Measured Electron Affinities Research Articles

Electron Collisions with Multielectron Atoms and Fullerene Molecules: Strong Polarisation Effects

Regge pole-calculated low-energy electron elastic total cross sections for multielectron atoms/fullerenes are characterised by ground, metastable and excited negative-ion formation, shape resonances and Ramsauer-Townsend minima. In this article, we demonstrate through the total cross sections for Eu, Au and At atoms and C60 fullerene the sensitivity of stable negative-ion formation to the crucial core-polarisation potential. The energy positions of the dramatically sharp resonances corresponding to the binding energies of the formed anions during the collisions agree excellently with the measured electron affinities of the atoms and C60. The sensitivity of Ramsauer-Townsend minima and shape resonances to the electronic structure and dynamics of Bk and Cf permits their first ever use as novel validation of the experimental observation that Cf is indeed a transitional element in the actinide series. Their electron affinities are also calculated.

Rigorous Negative Ion Binding Energies in Low-Energy Electron Elastic Collisions with Heavy Multi-Electron Atoms and Fullerene Molecules: Validation of Electron Affinities

Dramatically sharp resonances manifesting stable negative ion formation characterize Regge pole-calculated low-energy electron elastic total cross sections (TCSs) of heavy multi-electron systems. The novelty of the Regge pole analysis is in the extraction of rigorous and unambiguous negative ion binding energies (BEs), corresponding to the measured electron affinities (EAs) of the investigated multi-electron systems. The measured EAs have engendered the crucial question: is the EA of multi-electron atoms and fullerene molecules identified with the BE of the attached electron in the ground, metastable or excited state of the formed negative ion during a collision? Inconsistencies in the meaning of the measured EAs are elucidated and new EA values for Bk, Cf, Fm, and Lr are presented.

Electron affinities and lowest triplet and singlet state properties of para-oligophenylenes (n = 3-5): theory and experiment.

We apply photodetachment-photoelectron spectroscopy to measure the electron affinities and the energetics of the lowest excited electronic states of the neutral molecules para-terphenyl (p3P), para-quaterphenyl (p4P) and para-quinquephenyl (p5P), including especially the triplet states below S1. The interpretation of the experimental data is based on the comparison to calculated 0-0 energies and Dyson norms, using density functional theory and multireference configuration interaction methods, as well as Franck-Condon patterns. The comparison between calculated and experimental vibrational fine-structures reveals a twisted benzoid-like molecular structure of the S0 ground state and nearly planar quinoid-like nuclear arrangements in the S1 and T1 excited states as well as in the D0 anion ground state. For all para-oligophenylenes (ppPs) in this series, at least two triplet states have been identified in the energy regime below the S1 state. The large optical S0-S1 cross sections of the ppPs are rationalised by the nodal structure of the molecular orbitals involved in the transition. The measured electron affinities range from 380 meV (p3P) over 620 meV (p4P) to 805 meV (p5P). A saturation of the electron binding energy with the increasing number of phenyl units is thus not yet in sight.

Interface barrier-induced conversion of resistive switching mechanism in Mn-doped BiFeO3 memristor

Different from conductive filament (CF)-type counterparts, interface-type devices exhibit continuously gradual conductance changes, making them the potential for artificial synapses. In this paper, Mn-doped BiFeO3 (BFMO) devices with SrRuO3 and TiN bottom electrodes demonstrate the clear CF rather than the interface barrier type resistance-switching feature due to the high Schottky barrier. Considering the measured electron affinity of 3.52 eV and work function of 4.22 eV in the as-synthesized BFMO film (a weak n-type semiconductor, marked as n−), we fabricated a hetero-junction device with the Nb-doped SrTiO3 (NSTO) bottom electrode (a strong n-type semiconductor, marked as n+) exhibiting analog switch characteristics. The n−–n+ hetero-junction between BFMO and NSTO reverses the operation polarity and leads to a barrier transition-dominated conductive behavior in the BFMO-based memristor. The device shows a large ON/OFF ratio over 1200, favorable stability after 104 s, continual multi-value characteristics, symmetrical long-term potentiation and depression, and synaptic plasticity with about 80 ns time constant. The investigation of resistive switching features, band structure, and synapse performance in this work provides a reference for the application of BiFeO3 in the field of the memristor.

High Resolution Photoelectron Spectroscopy of the Acetyl Anion.

High-resolution photoelectron spectra of cryogenically cooled acetyl anions (CH3CO-) obtained using slow photoelectron velocity-map imaging are reported. The high resolution of the photoelectron spectrum yields a refined electron affinity of 0.4352 ± 0.0012 eV for the acetyl radical as well as the observation of a new vibronic structure that is assigned based on ab initio calculations. Three vibrational frequencies of the neutral radical are measured to be 1047 ± 3 cm-1 (ν6), 834 ± 2 cm-1 (ν7), and 471 ± 1 cm-1 (ν8). This work represents the first experimental measurement of the ν6 frequency of the neutral. The measured electron affinity is used to calculate a refined value of 1641.35 ± 0.42 kJ mol-1 for the gas-phase acidity of acetaldehyde. Analysis of the photoelectron angular distributions provides insight into the character of the highest occupied molecular orbital of the anion, revealing a molecular orbital with strong d-character. Additionally, details of a new centroiding algorithm based on finite differences, which has the potential to decrease data acquisition times by an order of magnitude at no cost to accuracy, are provided.

