning a variety of apparently disparate surface processes, and to promote cross-fertilisation between these different areas. In view of this aim, and the broad scope of the subject, no attempt is made to present a complete review of each of the fields considered. Instead, a cluster of examples is chosen, primarily from recent experimental studies, to illustrate the role played by negative ion resonance states in each of the dynamical processes; specifically, vibrational excitation in electron scattering, electron stimulated desorption, photon stimulated desorption, dissociative adsorption, negative ion formation and vibrational excitation in molecular beam scattering, and recombinative molecular desorption. The article concludes with a discussion of possible future developments in the field.

Organic polymer surface analysis by high-resolution electron energy loss spectroscopy: dipole versus nondipole character and resolution enhancement

High-resolution electron energy loss spectroscopy (HREELS) has been used to study the surface vibrational characteristics of ∼100-A-thick films of bisphenol-A-polycarbonate, polyethylene, and poly(2-vinylpyridine). Several vibrational modes of each polymer show significant intensity variations with primary beam energy, indicating electron interaction with the polymer surface via temporary negative-ion resonances. In high-density polyethylene, a simple dipole-allowed infrared-likc spectrum exists for beam energies <3 eV, whereas resonant enhancement of nondipole modes characterize the spectrum at higher beam energies

Low-energy electron scattering by SF6

In a crossed-beam experiment the authors have studied elastic and vibrationally inelastic scattering of electrons by SF6 over the energy range 50 meV to 1 eV at a fixed angle of 90'. A synchrotron photoionization source was used to form a primary beam of electrons of energy resolution FWHM of about 5 meV. Scattered electrons were analysed with an energy resolution of 11.5 meV allowing us to study the excitation of close lying vibrational modes independently. The authors present excitation functions for v1, v3, 2v1 and v1+v3. Excitation of v3 takes place through a direct dipole mechanism whereas the authors suggest that v1 involves a long-lived negative ion resonance.

New interpretation algorithm for molecular negative ion resonance states

AbstractA new interpretation algorithm for molecular negative ion resonance states is proposed. This algorithm may be used when there exists a set of resonance states where the energetic distance between the resonances coincides with the corresponding distance between ionization energies. This set arises by a mechanism of electron‐excited Feschbach resonance. In this mechanism, during the collision of electrons with a molecule, one electron from an occupied molecular orbital (MO) is promoted to an unoccupied MO and the striking electron is captured to the same MO. An assumed MO, excited in the set of resonances, is determined by quantum chemical calculations. This assumption is checked by the construction of correlation diagrams for electron states. The algorithm was applied to the monothiocarbonates and perfluoroalkenes and gave satisfactory results.

Adsorption of N 2 on the Pd(111) surface at 20 K : II. ARUPS and HREELS of the physisorbed phase

The adsorption of N 2 on Pd(111) has been investigated by means of angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and high-resolution electron energy-loss spectroscopy (HREELS). For N 2, which is adsorbed on top of the N 2 ML or on top of a Xe spacer layer, a vibrational fine structure of the lπ u level is observed, whose analysis reveals the existence of the delocalization of the photo-hole during the photoemission process. The 3σ g level shows a pronounced dispersion, whereas no band formation seems to occur in the case of the 2σ u level. The v(NN) stretching vibration of physisorbed N 2 lies at the gas-phase value of 289 meV. It is dipole-inactive, as it is in the gas phase and is excited through negative ion resonance (NIR) scattering. In agreement with a recent observation, it is found that the line shape of the losses observed through NIR scattering is characterized by an asymmetric tail at the high energy side. which is due to the simultaneous excitation of the ṽ (substrate-N 2) mode.

