Anion Ground State Research Articles

Low-energy electron scattering from fullerenes and heavy complex atoms: negative ions formation

Regge poles are generalized bound states. Our robust Regge pole methodology that embeds the crucial electron correlation effects and the vital polarization interaction is used to investigate negative ions formation in low-energy electron elastic scattering from the fullerenes Cn (n = 24, 28, 44, 60, 70, 74, 82, 94, 100, and 140) and the selected heavy lanthanide (Gd and Dy) and actinide (Pa and U) atoms through the elastic total cross sections (TCSs) calculations. All the TCSs are found to be characterized by Ramsauer–Townsend minima, shape resonances, and dramatically sharp resonances manifesting stable ground and metastable anionic formation during the collisions. The ground states anionic binding energies (BEs) for the Cn− (n = 24, 28, 44, 60, 70, and 82) match excellently the measured electron affinities (EAs). The thus benchmarked Regge pole methodology on the ground states anionic BEs for these fullerenes is then used to calculate the ground and the metastable elastic TCSs for the fullerenes, and the heavy atoms wherefrom their anionic BEs are extracted and compared with the measured EAs where they are available. Surprisingly, the C74 fullerene has the largest anionic ground state BE value among the investigated fullerenes in this paper and the ground state of the C140− anion has a large BE as well. Many of these fullerenes could be useful in nanocatalysis, sensor technology and organic solar cells through their ground and metastable anionic BEs. For the heavy atoms the extracted ground and metastable anionic BEs are compared with the measured and calculated EAs. These results particularly the ground states anionic BEs of the fullerenes and heavy atoms are expected to inspire and guide the long overdue experimental and theoretical explorations of electron attachment in low-energy electron scattering in these and related systems.

Spin State Energetics of VGe n -/0 (n = 5-7) Clusters and New Assignments of the Anion Photoelectron Spectra.

The geometrical structures, electron leading configurations, and relative energies of the low-lying states of VGe n -/0 (n = 5-7) clusters have been investigated with density functional theory and CASSCF/CASPT2 method. The pure GGA BP86 functional gave almost the same relative energy order for the low-lying states as the CASPT2 method. At the BP86 and CASPT2 levels, the ground states of VGe n -/0 (n = 5-7) clusters were proposed to be the 1 A1 and 2 A1 of A-VGe5 -/0 , 3 A″ and 2 A″, 2 A' (2 E2 ) of A-VGe6 -/0 , and 1 A' and 2 A' of A-VGe7 -/0 isomers. The adiabatic and vertical detachment energies (ADEs and VDEs) of the detachments of one electron from several orbitals of the anionic ground states were reported at the CASPT2 level. The calculated ADEs and VDEs corresponded well with the experimental values as observed in the 266 nm anion photoelectron spectra. © 2018 Wiley Periodicals, Inc.

Characterization of the vibrational properties of copper difluoride anion and neutral ground states via direct and indirect photodetachment spectroscopy.

Photoelectron spectra of 63CuF2- are reported at wavelengths 310 nm, 346.6 nm, and 350.1 nm, obtained via velocity map imaging. The photoelectron angular distributions allow for the unambiguous assignment of a 2Σg+ neutral CuF2 ground state. Vibrational analysis of the direct detachment transitions in the spectra enables accurate determination of the anion and neutral bond length difference (0.073 Å), adiabatic electron affinity of CuF2 (3.494 eV) and symmetric stretching (500 cm-1, anion, and 630 cm-1, neutral) and antisymmetric stretching (610 cm-1, anion, and 782 cm-1 neutral) frequencies of the ground electronic states. Strongly photon energy dependent intensities are also observed for select transitions. Equation-of-motion coupled-cluster singles and doubles calculations augmented by a complex absorbing potential reveal a metastable 1Πg anion state which is optically accessible due to Renner-Teller coupling. Mediation of the detachment process by this state allows measurement of the bending frequencies (177 cm-1, anion, and 200 cm-1, neutral) completing the inventory of experimentally measured vibrational properties of the ground electronic states.

Dynamics of electron attachment and photodissociation in iodide-uracil-water clusters via time-resolved photoelectron imaging.

