Negative Ion Resonance States Research Articles

Taming Negative Ion Resonances Using Nonlocal Exchange-Correlation Functionals.

The characterization of negative ion resonances poses a fundamental challenge to density functional methods due to the unbound nature of resonances. To overcome this challenge, we propose one-particle nonlocal exchange-correlation (xc) potentials combining the exact-exchange (EXX) and the random phase approximation (RPA) correlation potentials. The negative ion resonances are identified by perturbing the real Hermitian nonlocal xc potentials using complex absorbing local potentials. Our studies show that the nonlocal EXX+RPA potential significantly enhances the description of positions and widths of negative ion resonance states compared to potentials that exclude dynamic polarization in RPA or include only EXX. The use of low-scaling algorithms simplifies the computation of the RPA potential, thereby providing a practical solution for resonance-state characterization within the density functional framework. A theoretical framework and the underlying assumptions required for combining real Hermitian nonlocal xc potentials with complex local potentials are discussed.

Negative Ion Resonance States: The Fock-Space Coupled-Cluster Way.

The negative ion resonance states, which are electron-molecule metastable compound states, play the most important role in free-electron controlled molecular reactions and low-energy free-electron-induced DNA damage. Their electronic structure is often only poorly described but crucial to an understanding of their reaction dynamics. One of the most important challenges to current electronic structure theory is the computation of negative ion resonance states. As a major step forward, coupled-cluster theories, which are well-known for their ability to produce the best approximate bound state electronic eigen solutions, are upgraded to offer the most accurate and effective approximations for negative ion resonance states. The existing Fock-space coupled-cluster (FSCC) and the equation-of-motion coupled-cluster (EOM-CC) approaches that compute bound states are redesigned for the direct and simultaneous determination of both the kinetic energy of the free electron at which the electron-molecule compound states are resonantly formed and the corresponding autodetachment decay rate of the electron from the metastable compound state. This Feature Article reviews the computation of negative ion resonances using the FSCC approach and, in passing, provides the highlights of the equivalent EOM-CC approach.

Shape resonance of sulphur dioxide anion excited states using the CAP-CIP-FSMRCCSD method

We have studied the shape resonance of excited states of sulphur dioxide (SO2) anion by using the correlated independent particle Fock space multi-reference coupled cluster (CAP-CIP-FSMRCCSD) method augmented by complex absorption potential. These resonant states have been trapped experimentally in recent years by electron collision. In particular, we have investigated e–-SO2 scattering and computed the negative-ion resonance states of the anion responsible for the two resonances around 4.45 and 6.56 eV and compared the results with the existing experimental observations. From the computational results using the CAP-CIP-FSMRCCSD method, it has been observed that both the resonances near 4.45 and 6.56 eV result from A1 symmetries.

Electron attachment and quantum coherence in molecular hydrogen

Single electron attachment to a molecule may invoke quantum coherence in different angular momentum transfer channels. This has been observed in the 14 eV dissociative electron attachment resonance in molecular hydrogen where a coherent superposition of two negative ion resonant states of opposite parity is created, with the s and p partial waves of the electron contributing to the attachment process. Interference between the two partial wave contributions leads to a forward – backward asymmetry in the angular distribution of the product negative ions. Since these two resonant states dissociate to the same n = 2 state of H and H−, this asymmetry is further modified due to interference between the two paths of the dissociating molecular negative ion along different potential energy curves. This interference manifests as a function of the electron energy as well as isotopic composition. This case is akin to the quantum interference observed in photodissociation by one-photon vs two-photon absorption.

Dissociative electron attachment to some spinochromes: Fragment anion formation

Resonance attachment of low-energy (0–14eV) electrons to a series of natural polyphenolic naphthoquinones (so-called spinochromes) derived from sea urchins is studied by means of dissociative electron attachment (DEA) spectroscopy. The experimental findings are interpreted using quantum-chemical calculations of the empty level structure to predict the energies of negative ion resonant states as well as the thermodynamic energy thresholds for formation of fragments by DEA. A variety of fragment species is found to be formed at very low energies of incident electrons, thus modeling reductive conditions in living cells near the pathways of cellular electron transfer. In particular, the temporary molecular anions of the compounds under investigation dissociate efficiently through elimination of neutral H atoms or diatomic hydrogen molecules (in analogy with previous findings in flavonoids), this process being energetically possible owing to the presence of multiple hydroxyl groups usually associated with radical scavenging activity. A likely correlation between dehydrogenation of spinochromes stimulated by electron attachment and their well-documented antioxidant protective properties is briefly discussed.