Accurate electron affinity of Ti and fine structures of its anions.

The high-resolution photoelectron energy spectra of atomic titanium and its hydride anions were obtained on a slow-electron velocity-map imaging spectrometer equipped with a cold ion trap. The cold ion trap employed in the present measurement was found to be very helpful for reducing the interference from the titanium hydride anions. The electron affinity of Ti was determined to be 609.29(34) cm-1 or 75.54(4) meV. The accuracy was improved by a factor of 350 compared with the previous result. The fine structures of Ti- were clearly resolved: 70.0(12)(4F5/2), 165.2(15)(4F7/2), and 285.2(15) cm-1 (4F9/2) above its ground state 4F3/2. Moreover, the measured electron affinity and vibrational frequency of TiH can be reproduced well using the high level calculations.

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.

Conundrum in Measured Electron Affinities of Complex Heavy Atoms

Low-energy electron scattering from the lanthanide atoms Eu, Tb, Tm, Gd, and Nd including Nb is investigated through calculated electron elastic total cross sections using our robust Regge-pole methodology. The extracted binding energies of the resultant ground and metastable anions formed during the collisions are contrasted with the measured electron affinities (EAs). It is concluded that the measured EAs for these atoms require reinterpretation and new recommended values are presented.

Simple method for determining fullerene negative ion formation

A robust potential wherein is embedded the crucial core-polarization interaction is used in the Regge-pole methodology to calculate low-energy electron elastic scattering total cross section for the C60 fullerene in the electron impact energy range 0.02 ≤ E ≤ 10.0 eV. The energy position of the characteristic dramatically sharp resonance appearing at the second Ramsauer–Townsend minimum of the total cross section representing stable C60− fullerene negative ion formation agrees excellently with the measured electron affinity of C60 [Huang et al., J. Chem. Phys. 140, 224315 (2014)]. The benchmarked potential and the Regge-pole methodology are then used to calculate electron elastic scattering total cross sections for selected fullerenes, from C54 through C240. The total cross sections are found to be characterized generally by Ramsauer–Townsend minima, shape resonances and dramatically sharp resonances representing long-lived states of fullerene negative ion formation. For the total cross sections of C70, C76, C78, and C84 the agreement between the energy positions of the very sharp resonances and the measured electron affinities is outstanding. Additionally, we compare our extracted energy positions of the resultant fullerene anions from our calculated total cross sections of the C86, C90 and C92 fullerenes with the estimated electron affinities ≥3.0 eV by the experiment [Boltalina et al., Rapid Commun. Mass Spectrom. 7, 1009 (1993)]. Resonance energy positions of other fullerenes, including C180 and C240 are also obtained. Most of the total cross sections presented in this paper are the first and only; our novel approach is general and should be applicable to other fullerenes as well and complex heavy atoms, such as the lanthanide atoms. We conclude with a remark on the catalytic properties of the fullerenes through their negative ions.

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.

Binding energies of fullerene and complex atomic negative ions

SynopsisA robust potential which embeds fully the vital core polarization interaction has been used in the Regge-pole method to explore low-energy electron scattering from C60, Eu and Nb through the total cross sections (TCSs) calculation. From the characteristic dramatically sharp resonances in the TCSs manifesting negative ion formation in these systems, we extracted the binding energies (BEs) for the C60−, Eu− and Nb− anions. Our BE for C60− ground state is in outstanding agreement with the measured electron affinity (EA) of C60 [1]. For anionic Eu− and Nb− our BEs are compared with the measured EAs [2-4].

Resonances in low-energy electron elastic scattering from Fullerenes

SynopsisThe benchmarked Regge-pole method on the calculated low-energy electron elastic scattering total cross section (TCS) for C60 through the measured electron affinity (EA) [1, 2] is used to calculate TCSs for selected fullerenes, from C54 through C240 in the electron impact energy range 0.02 ≤ E ≤ 10.0 eV. From the characteristic dramatically sharp resonances representing long-lived ground state fullerene negative ion formation we extracted the binding energies (BEs) and compared them with the measured EAs, obtaining outstanding agreement.

