Negative Ion States Research Articles

Low-energy electron scattering cross sections of halofluorocarbons

The interaction of low-energy electrons with halogenated methanes is important in both their atmospheric and plasma-processing chemistry. In this work, the total electron scattering cross sections of mixed fluorohalomethanes (CFnX4−n) were measured for incident electrons in the energy range of 0.3–12 eV using electron transmission spectroscopy. Resonances in the scattering cross sections may be interpreted as the capture of low-energy electrons into unoccupied molecular orbitals. To aid in the assignments of the resulting negative ion states, we performed quantum-mechanical calculations of the electron attachment energies. The effect of halogen substitution on the orbitals participating in electron capture are examined.

Experimental study of vibrational excitation of allene by slow electron impact: Vibronic coupling in the short-lived negative ion states

Vibrational electron energy loss spectra, vibrational excitation functions, and angular dependence of vibrational energy losses were measured in relative units for propadiene (allene, H2C=C=CH2) in an incident energy range up to 16 eV. Resonant excitation via the 2 eV resonance is not very selective; symmetric and antisymmetric C–C–C stretch, CH2 twist and scissoring, CH stretch, and C–C–C bending are all excited. The antisymmetric C–C–C stretch and CH2 twist are excited by Jahn–Teller activity of the degenerate 2E resonance, the bending by vibronic coupling with higher lying resonances. The essential features of the excitation are qualitatively rationalized by a Hartree–Fock (HF)/6-31G* anion potential energy surface. Unspecific excitation of high vibrational levels, accompanied by detachment of slow electrons, is also observed to result from attachment of 2 eV electrons, and is rationalized as a consequence of temporary trapping of part of the nuclear wave packet on the bound (not autodetaching) part of the anion potential surface. Very broad resonance features are observed in the 2–16 eV range, mainly in the excitation functions of the C–H stretch and the CH2 scissoring vibrations. A moderately broad resonance peak at 11.5 eV, observed in the excitation of the symmetric and antisymmetric C–C–C stretch vibrations, is assigned to two overlapping σ* shape resonances. Absolute elastic cross sections are given for reference.

Electron emission in low-energy grazing collisions of N + and N 2+ with W(110) partially covered by Cs atoms

Electron energy spectra from slow nitrogen ions N + and N 2 + (50 and 500 eV) colliding under grazing incidence with W(110) surfaces are reported. The surface work function was varied by the exposure to cesium atoms. The electron energy spectra are interpreted in terms of various inter- and intra-atomic Auger processes: For N + Auger capture dominates up to a Cs coverage of about 0.2 ML (in terms of a completed monolayer (ML) at room temperature). For larger Cs coverages the resonant capture of one or two electrons from the surface leads to the formation of core excited states of the neutralized projectile (beyond 0.3 ML) and of the Feshbach resonance N−∗ (2s 22p 23s 2) (beyond 0.5 ML). These states manifest themselves by their decay via Auger deexcitation and autodetachment, respectively. We observe one additional rather sharp feature which we tentatively attribute to the decay of the metastable negative ion state N− (2s 22p 4 1D). For N 2 + besides Auger capture into the N 2 ground state also Auger deexcitation of excited N 2 molecules (B; a 3,1Π g) to the ground state seems to contribute to the spectra (below 0.2 ML). With increasing Cs coverage higher excited (Rydberg) states, in particular N 2∗ E 3∑ g +, become populated manifesting themselves by a variety of Auger deexcitation processes. For Cs coverages larger than 0.6 ML the resonant capture of an electron from the surface by the Rydberg state N 2 E 3∑ g + leads to the formation of the Feshbach resonance N 2−∗ E 2∑ g + (parent state N 2∗ E).

Calculation of the energies of .pi.* negative ion resonance states by the use of Koopmans' theorem

The use of Koopmans'theorem (KT) for the calculation of scaled energies of π * negative ion resonance states has been investigatcd by ab initio molecular theory as a function of basis set. HF/D95v energies calculated for HF/6-31G-optimized geometries have been found to correlate with experimental electron attachment energies for 56 π * negative ion states in 39 alkenes, polyenes, and benzenoid hydrocarbons with a correlation coefficient of 0.989 and with average and largest absolute errors of 0.11 and 0.32 eV, respectively. An equally good correlation was obtained at the same basis set for MP2/6-31G * -optimized geometries. Various factors that influence the quality of KT correlations are analyzed

Thermal electron attachment to C6F5X and C6H5X (X=I, Br, Cl, and F)