Dissociation dynamics in the dissociative electron attachment to carbon dioxide

Dissociative electron attachment (DEA) to gas phase ${\mathrm{CO}}_{2}$ has been probed using a velocity slice imaging technique. DEA to ${\mathrm{CO}}_{2}$ produces only an ${\mathrm{O}}^{\ensuremath{-}}$ ionic fragment and shows two major resonances located at 4.4 and 8.2 eV, respectively. The kinetic energy and angular distribution of the ${\mathrm{O}}^{\ensuremath{-}}$ ions are measured around the second resonance with higher efficiency and sensitivity that provide details of the DEA dynamics. The kinetic energy distributions are in good agreement with the previous reports. However, the distinct angular distributions show substantial difference from the two recent studies within the limited electron energies. Our angular distribution results show two negative ion resonant states are involved in the underlying DEA process at the entire electron energies over the second resonance. We discussed the recent conflicting findings in the angular distribution results. The forward-backward asymmetry observed in the angular distributions is explained due to the interference effect of different partial waves associated with the attaching electron.

Comment on “Coherent interference in the resonant dissociative electron attachment to carbon monoxide”

In a recent article [Phys. Rev. A 88, 012708 (2013)], Tian et al. claimed coherent interference among the various negative ion resonant states involved in the dissociative electron attachment (DEA) to carbon monoxide (CO) by investigating ${\mathrm{O}}^{\ensuremath{-}}$ angular distribution using the anion velocity time-sliced map imaging technique. However, our recent detailed study [Phys. Chem. Chem. Phys. 17, 7130 (2015)] on DEA to CO using the identical technique shows that the above claim can be questioned.

Electronically excited negative ion resonant states in chloroethylenes

The negative ion mass spectra of the resonant electron capture by molecules of 1,1-dichloroethylene, 1,2-dichloroethylene-cis, 1,2-dichloroethylene-trans, trichloroethylene and tetrachloroethylene have been recorded in the 0–12eV range of the captured electron energy using static magnetic sector mass spectrometer modified for operation in the resonant electron capture regime. As a result, several novel low-intensive dissociation channels were revealed in the compounds under study. Additionally, the negative ion resonant states were recorded at approximately 3–12eV, mostly for the first time. These resonant states were assigned to the electronically excited resonances of the inter-shell type by comparing their energies with those of the parent neutral molecules triplet and singlet electronically excited states known from the energy-loss spectra obtained by previous studies.

Scattering of electrons by a 1,2-butadiene (C4H6) molecule: measurements and calculations

We present the results of experimental and theoretical study on electron collisions with a 1,2-butadiene (H2C=C=CHCH3) molecule. Absolute grand-total cross sections (TCSs) were measured using a linear electron-transmission method for collision energies in the 0.5–300 eV range. Two distinct features in the TCS energy curve were detected: a narrow peak located at 2.3 eV and a broad enhancement centered around 9 eV. We attributed these features to the formation of negative-ion resonant states, based on comparisons with the existing data for other targets of similar structure. Experimental findings indicate a bonding effect on the TCS when confronting TCS curves for allenes with those for respective acetylenes. Clear differences in experimental TCS energy dependences for C4H6 isomers (1,2-butadiene, 1,3-butadiene, 1-butyne and 2-butyne) show on the isomeric effect. The independent atom approximation was employed to calculate the elastic (ECS) cross section from 25 to 3000 eV, while the binary-encounter-Bethe approach was used for computation of the ionization (ICS) cross section, from the threshold up to 3 keV. The sum of computed cross sections (ECS+ICS) quite reasonably reproduces the experimental TCS values above 35 eV. The reported TCS experimental values are not corrected for the forward-angle scattering effect.

Dissociative electron attachment resonances in ammonia: A velocity slice imaging based study

Negative ion resonance states of ammonia are accessed upon capture of electrons with energy 5.5 eV and 10.5 eV, respectively. These resonance states dissociate to produce H(-) and NH(2)(-) fragment anions via different fragmentation channels. Using the velocity slice imaging technique, we measured the angular and kinetic energy distribution of the fragment H(-) and NH(2)(-) anions with full 0-2π angular coverage across the two resonances. The scattered H(-) ions at both resonances show variation in their angular distribution as a function of the kinetic energy indicating geometric rearrangement of NH(3)(-*) ion due to internal excitations and differ from the equilibrium geometry of the neutral molecule. The second resonance at 10.5 eV shows strong forward-backward asymmetry in the scattering of H(-) and NH(2)(-) fragment ions. Based on the angular distributions of the H(-) ions, the symmetry of the resonances at 5.5 eV and 10 .5 eV are determined to be A(1) and E, respectively, within C(3v) geometry.

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.

Angular-momentum-dependent Fano profiles in excited zinc atoms

The Fano-equivalent profiles of two negative-ion resonant states have been observed in the integrated Stokes parameters of 636.2 nm photons emitted from the $3{d}^{10}4s4{d\phantom{\rule{0.2em}{0ex}}}^{1}{D}_{2}$ state of zinc. These profiles reflect the effects of electron exchange and the spin-orbit interaction via the angular-momentum-dependent properties of the alignment and orientation Stokes parameters. The resonant states have energies of $10.98\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $11.33\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and widths of $0.25\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $0.33\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, respectively, and are bound to an excited atomic state with a $3{d}^{9}$ core hole. The lower-energy resonance shows a strong spin-orbit interaction whereas the higher-energy resonance shows negligible spin-orbit interaction.