The dynamics of low energy electron attachment to monohydrated uracil are investigated using time-resolved photoelectron imaging to excite and probe iodide-uracil-water (I-·U·H2O) clusters. Upon photoexcitation of I-·U·H2O at 4.38 eV, near the measured cluster vertical detachment energy of 4.40 eV ± 0.05 eV, formation of both the dipole bound (DB) anion and valence bound (VB) anion of I-·U·H2O is observed and characterized using a probe photon energy of 1.58 eV. The measured binding energies for both anions are larger than those of the non-hydrated iodide-uracil (I-·U) counterparts, indicating that the presence of water stabilizes the transient negative ions. The VB anion exhibits a somewhat delayed 400 fs rise when compared to I-·U, suggesting that partial conversion of the DB anion to form the VB anion at early times is promoted by the water molecule. At a higher probe photon energy, 3.14 eV, I- re-formation is measured to be the major photodissociation channel. This product exhibits a bi-exponential rise; it is likely that the fast component arises from DB anion decay by internal conversion to the anion ground state followed by dissociation to I-, and the slow component arises from internal conversion of the VB anion.

Another Look at Photoelectron Spectra of the Anion Cr2O2-: Multireference Character and Energetic Degeneracy.

Geometrical and electronic structures of the chromium oxide dimer in both charge states Cr2O2-/0 were carefully and thoroughly studied using the restricted active space RASSCF/RASPT2 and density matrix renormalization group followed by second-order perturbation theory DMRG-CASPT2 methods in conjunction with large basis sets. The anionic and neutral clusters Cr2O2-/0 are characterized by a D2 h structure and high-spin ground states (10 and 9, respectively). RASSCF wave functions point out strong multireference characters of their electronic ground state. The two lowest-lying states 10Ag and 10B2g of the anion Cr2O2- are nearly degenerate, and both were proven to be populated and involved in the anion photoelectron spectra of Cr2O2-. The neutral cluster exhibits a 9B2g ground state. Within the active space of 20 electrons in 18 orbitals used, 30 electronic transitions starting from both lower-lying anionic states to corresponding neutral states were predicted. Transitions from the anionic ground state 10Ag cause most of visible bands in the anion photoelectron spectra of Cr2O2-, while the first band with low intensity is determined to arise from a transition starting from the nearly degenerate state 10B2g. Furthermore, multidimensional Franck-Condon factor simulations were carried out to further support our band assignments.

The Electronic Structures of CoGe n-/0 ( n = 1-3) Clusters from Multiconfigurational CASSCF/CASPT2 and RASSCF/RASPT2 Calculations.

Density functional theory and multiconfigurational CASPT2 and RASPT2 methods are employed to investigate the low-lying states of CoGe n-/0 ( n = 1-3) clusters. With the RASPT2 approach, the active space is extended to 14 orbitals for CoGe-/0, 17 orbitals for CoGe2-/0, and 20 orbitals for CoGe3-/0. These active spaces include the 3d, 4s, and 4d of Co and 4p of Ge. The 4d of Co is incorporated into these active spaces in order to account for the important double-shell effect of Co. The structural parameters, vibrational frequencies, and relative energies of the low-lying states of CoGe n-/0 ( n = 1-3) are reported. The ground states of CoGe n- ( n = 1-3) are computed to be 3Φ of linear CoGe-, 3B1 of cyclic CoGe2-, and 3B1 of cyclic CoGe3- isomer. The ground states of the neutral clusters are calculated to be 2Δ of linear CoGe, 4B1 of cyclic CoGe2, and 4A″ of tetrahedral CoGe3 isomer. The calculated adiabatic and vertical detachment energies of the anionic ground states are in agreement with the experimental values as observed in the 266 nm anion photoelectron spectra.

Photoelectron spectroscopy of anthracene and fluoranthene radical anions.