Resonances in electron scattering by molecules on surfaces

This article reviews the discovery, exploration, and application of negative-ion resonances in inelastic electron scattering by molecules adsorbed on surfaces. A major theme of the review is the degree to which the properties of resonances in free molecules are perturbed by adsorption. The influence of the surface upon the energy, lifetime (width), symmetry, and decay channels of molecular resonances is discussed, in the light of both experimental and theoretical studies of a wide range (from diatomic molecules to polymers) of both weakly bound (physisorbed) and strongly bound (chemisorbed) molecules. The metallic image potential, electron scattering by the atoms of the surface, and chemical bonding in chemisorption systems are found to be key factors in determining the energy, width, and symmetry of resonances in molecular adsorbates. In the case of oriented adsorbed molecules, the angular distribution of scattered electrons is found to reflect not only the symmetry of the resonant state (as in the gas phase), but also the orientation of the molecular axis. Coherent elastic electron scattering by the surface can modulate the angular distributions, as well as the shape of the resonance profile. Selection rules that govern the observed resonance behavior are discussed. A further consequence of adsorption is the enrichment of the range of channels into which resonances may decay, and the excitation of both molecule-surface and intermolecular vibrational modes has been established. The article concludes with an evaluation of future prospects for the investigation and application of resonances in adsorbed molecules.

Resonance behavior of electron scattering from nitrogen molecules adsorbed on metallic surfaces

We use the coupled angular modes theory of electron scattering from molecules to identify the important role played by substrate multiple scattering in determining the lifetime of transient negative ions in adsorbed molecules. For the 2Π g negative ion resonance in N 2, we demonstrate the phenomenon of complete resonance quenchingwhich occurs when the resonance energy is such that the probe electron is strongly reflected from the substrate. This situation may occur when the resonance energy lies in a band-gap of the unoccupied states of a metallic substrate. It is proposed that the simple resonance energy shift and lifetime reduction observed in previous experimental and theoretical studies of resonance electron scattering from adsorbates is by no means a universal phenomenon. The importance of the unoccupied metallic band structure of the substrate in determining the properties of negative ions in adsorbed molecules is discussed.

Resonant electron-stimulated desorption of O − ions from oriented O 2 on graphite

We present measurements of the yield of O − ions in the resonant dissociative attachment reaction e −+O 2→O 2 −→O+O − in ζ-phase (upright) O 2 physisorbed on graphite. The yield of O − as a function of the electron impact energy shows three resonances at 8, 10, and 13 eV. Angular distribution measurements were performed on the O − ions desorbed from each resonance by rotating the substrate between the electron source and the ion detector. In each case we find that the desorbed O − ions are confined to an angular range close to the surface normal. Two important factors contributing to these angular profiles are: (i) the cross-section for capture of the incoming electron into the negative-ion resonance state, and (ii) the probability that the desorbing O − ion in emitted in the direction of the detector. In the present case, the narrow O − emission profile dominates the angular distributions.

The influence of negative ion resonances on the electron impact excitation of the 33D state of helium near 60 eV

The authors have studied the electron impact excitation of the 33D state of helium at incident energies near the well known negative ion resonances He-(2s2 2p) 2P and He-(2s2p2) 2D at 57.2 and 58.3 eV respectively. This was done by measuring the polarization degree of the radiation emitted in the subsequent 33D to 23P transition both in a non-coincidence and a coincidence experiment. As the direct excitation process of 33D and the indirect excitation process via a negative ion resonance are indistinguishable, these processes interfere. Analysis of the present and previous results indicate that the effects of interference play a minor role for the polarization degree as measured in the non-coincidence experiment. For the coincidence experiment the effects of interference are important.

Surface analysis of polycarbonate thin films by High-Resolution Electron Energy Loss Spectroscopy. Negative ion resonances and surface vibrations

Surface analysis of polycarbonate thin films by High-Resolution Electron Energy Loss Spectroscopy. Negative ion resonances and surface vibrations

Physisorption of CO on Ag(111): investigation of the monolayer and the multilayer through HREELS, ARUPS, and TDS

