Negative Ion States Research Articles

Lifetimes of negative-ion states near metal surfaces.

Energy shifts and broadening of negative-ion states near metal surfaces are calculated using the complex-scaling technique. The affinity levels of the atoms are described using a nonlocal pseudopotential derived with use of the separable-potential method. The affinity levels shift downward with decreasing atom-surface separation. The widths of the affinity levels are found to be narrow compared to the width of typical excited states.

ETS study of the negative ion states of bicyclo[1.1.1]pentane, and its mono- and di-ethynyl derivatives: through-bond coupling of π* orbitals mediated by the staffane bridge

Electron transmission spectroscopy was used to study the through-bond coupling capacity of the bicyclo[1.1.1]pentane bridge for π* orbitals. Electron attachment energies of 1.66 and 2.78 eV were measured for 1,3-diethynylbicyclo[1.1.1]pentane, resulting in a π* splitting of 1.1 eV. This is larger than the splitting of the π levels determined earlier by PES (0.68 eV) and indicates a sizeable through-bond coupling. To aid in the interpretation, attachment energies were also measured for bicyclo[1.1.1]pentane (5.35 eV), for 1,1′-bi(bicyclo[1.1.1]pentane) (4.29 and 5.92 eV), and for 1-ethynylbicyclo[1.1.1]pentane (2.41 and 5.02 eV). The results are interpreted with the aid of qualitative arguments and of AM1 and Hartree—Fock virtual orbital energies within the context of Koopmans' theorem. The bicyclopentane bridge has σ and σ* orbitals to interact with both the in-phase and out-of-phase combinations of the acetylenic π* orbitals, and the observed splitting is thus a measure of the difference of the couplings. A minimal basis set is inadequate to reproduce the size of the splitting and we conclude that the trough-space interaction of the p π-basis orbitals on the C 1 and C 3 carbon atoms of the bicyclo[1.1.1]pentane bridge plays an important role.

Resonant electron capture in atom metal collisions: HAl(111)

Resonant electron capture in fast grazing HAl(111) collisions is studied with a new nonperturbative method, the coupled angular mode (CAM) method. The quantities needed to describe the electron capture, namely the energy position and autodetachment width of the H− negative ion state are determined. This static study, in contrast with an earlier perturbation treatment, is in agreement with data extracted from experimental results on electron capture by Wyputta et al.

Selective resonance population in EELS of physisorbed O2

We have observed low-energy electron scattering via two different transient negative-ion states (4Σuand2πu) in the same phase of O2 physisorbed on graphite. Resonant electron scattering via either of the two states can be highlighted by choice of a particular scattering geometry. The results manifest the distinctive symmetry of each negative-ion state, to which the incident and emitted electron wavefields couple. We find no evidence that the interaction of the O2 molecule with the substrate quenches either molecular resonance.

Dissociative electron attachment to singlet oxygen

Cross sections and O − kinetic energy measurements are reported for the dissociative electron attachment to O 2( 1Δ g) via the 2Π u and 2Σ + g negative ion states of O − 2.

Polycyclic aromatic hydrocarbons: negative ion formation following low energy (0–15 eV) electron impact

Long-lived (autodetachment lifetimes > 10 −6 s) parent negative ions of a number of polycyclic aromatic hydrocarbons (M) have been observed when the electron affinity (EA) of M exceeds a few tenths of an electronvolt. For the larger hydrocarbons with EA (M) ⩾ 0.5 eV the long-lived parent negative ion was found to form via as many as three negative ion states above 0.0 eV. The production of long-lived parent negative ions at electron energies well above 0.0 eV and with much larger cross sections than at 0.0 eV is attributed to electron-excited Feshbach resonances (i.e., two-electron negative ion states). Results are also reported on the formation of (MH) − from M; (MH) − is the only dissociative attachment negative ion observed.

Temporary negative ions and vibrational excitation in CH3Cl and CD3Cl

Electron energy loss spectroscopy is used to study the vibrational modes of CH3Cl and CD3Cl excited through the formation of a temporary negative ion state centered at 3.5 eV. Using symmetry and charge distribution arguments together with the experimentally determined excitation functions for several vibrational modes, we confirm that this resonance results from filling of the 8a1(C–Cl) σ* molecular orbital. Evidence for a broad higher-lying resonance which we attribute to the unfilled 4e orbital is also presented.