We report the slow electron velocity map imaging spectroscopy of cryogenically cooled anthracene and fluoranthene radical anions, two similarly sized polycyclic aromatic hydrocarbon molecules. The results allow us to examine the lowest energy singlet and triplet states in the neutral molecules on equal footing from the anionic ground state. The analysis of the experimental spectra is aided by harmonic calculations and Franck-Condon simulations, which generally show good agreement with experimental values and spectra. The electron affinity of fluoranthene is measured to be 0.757(2) eV, which is larger than that of anthracene at 0.532(3) eV. The lowest energy triplet state in anthracene is observed at 1.872(3) eV above the singlet ground state, while that of fluoranthene is observed at 2.321(2) eV above its singlet ground state. Comparisons of experimental and calculated spectra show that in addition to the Franck-Condon active modes, there is a clear presence of vibrational modes that gain intensity via vibronic coupling in both the singlet and triplet states in both molecules. In addition, the triplet state generally exhibits increased vibronic coupling compared to the singlet state, with the fluoranthene triplet state exhibiting evidence of distortion from C2v symmetry.

An Electron Density Source-Function Study of DNA Base Pairs in Their Neutral and Ionized Ground States†.

The source function (SF) decomposes the electron density at any point into contributions from all other points in the molecule, complex, or crystal. The SF "illuminates" those regions in a molecule that most contribute to the electron density at a point of reference. When this point of reference is the bond critical point (BCP), a commonly used surrogate of chemical bonding, then the SF analysis at an atomic resolution within the framework of Bader's Quantum Theory of Atoms in Molecules returns the contribution of each atom in the system to the electron density at that BCP. The SF is used to locate the important regions that control the hydrogen bonds in both Watson-Crick (WC) DNA dimers (adenine:thymine (AT) and guanine:cytosine (GC)) which are studied in their neutral and their singly ionized (radical cationic and anionic) ground states. The atomic contributions to the electron density at the BCPs of the hydrogen bonds in the two dimers are found to be delocalized to various extents. Surprisingly, gaining or loosing an electron has similar net effects on some hydrogen bonds concealing subtle compensations traced to atomic sources contributions. Coarser levels of resolutions (groups, rings, and/or monomers-in-dimers) reveal that distant groups and rings often have non-negligible effects especially on the weaker hydrogen bonds such as the third weak CH⋅⋅⋅O hydrogen bond in AT. Interestingly, neither the purine nor the pyrimidine in the neutral or ionized forms dominate any given hydrogen bond despite that the former has more atoms that can act as source or sink for the density at its BCP. © 2018 Wiley Periodicals, Inc.

Spin-Orbit Splittings and Low-Lying Electronic States of AuSi and AuGe: Anion Photoelectron Spectroscopy and ab Initio Calculations.

We measured the photoelectron spectra of diatomic AuSi- and AuGe- and conducted calculations on the structures and electronic properties of AuSi-/0 and AuGe-/0. The calculations at the CASSCF/CASPT2 level confirmed that experimentally observed spectra features of AuSi- and AuGe- can be attributed to the transitions from the 3Σ- anionic ground state to the 2Π (2Π1/2 and 2Π3/2), 4Σ-, 32Σ+, and 42Σ+ electronic states of their neutral counterparts. The electron affinities (EAs) of AuSi and AuGe are determined by the experiments to be 1.54 ± 0.05 and 1.51 ± 0.05 eV, respectively. The spin-orbit splittings (2Π1/2-2Π3/2) of AuSi and AuGe measured in this work are in agreement with the literature values. The energy difference between the 4Σ- (A) and 2Π1/2 states of AuSi obtained in this work is in reasonable agreement with the literature value, while that of AuGe obtained in this work by anion photoelectron spectroscopy is slightly larger than the literature value by neutral emission spectroscopy. The term energies of the 32Σ+ (B) and 42Σ+ (C) states of AuSi and AuGe were also determined based on the photoelectron spectra. Because of the different bond lengths between the anionic and neutral states, the electronic state terms energies of AuSi and AuGe estimated from the anion photoelectron spectra might be slightly different from those obtained from the neutral emission spectra.

