Negative Ion Resonances Research Articles

Dynamics of the dissociative electron attachment in H2O and D2O: The A1 resonance and axial recoil approximation#

The dynamics of the formation and decay of negative ion resonance of A1 symmetry at 8.5 eV electron energy in the dissociative electron attachment (DEA) process in H2O and D2O are investigated using the velocity slice imaging technique. While the highest energy hydride ions formed by DEA show angular distributions characteristic to the A1 symmetry, those formed with low-kinetic energy show considerably different angular distributions indicating changes in the orientation of the dissociating bond due to bending mode vibrations. Our observations are quite different from the recently reported measurements, but consistent with the fully quantum calculations. Dynamics of the 2A1 negative ion resonance in water is probed using velocity slice imaging of H– from H2O and D– from D2O. While the momentum distributions of the highest energy ions agree with quantum calculations, those of low energy ions indicate the role of bending mode excitations before dissociation.

On relativistic shifts of negative-ion resonances

Results from a Dirac approach to electron- (neutral) atom resonance calculations are compared with those from nonrelativistic ones. A recently developed amplitude-phase method for the Dirac equations that is valid also in the non-relativistic limit is applied. The main conclusion is that significant energy differences are expected for long-lived resonances.

Switching ability of nitro-spiropyran on Au(111): electronic structure changes as a sensitiveprobe during a ring-opening reaction

Spiropyran is a prototype molecular switch which undergoes a reversible ring-opening reactionby photoinduced cleavage of a C–O bond in the spiropyran (SP) to the merocyanine (MC)isomer. While the electronic states and switching behavior are well characterized insolution, adsorption on metal surfaces crucially affects these properties. Using two-photonphotoemission and scanning tunneling spectroscopy, we resolve the molecular energy levelson a Au(111) surface of both isomeric forms. Illumination at various wavelengths does notyield any observable switching rate, thus evidencing a very small upper limit of thequantum efficiency. Electron-induced switching from the SP to the MC isomer viageneration of a negative ion resonance can be detected with a quantum yield of(2.2 ± 0.2) × 10−10 events/electron in tunneling spectroscopy. In contrast, the back reaction could not be observed. This studyreveals that the switching properties of surface-bound molecular switches can be verydifferent compared with free molecules, reflecting the strong influence of the interactionwith the metal substrate.

Semtex 1A and H negative ion resonances for explosives’ detection

We present the results obtained in dissociative electron attachment to Semtex in the gas phase by making use of a crossed electron-molecular beam experiment. Energy dependence of partial cross sections has been measured in an electron energy range from 0 to 12 eV with an energy resolution of ∼80 meV. Semtex 1A and H are mixtures of two other explosives, i.e., royal demolition explosive – RDX (hexogen) and pentaerythritol tetranitrate – PETN (pentrite), appearing in different fractions. In fresh samples product anions from both explosives can be detected together with anions originating from volatile markers. After some time we only observe product ions formed upon dissociative electron attachment to PETN, which are not observed for a mixture of pure RDX and PETN. For both Semtex samples, an additional anionic feature at 210 m/ z can be assigned to the binder butyl rubber.

Elastic scattering of slow electrons from Y, Ru, Pd, Ag and Pt atoms: search for nanocatalysts

Electron elastic scattering total cross sections (TCSs) for Y, Ru, Pd, Ag and Pt atoms are investigated in the electron impact energy range 0 ⩽ E ⩽ 7.0 eV. The complex angular momentum method that incorporates the electron–electron correlations and core-polarization interactions, crucial for the existence and stability of most negative ions, is used for the calculations. The TCSs for these atoms are found to be characterized by shape resonances (short-lived resonances), Ramsauer–Townsend minima and very sharp resonances (long-lived resonances) that correspond to the existence of stable bound states of the negative ions formed during the collision as Regge resonances. Using the configuration of the negative-ion resonances and Ramsauer–Townsend minima in the Au TCS as the benchmark for nanocatalysts, we conclude that the Y, Ru, Pd, Ag and Pt atoms represent excellent candidates for nanocatalysts individually or in various combinations. Calculated electron elastic TCSs for Y, Ru, Pd, Ag and Pt atoms are presented as illustrations.