Mechanisms of anion formation in O2, O2/Ar and O2/Ne clusters; the role of inelastic electron scattering

Measurements are reported on the formation of negative ions in O2, O2/Ar and O2/Ne clusters aimed at establishing the mechanisms of anion formation and the role of inelastic electron scattering by the cluster constituents on negative ion formation in clusters. In the case of pure O2 clusters the main anions we detected are of two types: O−(O2) n≧0 and (O2) n ≧1− . The yields of O−(O2) n showed maxima at 6.3, 8.0 and ∼14.0 eV and the data suggest O− as their precursor; the maxima at 8 and ∼14 eV are due to the production of O− via symmetry forbidden dissociative attachment processes in O2 at these energies which become allowed in clusters. The yields of (O2) n − showed a strong maximum at near-zero energy (∼0.5 eV) and also at 6.3, 8 and ∼14 eV. With the exception of the near-zero energy resonance, the (O2) n − anions at 6.3, 8 and ∼14 eV are attributed to nondissociative attachment of near-zero energy “secondary” electrons to O2 clusters. The slow secondary electrons result predominantly from scattering via the O 2 − negative ion states of incident electrons with energies in their respective regions. Similar results were obtained for the mixed O2/rare gas clusters except that now a feeble and distinctly structured contribution in the yields of O−(O2) n , (O2) n − (and Ar(O2) n − ) was observed at energies >10 eV. These anions are believed to have the lowest negative ion states of Ar*− (Ne*−) as their precursors.

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.

Electron affinities, shake-up affinities, and ionization energies of S4 isomers

Nine isomers of S4 (D∞h,D2h,D4h,D3h,D2i,C2v, C2h,C3v,Cs) have been studied with large basis sets in Green function, CASSCF, and single and multi reference CI approaches. The C2v and D2h isomers are found to be nearly equally stable with the C2v isomer 1 kcal/mol below the D2h form and the C2h isomer about 10 kcal/mol higher in energy. All isomers have an appreciable electron affinity and some have several bound negative ion states. Besides normal electron affinities they have shake-up affinities, which may also be strongly positive. The lowest ionization energies of the isomers range from 7.5 to 9 eV.

Ionization energy calculations of MCl 4 (MTi, Zr and Hf)-type molecules by the MS Xα method

The ionization energies of core and valence electrons of MCl 4 (where MTi, Zr and Hf)-type complexes have now been calculated using the MS Xα method and comparison made with previously calculated values and experimental values reported earlier. This method helps to predict the ionization energies of the MCl 4 molecules. The periodic trend in the relativistic effect on MCl 4 has been discussed. The electron affinities computed by this method suggest that the negative-ion states of TiCl 4 seem to be more stable than the other two tetrachlorides.

Formation of negative ions of water clusters by electron transfer from high-Rydberg atoms

Negative ions of H 2O and D 2O clusters were produced by collisional electron transfer from highly excited Rydberg atoms, where no evaporation of monomers after electron transfer is expected. The threshold sizes observed in the mass spectra of (H 2O) − n and (D 2O) − n , n ⩾ 11 and 13 respectively, are interpreted as the critical sizes beyond which the vertical electron affinities of water clusters are positive. A comparison with previous reports shows that the negative-ion states of water clusters with near-threshold sizes are likely to be far from the solvated electron state.

Semiclassical study of He− doubly excited Rydberg states

The rovibronic spectra of highly excited Wannier-ridge states of negative helium ions have been calculated within the semiclassical theory, beyond the first-order perturbation theory. A thorough numerical analysis of the stability of classical configuration has been carried out and an estimate of the core electron impact on the lifetime of the resonance states is made. The calculations resolve degeneracies, both with respect to angular momentum and vibrational quantum numbers, which are different from previous analyses. The overall effect of the 1s electron motion on the correlated outer electron motions is found to be negligible.