New insights in low-energy electron-fullerene interactions

The robust Regge-pole methodology has been used to probe for long-lived metastable anionic formation in Cn (n = 20, 24, 26, 28, 44, 70, 92 and 112) through the calculated electron elastic scattering total cross sections (TCSs). All the TCSs are found to be characterized by Ramsauer-Townsend minima, shape resonances and dramatically sharp resonances manifesting metastable anionic formation during the collisions. The energy positions of the anionic ground states resonances are found to match the measured electron affinities (EAs). We also investigated the size-effect through the correlation and polarization induced metastable resonances as the fullerene size varied from C20 through C112. The C20 TCSs exhibit atomic behavior while the C112 TCSs demonstrate strong departure from atomic behavior attributed to the size effect. Surprisingly C24 is found to have the largest EA among the investigated fullerenes making it suitable for use in organic solar cells and nanocatalysis.

Probing the temperature-dependent changes of the interfacial hydration and viscosity of Tween20 : cholesterol (1 : 1) niosome membrane using fisetin as a fluorescent molecular probe.

A detailed photophysical study of fisetin in a Tween20 : cholesterol (1 : 1) niosome membrane has been carried out. Fisetin is found to partition well into the Tween20 : cholesterol (1 : 1) niosome membrane at low temperature (Kp = 2.7 × 104 M-1 at 10 °C). Cetylpyridinium chloride quenching study confirms the location of fisetin molecules in the interfacial domain of Tween20 : cholesterol (1 : 1) niosome membrane. The emission from the prototropic forms of fisetin (neutral form, excited state anion, ground state anion and phototautomer form) is found to sensitively reflect the local heterogeneities in Tween20 : cholesterol (1 : 1) niosome membrane. The shift in anionic emission maximum with variation in temperature shows the sensitivity of fisetin towards water accessibility at the interfacial domain of Tween20 : cholesterol (1 : 1) niosome membrane. Zeta potential value confirms that there is no role of surface charge in the multiple prototropism of fisetin in Tween20 : cholesterol (1 : 1) niosome membrane. The microviscosity changes with temperature, as reflected in fluorescence anisotropy values of fisetin phototautomeric species FT*, give information about the temperature-induced changes in the motional resistance offered by the interfacial domain of the niosomal membrane to small molecules. A temperature-dependent fluorescence lifetime study confirms the distribution of FT* in the two different sites of niosomal interfacial domain, i.e. water-deficient inner site and water-accessible outer site. This heterogeneity in distribution of FT* is further confirmed through time-resolved fluorescence anisotropy decay resulting in two different rotational time constants (faster component of ∼1.04 ns originates from water-accessible outer site and slower component of ∼16.50 ns originates from water-deficient inner site). The interfacial location of fisetin in Tween20 : cholesterol (1 : 1) niosome membrane has an important implication with regards to antioxidant activity as confirmed from a DPPH radical scavenging study.

A CASSCF/CASPT2 investigation on electron detachments from ScSi n- (n=4-6) clusters.

In this work, the low-lying states of several isomers of ScSi n-/0 (n=4-6) were investigated with the B3LYP functional and CASPT2 method. The ground states of the anionic clusters were predicted to be the singlet states of the trigonal bipyramid ScSi4- (A-ScSi4-), the face-capped trigonal bipyramid ScSi5- (A-ScSi5-), and the pentagonal bipyramid ScSi6- (A-ScSi6-) isomer. Based on the anionic ground states, all the relevant electron detachment processes were identified. The corresponding adiabatic and vertical detachment energies (ADEs and VDEs) of the anionic clusters were computed at the CASPT2 level. The calculated results were used to interpret all the important features in the photoelectron spectra of ScSi n- (n=4-6) clusters. Franck-Condon factor simulations were also performed based on the B3LYP geometries, vibrational frequencies, and normal modes to explain the shapes of the first bands in the spectra.

Insights into Geometric and Electronic Structures of VGe3-/0 Clusters from Anion Photoelectron Spectrum Assignment.

The global minima of both neutral and anionic clusters of VGe3-/0 were determined using different quantum chemical methods (DFT, RCCSD(T), CASSCF/CASPT2). On the basis of the ground states identified, most excited bands in the anion photoelectron spectrum of VGe3- were assigned. The tetrahedral isomers of both charged states are the most stable ones. A singlet state (Cs,1A') of the tetrahedral isomer has the globally lowest energy on the potential hypersurface of VGe3-. Two states 12A' and 12A″ of the neutral tetrahedral isomer are nearly degenerate and identified as the competing ground state of VGe3. From the anionic ground state, four of five bands in the anion photoelectron spectrum of VGe3- were determined to be the consequences of one-electron transitions starting from the anionic ground state 1A'. Both nearly degenerate neutral ground states are responsible for generation of the first band. Two different transitions from the anionic ground state 1A' to the first two nearly degenerate excited states (22A' and 22A″) of the neutral underlie the second lowest ionization band. Two higher levels of ionization recorded in the spectrum were assigned to the two higher excited states 42A' and 52A' of the neutral. Franck-Condon factor simulations of the first band were performed to obtain more insights into the experimental bands of the spectrum.