Dissociative electron attachment to H2S probed by ion momentum imaging

The dissociation dynamics of negative ion resonance states in H(2)S formed upon electron attachment are studied using momentum imaging of the fragment H(-) and S(-) ions and compared with similar resonances in water. The H(-) momentum images show that dissociation dynamics at the 5.2 eV resonance are very similar to those of the 6.5 eV (B(1)) resonance in water. Unlike the 8.5 eV resonance in water, which has A(1) symmetry but is found to display considerable deviation from the axial recoil approximation in the momentum distribution of H(-) ions, the distribution from the corresponding resonance in H(2)S at 7.5 eV is found to follow the axial recoil approximation fairly well. The resonance state with B(2) symmetry at 10 eV is found to decay via four dissociation channels viz.-H(-) + H + S, H(-) + SH(A(2)Σ), H(-) + SH(X(2)Π) and S(-) + H + H channels, similar to those that were seen in the B(2) resonance in water at 12 eV, including sequential fragmentation in the multiple fragmentation channels. However, the angular distributions for the fragment ions from this resonance are found to be distinctly different from those in water, even while displaying considerable deviation from the axial recoil approximation similar to that in water.

A New Mechanism of Atomic Manipulation: Bond-Selective Molecular Dissociation via Thermally Activated Electron Attachment

We report a new mechanism of (bond-selective) atomic manipulation in the scanning tunneling microscope (STM). We demonstrate a channel for one-electron-induced C-Cl bond dissociation in chlorobenzene molecules chemisorbed on the Si(111)-7 × 7 surface, at room temperature and above, which is thermally activated. We find an Arrhenius thermal energy barrier to one-electron dissociation of 0.8 ± 0.2 eV, which we correlate explicitly with the barrier between chemisorbed and physisorbed precursor states of the molecule. Thermal excitation promotes the target molecule from a state where one-electron dissociation is suppressed to a transient state where efficient one-electron dissociation, analogous to the gas-phase negative-ion resonance process, occurs. We expect the mechanism will be obtained in many surface systems, and not just in STM manipulation, but in photon and electron beam stimulated (selective) chemistry.

Resonance phenomena in electron scattering by atomic oxygen from9to11.4eV

The momentum-space coupled-channels optical method has been used to study negative-ion resonance phenomena in electron impact on atomic oxygen in the energy region of 9-11.4 eV. Total scattering cross sections have been calculated, and negative-ion resonances have been located. Most of the known resonances of available experimental observations and theoretical results have been confirmed, and some new resonance features have been found in the present calculation.

Relativistic convergent close-coupling method applied to electron scattering from mercury

We report on the extension of the recently formulated relativistic convergent close-coupling (RCCC) method to accommodate two-electron and quasi-two-electron targets. We apply the theory to electron scattering from mercury and obtain differential and integrated cross sections for elastic and inelastic scattering. We compared with previous nonrelativistic convergent close-coupling (CCC) calculations and for a number of transitions obtained significantly better agreement with the experiment. The RCCC method is able to resolve structure in the integrated cross sections for the energy regime in the vicinity of the excitation thresholds for the $(6s6p){ }^{3}{P}_{0,1,2}$ states. These cross sections are associated with the formation of negative ion (${\mathrm{Hg}}^{\ensuremath{-}}$) resonances that could not be resolved with the nonrelativistic CCC method. The RCCC results are compared with the experiment and other relativistic theories.