Rate constants have been measured for thermal electron attachment to C6F5X (X=I, Br, Cl, F, and H) and C6H5X (X=I, Br, Cl, and F) at room temperature in N2 buffer gas (1–100 Torr) using the pulse-radiolysis microwave cavity method. For all the compounds studied, the rate constants are of the two-body type. Unexpectedly, the values for C6F5X except C6F5H are all the same (∼2×10−7 cm3 molecule−1 s−1), which are higher than most of the previous values, while that for C6F5H, measured in Xe and Ar buffer gases, is very low (7×10−12 cm3 molecule−1 s−1). For C6H5X, the value decreases dramatically with varying X from I to Br to Cl as 1.0×10−8 to 6.5×10−12 to 3×10−14 cm3 molecule−1 s−1, and that for C6H5F must be much lower than 10−13 cm3 molecule−1 s−1. These results for the magnitude of the rate constant are rationalized by the variation in the energy of a transient negative-ion state of each molecule, which results from a combination of the electron affinities of constituents (halogen atom X and C6F5 radical) and the strength of the C6F5–X (or C6H5–X) bond.

Temporary negative-ion resonances in the NiO(100) high-resolution electron-energy-loss spectrum.

The NiO(100) Fuchs-Kliewer phonon spectrum has been investigated by high-resolution electron-energy-loss spectroscopy (HREELS) at beam energies of 0.8--17 eV and the intensity distribution of the phonons relative to the dielectric properties of the NiO substrate has been considered. Over most of the range, the spectrum is well behaved, with the single-loss phonon energy at -70.5 meV and multiple excitations decreasing according to the predicted Poisson distribution. At 5.0-, 8.5-, and 13.5-eV primary beam energies, however, a substantial resonant enhancement in the phonon cross section is observed. The enhancement is accompanied by a shift from the Poisson intensity distribution to favor higher multiple excitations and an increased scattering away from the specular lobe. The energies correspond to maxima in the nickel oxide unfilled density of states and the resonances are assigned as temporary negative-ion states in which the electrons of the HREELS beam are trapped briefly into these levels.

Theoretical study of the electronic spectrum of the CoH molecule

Calculated potential energy curves and spectroscopic parameters of the ground and various low-lying excited states of the cobalt hydride molecule are presented. Over 30 electronic states of singlet, triplet, and quintet multiplicity have been obtained with adiabatic excitation energies below 4 eV. In addition, the electronic structure of several negative ion states has been determined. CoH− possesses a 4Φ electronic ground state and at least three other electronic states that are stable with respect to electron autodetachment. The calculations include relativistic effects variationally by employing a one-component no-pair operator with external-field projectors. Electron correlation is accounted for by using multireference single and double excitation configuration interaction methods. Several experimentally observed bands in the optical spectrum of CoH are ascribed to transitions between the X 3Φ ground state and excited states of 3Φ, 3Δ, and 1Γ symmetry.

New resonances in high resolution EELS of adsorbed molecules: electronic excitation of physisorbed O 2

We present an electron energy loss study of the X 3Σ g − → a 1Δ g electronic excitation of O 2 monolayers physisorbed on Pt(111) and graphite. We find negative ion resonances at incident electron energies of ∼6 and 13 eV. The resonance at 13 eV has not previously been observed in either gas phase or adsorbed O 2. The angular variation of the a 1 Δ g intensity indicates that the 13 eV resonance most likely corresponds to the 2 Δ u negative ion state. This assignment contrasts with the assignment of the 13 eV resonance reported in electron stimulated desorption studies of condensed O 2.

Bound excited states of chloroethylene anions studied by electron capture negative ion mass spectrometry

Electron detachment from the ground state of negative ions of chloroethylenes and dissociation from a bound excited state have been simultaneously observed by measuring the temperature dependence of [Cl - ]/[M - ] in a thermal electron attachment chemical ionization mass spectrometer source. The data show two slopes in a typical reciprocal temperature plot. The lower slope is the difference in activation energies for attachment to the two negative ion states, and the higher slope gives the detachment energy from the ground state. The detachment energy for C 2 Cl 4 - is 0.5 eV while that for C 2 HCl 3 - is 0.3 eV

Precise nonvariational calculation of the two-photon annihilation rate of the positronium negative ion.