Lifetimes of bound excited states of Pt−

SynopsisMeasurements of the radiative lifetimes of the two excited states of the platinum anion Pt− are presented. Pt− ions stored in the cryogenic ion storage ring DESIREE were photodetached at different photon wavelengths and the resulting yield of neutral Pt measured as a function of time was recorded. Analysis of the neutral decay curves show a 2.54 ± 0.10 s lifetime for the higher-lying 5d106s 2S1/2 excited state and a lifetime in the range of 50–200 ms for the lower-lying 5d96s2 2D3/2 excited state. This is the first study to report the lifetime of a bound anion excited state with an electron configuration different from that of the anion ground state.

Titanium Digermanium: Theoretical Assignment of Electronic Transitions Underlying Its Anion Photoelectron Spectrum.

Electronic structures of both the anionic and neutral triatomic species TiGe2-/0 were theoretically studied employing single-reference (DFT and RCCSD(T)) and multiconfigurational (CASSCF/CASPT2 and CASSCF/NEVPT2) methods with large basis sets. The ground state of TiGe2- (C2v) was identified to be 4B1, but the 2A1 state is nearly degenerate, whereas the 3B1 is clearly the ground state of the neutral TiGe2 (C2v). On the basis of the computed ground and excited states of both neutral and anionic structures, all electronic transitions giving rise to experimental anion photoelectron bands in the spectrum of TiGe2- can now be assigned. The X band of the anion photoelectron spectrum is attributed to a one-electron transition between two ground states 4B1 → 3B1. Three neutral excited states 23A2, 25B1, and 35B1 are energetically responsible for the B band upon one-electron photodetachement from the anionic ground state 4B1. The C band is assigned to the transition 4B1 → 25A1. A transition from the nearly degenerate ground state 2A1 of the anion to the low-spin 1A1 of the final neutral state can be ascribed to the A band. Furthermore, the first two bands' progressions, whose normal vibrational modes were accessible from CASSCF/CASPT2 calculations, were also simulated by determination of multidimensional Franck-Condon factors.

Vibronic eigenstates and the geometric phase effect in the 2E″ state of NO3.

The 2E″ state of NO3, a prototype for the Jahn-Teller effect, has been an enigma and a challenge for a long time for both experiment and theory. We present a detailed theoretical study of the vibronic quantum dynamics in this electronic state, uncovering the effects of tunnelling, geometric phase, and symmetry. To this end, 45 vibronic levels of NO3 in the 2E″ state are determined accurately and analyzed thoroughly. The computation is based on a high quality diabatic potential representation of the two-sheeted surface of the 2E″ state developed by us [W. Eisfeld et al., J. Chem. Phys. 140, 224109 (2014)] and on the multi-configuration time dependent Hartree approach. The vibrational eigenstates of the NO3- anion are determined and analyzed as well to gain a deeper understanding of the symmetry properties of such D3h symmetric systems. To this end, 61 eigenstates of the NO3- anion ground state are computed using the single sheeted potential surface of the 1A1 state published in the same reference quoted above. The assignments of both the vibrational and vibronic levels are discussed. A simple model is proposed to rationalize the computed NO3 spectrum strongly influenced by the Jahn-Teller couplings, the associated geometric phase effect, and the tunnelling. Comparison with the available spectroscopic data is also presented.

Electronic Structure of Neutral and Anionic Scandium Disilicon ScSi2-/0 Clusters and the Related Anion Photoelectron Spectrum.