Excitation dynamics and post-collision interaction for lowest autoionizing states in Li and Li2 excited by electron impact

The ejected-electron spectra in the region of the lowest autoionizing states in Li and have been studied at an observation angle of 54.7° over the electron-impact energy range 0–6 eV above the excitation threshold at 56.4 eV. The strong near-threshold resonance excitation has been revealed for the and (1s → 3σg)1Σ+g molecular transitions. For the atomic , and molecular states the post-collision interaction (PCI) energy shift of corresponding lines has been measured and compared with electron-impact excitation functions for these states. The general appearance of the PCI data for the and states reveals a close agreement with the classical and semi-classical predictions, showing a minimal correlation between resonance excitation of the subshell and the PCI. Meanwhile, in the energy region of 0–1 eV the energy shift of the line reveals an abrupt decrease to nearly zero values, similar to that observed earlier for the (np5(n+1)s2)2P3/2 lines in the ejected-electron spectra of Na (n = 2) and K (n = 3) atoms. The calculations show the possibility for extrema of the PCI shift to appear in those energy regions where the negative-ion resonances are present in the excitation function of the state.

Spin-polarised Stokes parameters of open and closed shell transitions and their resonances in zinc atoms

The formation and decay processes of negative ion resonances are explored for zinc atoms using spin-polarised incident electrons with observation of visible photons from the resonance decay into the 6s 1S, 4d 1,3D and 4p 1,3P states. For Fano resonances, near singly and doubly excited neutral atom states and two ionic states, the Stokes parameters of the radiation indicate markedly different effects of electron exchange and spin-orbit interactions as well as the angular momentum effects of closed and open 3d10 core states.

Negative-ion resonances in total cross sections for slow electron-sodium collisions

The interaction of electrons and atomic sodium was studied by using the momentum space coupled-channel optical method at low-impact energies. Total scattering cross sections have been calculated, and negative-ion resonances have been located. The present calculation results are in good agreement with available experimental data and previous theoretical results. We have obtained resonance features at $4.166$ and $4.502$ eV and also found possible resonance structures near the ionization threshold in the total cross sections.

Negative-ion resonances and cusps in4 1,3Pelectron excitation functions of the zinc atom

Negative-ion resonance structures, cusps, and possibly broad shape resonances were observed in the electron impact excitation of the $4s4p {}^{1}{P}_{1}$ and $4s4p {}^{3}{P}_{0,1,2}$ states of zinc near the thresholds of the lowest autoionizing states. Angle differential scattered electron excitation functions show strong contributions from the decay of negative-ion resonances built on a $3d$ excited ion core with final state spin-dependent decay. Most resonances have a $3{d}^{9}{\mathit{ns}}^{2}4{p}^{2}$ electron configuration, and they appear in one or both of the ${}^{1}{P}_{1}$ and ${}^{3}{P}_{0,1,2}$ states but with different branching ratios and angular behaviors.

Electron correlations and spin effects activated by excitation of an inner 3d electron in zinc

Excitation of the 4s4p3P0,1,2 states of zinc was studied in the energy region of the lowest 3d−1 autoionizing state using spin polarized incident electrons. The energy dependence of angular differential excitation intensities was measured for incident electrons with spin up and down. Negative ion resonances with a 3d94s24p2 electron configuration show prominent structure in the angular differential cross section and also show significant spin dependence which is prominent especially at 90°.

The post-collision energy shift of the (np5(n + 1)s2)2P3/2 lines in autoionizing spectra of sodium (n = 2) and caesium (n = 5) atoms excited by electron impact