The interaction of CO with Ag(111) has been investigated at temperatures below 35 K through high-resolution electron energy-loss spectroscopy (HREELS), angle-resolved UV photoelectron spectroscopy (ARUPS), and thermal desorption spectroscopy (TDS). From the dipole activity of the v(C-O) stretching mode observed in HREELS and from the application of the dipole selection rule in ARUPS we conclude that the molecules are adsorbed with a random orientation of the molecular axis in the monolayer. No dispersion of the valence levels is observed in the monolayer. In the multilayer the 5σ level exhibits a split which amounts to 0.65 eV at the \\ ̄ gG point and vanishes at k ∥ = 0.8 Å −1 indicating the formation of an ordered overstructure with more tha one molecule in the unit cell. No split at all could be resolved for the 4σ level. The negative ion resonance scattering, known from HREELS measurements in the gas phase, is observed only when the coverage of one monolayer is exceeded, indicating that even in the case of physisorption the metal surface influences the unoccupied 2π level of CO.

Resonance and dipole electron-scattering in physisorbed mono- and multilayers on Al(111) and Ag(111) surfaces

High-resolution electron energy-loss spectroscopy (HREELS) has been used with UV photoelectron spectroscopy (UPS) and thermal desorption spectroscopy (TDS) to investigate physisorbed N2 and CO on Al(111) and on Ag(111) with and without H2O coadsorption. Electron scattering occurs for N2 at all coverages via a negative-ion resonance (NIR). This can be recognized from the appearance of the characteristic features of this scattering mechanism: (a) the resonant dependence of the cross section on the primary energy, (b) the peculiar angular distribution of the scattered electrons, (c) the strong enhancement for the excitation probability of dipole-forbidden overtones and (d) the asymmetric lineshape of the stretching features due to the excitation of molecule against surface vibrations. No dependence of the scattering mechanism on the layer thickness can be established for N2, whereas for CO the critical coverage of one monolayer (ML) must be exceeded in order that the NIR scattering mechanism becomes active. The coadsorption of submonolayer quantities of H2O with a ML N2 on Al(111) leads to the formation of a mixed phase in which the H2O molecules are embedded in the N2 matrix and the NIR is quenched.

Resonance electron scattering by O2 monolayers on graphite

We present a high resolution electron energy loss spectroscopy study of δ and ζ2 phase O2 monolayers physisorbed on graphite. Various molecular negative ion resonances are observed in the cross sections for vibrational and electronic excitation of the O2 molecules for electron beam energies in the range 4 <Ei < 10 eV. In particular, vibrational excitation is found to proceed via the 4∑u transient negative ion state in the δ phase, but via the 2Πu negative ion in the ζ2 phase. This “selective resonance population” is shown to arise because the resonance scattering cross sections are functions of the adsorbate molecular orientation in each phase, as well as the resonance symmetry and scattering geometry. Electronic excitation of the O2 molecule (X3σ−g → a1Δg) is proceed via the X2Πg negative ion resonance in both δ and ζ2 phase, though the resonance is shifted in energy between the two phases due to multiple electron scattering in the surface. The angular distributions of scattered electrons are used to study the structure of the O2 monolayer phases. The postulated molecular orientations in the δ phase (molecular axes approximately parallel to the surface) and in the ζ2 phase (approximately upright) are directly confirmed. Further, multiple electron scattering calculations, performed to allow a quantitative analysis of the measured angular distributions, are consistent with substantial dynamical tilting of the O2 molecules away from their equilibrium orientations in both phases, with an estimated mean tilt angle of 20° in the δ phase and 10° in the ζ2 phase.

H 2O-induced quenching of the negative-ion resonance scattering for N 2 physisorbed on Al(111)

Monolayers of N 2 physisorbed on the Al(111) surface at 20 K, were investigated by means of high-resolution electron energy-loss spectroscopy (HREELS) and UV photoelectron spectroscopy (UPS). The N 2 stretching vibration at 290 meV is excited by negative-ion resonance scattering. By coadsorption of a 0.16 monolayer of H 2O, the N 2 stretching mode is strongly suppressed. This is explained as due to a complete quenching of the negative-ion resonance of N 2 by the H 2O molecules.