A direct (nonvariational) solution of the Schr\"odinger equation for the ground state of the positronium negative ion is obtained with the correlation-function hyperspherical-harmonic (CFHH) method. Given the proper correlation function chosen from physical considerations, the CFHH method generates wave functions accurate in the whole range of interparticle distances that lead, in turn, to precise estimates of the expectation values of the Hamiltonian and of different functions of interparticle distances. The correlation function used was chosen to have proper electron-positron and electron-electron cusps as well as asymptotic behavior. The inclusion of 225 hyperspherical functions yields the nonextrapolated ground-state energy value of 0.262 005 058 atomic units, which is lower than the nonextrapolated energy values 0.262 004 895 and 0.262 005 056 calculated in works of Ho [J. Phys. B 16, 1503 (1983)] and Bhatia and Drachman [Phys. Rev. A 28, 2523 (1983)] but higher than the best variational value 0.262 005 069 obtained by Petenlenz and Smith [Phys. Rev. A 36, 5125 (1987)]. The accuracy of our value of 2.086 10 \ifmmode\pm\else\textpm\fi{}0.000 06 ${\mathrm{nsec}}^{\mathrm{\ensuremath{-}}1}$ for the two-photon annihilation rate is higher by an order of magnitude than obtained in the previous literature.

Crystallographic dependence of recoiledO−-ion fractions from Ni{100}c(2×2)-O and NiO{100} surfaces

The negative-ion fractions of oxygen directly recoiled from a Ni{100} c(2×2)-O and a NiO{100} surface have been measured as a function of the crystal azimuthal angle and the recoiled atom exit trajectory from the surface. The data exhibit a pronounced angular dependence of the ion fractions, i.e., the recoiled O - ion survival probability depends on both its perpendicular velocity and the distance between the recoiled O - and the chemisorbed O atoms and Ni substrate atoms. A theoretical analysis of the data suggests that at steep exit angles, the angular dependence of the ion fraction is dominated by the lateral dependence of the survival probability of the negative-ion state oriented perpendicular to the surface

Differential cross sections for elastic scattering of electrons from N2 at 0.55, 1.5 and 2.2 eV

Differential cross sections for vibrationally elastic scattering of electrons from N2 have been measured at three energies using the relative flow technique and normalizing to the cross sections in helium. At 1.5 eV the authors' results generally support the observations of Brennan et al. (1992), who find substantially larger cross sections than previously reported by Shyn and Carignan (1980) and by Sohn et al. (1986). At 0.55 eV, the authors' results are also larger than those of Sohn et al. but in good agreement with the calculations of Morrison et al. (1987). A measurement is also reported at the second peak of the 2 Pi g temporary negative ion state, and several difficulties in making comparisons with theory and experiment are elucidated.

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.

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.

Role of inelastic electron scattering by molecular nitrogen in the formation of oxide (O2)n-) anions in mixed molecular oxygen/molecular nitrogen clusters

Negative ion formation following electron capture by mixed O[sub 2]/N[sub 2] clusters was studied in a supersonic beam experiment. Inelastic electron scattering via the 2.3-eV negative ion state of N[sub 2] greatly affects the formation of O[sub 2][sup [minus]] and (O[sub 2])[sub 2][sup [minus]]. 21 refs., 4 figs., 1 tab.

Autoionization and autodetachment in collisions of slow inert gas ions with partially K covered W(110) surfaces

We report the spectra of the electrons emitted after slow collisions (50 to 1000 eV) of inert gas ions (He +, Ar +, Xe +) with partially K covered W(110) surfaces. At collision energies below 250 eV autodetachment of the excited negative ion states X −∗ (He −∗(1s2s 2), Ar −∗(3p 54s 2), Xe −∗(5p 56s 2)), formed by resonant two-step capture of two surface electrons, is collisions the formation of the He −∗ resonance could be confirmed by model calculations which simulate the electron spectra. At higher energies in addition the autoionization of Ar ∗∗(3p 44s 2) and K ∗∗(3p 54s 2) is detected. These states are formed via promotion in inert gas ion collisions with the surface adsorbed K atoms. Besides these intra-atomic Auger processes also interatomic Auger processes involving one or two electrons of the surface (Auger de-excitation and Auger capture) are observed.

Ca negative-ion binding energy.

We report our recent results of the measurement of the binding energy of the ground state of the calcium negative ion using electric field dissociation and accelerator mass spectrometry. The electron affinity of Ca was measured to be 17.${5}_{\mathrm{\ensuremath{-}}2}^{+4}$ meV for a p-wave electron emission, which is in good agreement with recent photodetachment experiments performed by Walter and Peterson [Phys. Rev. Lett. 68, 2281 (1992)].

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.

Temporary anion states of hydrogen cyanide, methyl cyanide, and methylene dicyanide, selected cyanoethylenes, benzonitrile, and tetracyanoquinodimethane

The vertical electron affinities of temporary negative ion states of HCN, CH{sub 3}CN, CH{sub 2}(CN){sub 2}, acrylonitrile, fumaronitrile, benzonitrile, tetracyanoquinodimethane, and tetracyanoethylene are determined using electron transmission spectroscopy and molecular orbital calculations.