SFG Spectroscopy of CO/Ni(111): UV Pumping and the Transient Hot Band Transition of Adsorbed CO

Abstract A UV excitation by a picosecond pulse at 266 nm induced an unusual shoulder on the νCO = 1 ← 0 resonance peak of CO/Ni(111) monitored by sum-frequency generation (SFG) of visible and IR pulses. The observed line shape was reproduced by the use of a dipole-dipole interaction model with the coherent potential approximation (CPA) where the hot band transition with a population ratio of 0.3 to 0.7 (v = 1 to v = 0) was assumed. Neither the transition to the two-phonon bound state nor the coupling with the low-frequency phonon modes explained the observed changes. The shoulder appeared only during the UV excitation, which indicated that the electronically driven excitation, presumably by the hot electrons generated by the irradiation, dominated the process. As possible mechanisms, the involvement of an intermediate negative ion resonance state and/or the non-adiabatic coupling of electronic states with C–O stretching mode were considered.

Negative ion resonance states of SF 6 physisorbed on the graphite surface investigated by high-resolution electron energy loss spectroscopy

The study of the adsorption and electron driven reactions of etching molecules at surfaces is relevant both to the field of microfabrication and to the characterisation of the basic physics of molecule-surface dynamics. We have investigated the negative ion resonance states of sulphur hexafluoride (SF 6) physisorbed upon graphite (HOPG) at ∼85 K using high-resolution electron energy loss spectroscopy (HREELS) for both monolayer and multilayer coverages. The resonant energy profile of the ν 3 vibrational mode of SF 6 exhibits two peaks in the monolayer regime, at 1.75±0.5 eV and at 5.5±0.5 eV. The lower-lying resonant state is attributed to capture into the 6a 1g molecular orbital, and higher-energy resonance is assigned to the 6t 1u state. In the multilayer regime, there is a positive shift of ∼1 eV in the energy profile of these two peaks, which is attributed to the reduced influence of the image potential. No evidence for phase transitions has been found.

HREELS study of alkanethiol passivated gold clusters on graphite

Passivated metal (e.g. gold) nanoclusters consist of a metal particle core surrounded by a ligand coat. We have investigated films of gold particles passivated with alkanethiol (C 8H 17S) ligands deposited from solution onto a graphite surface using high-resolution electron energy loss spectroscopy (HREELS). Vibrational modes of the CH x groups of the ligands are observed, together with their overtones. Incident energy dependent measurements show an enhancement in intensity of these vibrational modes at a specific energy, ∼10 eV, consistent with a negative ion resonance state. This observation is compared with previous studies of self-assembled monolayers of thiols on the Au (111) surface.

Depletion of Molecular Vibrational Modes through Electron Impact and Evidence for Resonance Dissociation to Neutrals in CondensedCH3Cl/Graphite

We have employed high resolution electron energy loss spectroscopy to measure the depletion of the vibrational mode intensities of an adsorbed target molecule as a result of electron impact and as a function of incident energy. This new technique sums over all molecular decay channels in which a (specific) molecular bond is broken by the incident electron, irrespective of the fate of the fragments (ions or neutrals). In the case of the model system which we consider, physisorbed ${\mathrm{CH}}_{3}\mathrm{Cl}/\mathrm{graphite}$, we isolate a resonance in the depletion cross section at 3 eV, which also provides the first direct evidence for the dissociation of a negative ion resonance state into neutral fragments.

Quantum-mechanical wave packet calculation of photoinduced surface reaction: O2/Pt(111)

We present a two-dimensional quantum-mechanical wave packet study of photoinduced reaction of O2 on Pt(111) within a mechanism of hot electron/hole excitation of a molecular resonance. Based on three constructed potential energy surfaces including the molecule-surface and intra-molecular coordinates, photoexcitation is simulated by nonadiabatic electronic transitions between the ground state (the chemisorbed O2−) and a negative ion resonance state (the O22− shape resonance) or a neutral O2 state. The wave packet dynamics exhibits a fast energy exchange between the two bonds, giving a comparable yield for desorption and dissociation. The calculated branching ratio, BR=0.5–1.0, between desorption and dissociation and the mean kinetic energy of the desorbed molecules 〈Ekin〉/2kB= 990 K are comparable with the reported experimental data measured in desorption by nanosecond laser pulses, while the vibrational temperature is first predicted by this calculation. In addition our results indicate the importance of a proper treatment of damping effect in a coherent wave packet after deexcitation.

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

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.

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.