Accuracy of three-body wave functions obtained with the correlation-function hyperspherical-harmonic method.

The local convergence and accuracy of wave functions obtained by direct solution of the Schr\"odinger equation with the help of the correlation-function hyperspherical-harmonic method are analyzed for ground and excited states of the helium atom and for the ground state of the positronium negative ion. The inclusion of the cusp conditions into the correlation function is shown to be of crucial importance, not only near the coalescence points, but also away from them. The proper inclusion of all cusps yields for the ground state of the helium atom the local wave-function accuracy of about ${10}^{\mathrm{\ensuremath{-}}7}$ for different interparticle distances. The omission of one of the cusps in the excited helium atom reduces the wave-function precision to ${10}^{\mathrm{\ensuremath{-}}2}$ near the corresponding coalescence point and to ${10}^{\mathrm{\ensuremath{-}}4}$--${10}^{\mathrm{\ensuremath{-}}5}$ away from it.

Position and width of a negative ion state in front of a surface: formation of C −( 4S) ions by electron capture

A new method for determining the position and width of negative ion states in front of a metal surface is presented. This coupled angular mode (CAM) method consists of studying the electron scattering by the superposition of an e −-atom (molecule) interaction and an e −-surface interaction. The surface interaction destroys the spherical symmetry of the e −-atom (molecule) scattering and mixes the various angular scattering modes. The e −1-atom (molecule) interaction is treated in the effective range approximation and then the present method is an extension of ERT to anisotropic environments; typically, it describes a negative ion in the field of its image charge. An application to the case of C − ion is presented. This system presents multichannel and orientation effects. Indeed, the partial widths of C − corresponding to the various orientations of C are found to differ by an order of magnitude. These results are used to discuss the experimental results of van Pinxteren et al. [Europhys. Lett. 10 (1989) 715] on C − formation by collision on a cesiated tungsten surface.

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.

Selective resonance population in electron scattering by adsorbed molecules.

We have investigated vibrational excitation of physisorbed ${\mathrm{O}}_{2}$ by electron impact in the 4--14-eV range, where a single broad resonance is observed in gas-phase studies. We show that by orienting the molecule in two different alignments on the surface of graphite it is possible to selectively populate two different negative-ion resonance states in this energy range, the $^{4}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}$ state (at \ensuremath{\sim}8.5 eV) and the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$ state (at \ensuremath{\sim}6 eV), each of which leads to molecular vibrational excitation. Calculations show that this selective resonance population arises from the distinctive differential electron-capture cross sections of the two molecular states, into which electrons are ``fed'' by the surrounding surface structure.

Low energy resonance states of negative molecular ions. Five-membered ring heterocycles and cyclopentadiene

Low energy resonance states of negative molecular ions. Five-membered ring heterocycles and cyclopentadiene

Electron scattering cross sections and negative ion states of silane and halide derivatives of silane

The interaction of low-energy electrons with silane and its halogenated derivatives is a fundamental step in many plasma chemical processes. The total scattering cross-section for electrons in the 0.2–12 eV range are observed to range up to 10−14 cm2. Scattering resonances are assigned to temporary negative ion states on the basis of correlations with analogous inner shell excitation resonances.

Physisorption of N 2 and CO on Al(111): A combined HREELS-UPS investigation

Mono- and multilayers of CO and N 2 physisorbed on Al(111) at 20 K were investigated by means of high-resolution electron energy-loss spectroscopy (HREELS) and UV photoelectron spectroscopy (UPS). From the desorption temperature an upper limit for the bonding energy to the Al(111) surface of 0.2 eV is estimated for both CO and N 2. Essentially, UPS and HREELS reproduce the gas phase data. The UP spectra are considerably broadened and shifted by 1 to 2 eV to smaller electronic binding energies. The energies of the molecular stretching vibrations are equal to the gas phase value within ± 1 meV. At the CO monolayer the electrons are inelastically scattered in the dipole mode. From the strong scattering cross section a complete in-plane orientation of CO can be excluded. The strong cross section for N 2 is due to resonance scattering via a known temporary negative-ion state.