Several quantum chemical methods including DFT (B3LYP, BP86 functional), coupled-cluster theory (RCCSD(T)), and complete active space multiconfigurational methods (CASSCF/CASPT2) were used to study the geometric and electronic structures of the scandium disilicon cluster in both neutral and anionic states, ScSi2-/0. On the basis of the computed ground and lower-lying electronic states, and ionization energies of the anion, all the experimental bands in the anion photoelectron spectrum of ScSi2- can now fully be elucidated. The 3B2 and 2B2 states are determined to be the ground states of the anionic and the neutral triatomic species, respectively. The transition 3B2 → 2B2 is thus assigned to be responsible for the X band in the photoelectron spectrum. The 2A1 neutral state is the final state corresponding to the A band. Although the first two bands arise from ionizations of scandium's 4s and 3d orbitals, all three remaining bands with higher ionization energies are the results of one-electron removals from the Si2 moiety orbitals of the anionic ground state 3B2. Two electronic states of the same representations 14B2 and 24B2 are ascribed to be the carriers of the B and C bands, whereas the excited state 4A2 is attributed to the last band D of the experimental photoelectron spectrum of ScSi2-. From all accessible vibrations of the ground and excited states computed at the B3LYP level, a simulation of band progressions in the photoelectron spectrum was also carried out and used to provide more insights into the experimental bands.

Deciphering the Structural Evolution and Electronic Properties of Magnesium Clusters: An Aromatic Homonuclear Metal Mg17 Cluster

The structures and electronic properties of low-energy neutral and anionic Mgn (n = 3-20) clusters have been studied by utilizing a widely adopted CALYPSO structure searching method coupled with density functional theory calculations. A large number of low-energy isomers are optimized at the B3PW91 functional with the 6-311+G(d) basis set. The optimized geometries clearly indicate that a structural transition from hollow three-dimensional configurations to filled-cage-like structures occurs at n = 16 for both neutral and anionic clusters. Based on the anionic ground state structures, photoelectron spectra are simulated using time-dependent density functional theory (TD-DFT) and compared with experimental results. The good agreement validates that the current ground state structures, obtained from the symmetry-unconstrained searches, are true global minima. A detailed chemical bonding analysis distinctly indicates that the Mg17 cluster is the first neutral locally π-aromatic homonuclear all-metal cluster, which perfectly satisfies Hückel's well-known 4N + 2 rule.

Structure and energetics in dissociative electron attachment to HFeCo3(CO)12

Here we report structural parameters on the heteronuclear transition metal complex HFeCo3(CO)12 and its anion formed upon electron attachment, as well as the thermochemical thresholds for sequential CO loss and the loss of the apical group (as Fe(CO)- 3 and Fe(CO)- 4). Geometrical parameters from single crystal X-ray diffraction are compared with calculated values from density functional theory calculations, for the neutral and anionic ground state of this transition metal cluster. Further, experimental appearance energies for sequential CO loss and the formation of Fe(CO)- 3 and Fe(CO)- 4 are compared to the respective calculated threshold values. Geometry optimizations were performed at the BP86/def2-TZVP level of theory while the threshold energies were calculated at the PBE0/ma-def2-TZVP level of theory. The SOMO of the anion is found to have a clear Fe-Co anti-bonding character resulting in elongation of the Fe-Co bonds and the transformation of one of the terminal Co-CO groups to a bridging Co-CO-Fe group upon electron attachment. The thermochemical threshold PBE0 calculations are concordant with the observed appearance energies and structural parameters from single crystal X-ray diffraction for the neutral molecule are well reproduced at the BP86/def2-TZVP level of theory.

First spectroscopic observation of gold(i) butadiynylide: Photodetachment velocity map imaging of the AuC4H anion.

The velocity map imaging technique was used in the investigation of gold(i) butadiynylide, AuC4H(-), with images recorded at two excitation wavelengths. The resultant photodetachment spectra show a well defined vibrational progression in the neutral with an energy spacing of 343 ± 3 cm(-1). The adiabatic electron affinity was determined to be 1.775 ± 0.005 eV and assigned to the X(1)Σ(+)←X(2)Σ(+) transition between the anionic and neutral ground states. Franck-Condon simulations performed on density functional theory optimized geometries assisted the assignment of linear geometries to the neutral and anion and the observed vibrational progression to that of the Au-C4H stretch.