The energy shift of lines arising from the decay of the (np5(n + 1)s2)2P3/2 autoionizing states in Na (n = 2) and Cs (n = 5) atoms has been studied for the first time in ejected-electron spectra measured over the excess energy range −0.1 to 5.4 eV. In Na spectra, the energy dependence of the shift possesses a clear oscillatory-like character. In particular, the shift becomes of negative or positive sign depending on the presence or absence of a negative-ion resonance structure in the excitation function of the (2p53s2)2P3/2 state. Earlier, we reported a similar behaviour of the line shift for the (3p54s2)2P3/2 autoionizing state in K atoms. Though in Cs atoms the strong negative-ion resonance is also present close to the excitation threshold of the (5p56s2)2P3/2 state, in contrast to Na and K data, the energy dependence of the line shift possesses a smooth asymptotic behaviour described well by the classical model of the post-collision effect. The comparative analysis of the spectroscopic classification of the resonance structure in the excitation of the (np5(n + 1)s2)2P3/2 autoionizing states in Na, K and Cs atoms has shown that the observed effect most probably reflects the complication of the compound character of resonances. In whole, the obtained data show that under the condition of the high polarizability of a resonantly excited target the relation between polarization attraction, centrifugal and Coulomb repulsion becomes a crucial factor which determines the final energy distribution of outgoing electrons.

Low-energy vibrational excitation cross section of multilayers of CO molecules on graphite using electron scattering

This paper presents a low-energy study of the vibrational excitation cross section of 50 layers of CO molecules on graphite at 23 K using the electron energy loss spectroscopy technique. The vibrational excitation cross section or the excitation curve is presented for two different scattering angles. The results show an energy dependence of the vibrational excitation cross section, at both angles, which indicates the formation of negative ion resonance in this adsorbed phase of CO on graphite. The data show an oscillatory structure in the region between 0.5 and 3 eV that is associated with the excitation of the 2Π state.

Scattering asymmetries in excitation of zinc by spin polarized electrons

Excitation of the 4s4p 3,1P states in zinc at energies corresponding to the 3d94s24p autoionizing states was studied using the spin polarized electrons. The spin dependent angle differential scattered electron intensities were measured at 30°, 54° and 90° and corresponding spin up/down scattering asymmetries evaluated. Significant nonzero asymmetries have been observed at energies where 3d core excited negative ion resonances affect electron scattering. The non-zero asymmetries observed for the 3P0,1,2 excitation, inspite the fact that the fine structure of this state was not resolved, indicate to the significance of electron correlations and possible configuration mixing.

Inner-shell photodetachment from Se− negative ions at the ALS

The photodetachment spectrum of Se− from 50 – 62 eV has been investigated using the merged ion-photon beam photodetachment technique. Se+ and Se++ ions formed by double and triple detachment were detected as a function of photon energy. Correlations in short-lived negative ion resonances formed by completely filling the valence 4p shell in Se- by photoexcitation of 3d electrons lead to three resonance structures. The resonances are observed in both the Se+ and Se++ decay channels bound by several eV below the 3d detachment threshold near 54 eV.

Post-collision interaction in electron impact excitation of the p6 outer subshell in Na, K, and Cs atoms

The post-collision interaction (PCI) energy shift of lines resulting from the decay of the np5(n+1)s22P3/2 lowest autoionizing states has been measured in the ejected-electron spectra of Na, K, and Cs atoms. The data reveal several new features in the energy dependences of the line shift. Particularly, contrary to the smooth asymptotic character predicted by the known models of the PCI, the oscillations of the shift are observed in Na and K atoms. The oscillations appear in those energy regions where the strong negative-ion resonances are present in excitation functions for the np5(n+1)s22P3/2 states. Despite the strong resonance excitation of the 5p56s22P3/2 state in Cs atoms, the respective PCI shift satisfies the classical description of the effect.

Energy shift of the (2p53s2)2P3/2 line in ejected-electron spectra of sodium atoms excited by low-energy electron impact

The energy shift of the line resulting from the decay of the (2p53s2)2P3/2 lowest autoionizing state in Na atoms has been measured in the ejected-electron spectra obtained at near-threshold electron impact energies. Contrary to the smooth asymptotic behavior predicted by the known models of the PCI effect, the data reveal pronounced oscillatory character of the energy dependence of the shift in the energy regions where the strong negative-ion resonances are present in the excitation function of the 2P3/2 state.