Cyanation of Fullerene(CF3) n Does Not Always Increase Electron Affinity: Experimental and Theoretical Study

Trifluoromethylfullerenes have highly tunable electronic properties and can be used as adaptable building blocks for advanced organic electronics. Previously, our group has shown that the frontier orbitals of a series of C60(CF3) n are greatly influenced by the addition pattern of CF3 groups, which in turn, has a profound influence on electron affinities.1 Judicious additions of strong electron-withdrawing groups, such as CN or CF3, to fullerene cages are typically expected (and theoretically predicted) to increase electron affinity.Indeed, regioselective synthesis of a new SPP derivative with three unique addends, C60(CF3)4(CN)H, made possible due to the use of a new isomer of C60(CF3)4 as a substrate, resulted in the experimental and theoretical identification of a family of superhalogen fullerene radicals, species with the gas-phase electron affinity higher than that of the most electronegative halogens, F and Cl.2 The addition of one CN group to C60(CF3)4 increases the measured gas-phase electron affinity by more than 1 eV to 4.28(1) eV; such a large increase is likely due to a significant stabilization of the skew pentagonal pyramid pattern (SPP), and electron-withdrawing effect of CN group. In contrast, the measured electron affinity of another SPP open shell molecular species, C60(CF3)4H, is considerably lower, 3.96(1) eV.Regioselective cyanation of several C70(CF3) n compounds has also been carried out using recently described technique.3 In the case of C 1-C70(CF3)10, the addition of two CN groups results in an expected increase in the measured gas-phase electron affinity by 0.22 eV. Surprisingly, the addition of two electron-withdrawing CN groups to Cs -C70(CF3)8 results in a decrease in the measured gas-phase electron affinity! This decrease is observed for two different CN addition patterns. The underlying reasons and implications for the design of electrophilic fullerene derivatives are rationalized using DFT calculations.References 1 Popov, A. A.; Kareev, I. E.; Shustova, N. B.; Stukalin, E. B.; Lebedkin, S. F.; Seppelt, K.; Strauss, S. H.; Boltalina, O. V.; Dunsch, L. J. Am. Chem. Soc. 2007, 129, 11551. 2 Clikeman, T. T.; Deng, S. H. M.; Avdoshenko, S.; Wang, X.-B.; Popov , A. A.; Strauss, S. H.; Boltalina, O. V. Chem. Eur. J. 2013, 19, 15404. 3 Clikeman, T. T.; Kuvychko, I. V.; Shustova, N. B.; Chen, Y.-S.; Popov, A. A.; Boltalina, O. V.; Strauss, S. H. Chem. Eur. J. 2013, 19, 5070.

Photoelectron spectroscopic study of the ethyl cyanoacrylate anion

Anion photoelectron spectroscopy and density functional theory have been utilized to study the parent, ethyl cyanoacrylate molecular anion, ECA−. The measured electron affinity (0.9±0.2eV), vertical detachment energy (1.3±0.1eV), and anion-to-triplet neutral, photodetachment transition energies (4.0±0.1eV and 4.5±0.1eV) all compare well with their calculated values. The relatively high electron affinity of the ECA monomer is responsible for the fact that its “anionic” polymerization mechanism proceeds even with weak nucleophiles, such as water.

Slow electron elastic scattering cross sections for In, Tl, Ga and At atoms

The complex angular momentum (CAM)-calculated, low-energy 0 ≤ E ≤ 5 eV electron elastic total cross section (TCS) for In is benchmarked through its recently measured electron affinity in Walter et al (2010 Phys. Rev. A 82 032507). The TCSs for Tl, Ga and At atoms are then calculated using the CAM method. From the dramatically sharp resonances in the TCSs, binding energies for Tl−, Ga− and At− negative ions formed during the collisions as Regge resonances are extracted and compared with the existing experimental and theoretical values.

An interface dipole predictive model for high-k dielectric/semiconductor heterostructures using the concept of the dipole neutrality point

This study introduces a predictive model involving the concept of the dipole neutrality point (DNP) for dipoles at the high-k dielectric/semiconductor interface. The DNP allows a simple and intuitive prediction of interface dipoles using electronegativity (EN). This concept is formulated based on a negative correlation between the dielectric work function and EN observed for many high-k oxides using both our measured electron affinity values and those available from the literature. Good agreement in the interface dipoles prediction is observed for our measured dipoles and the flatband voltage shifts from other reports. The simple model will be beneficial for investigations involving threshold voltage adjustment in metal-oxide–semiconductor devices using high-k dielectric materials, which is important for advanced gate stacks development.

Superhalogen properties of CumCln clusters: Theory and experiment

Using a combination of density functional theory and anion photoelectron spectroscopy experiment, we have studied the structure and electronic properties of CuCl(n)(-) (n = 1-5) and Cu(2)Cl(n)(-) (n = 2-5) clusters. Prominent peaks in the mass spectrum of these clusters occurring at n = 2, 3, and 4 in CuCl(n)(-) and at n = 3, 4, and 5 in Cu(2)Cl(n)(-) are shown to be associated with the large electron affinities of their neutral clusters that far exceed the value of Cl. While CuCl(n) (n ≥ 2) clusters are conventional superhalogens with a metal atom at the core surrounded by halogen atoms, Cu(2)Cl(n) (n ≥ 3) clusters are also superhalogens but with (CuCl)(2) forming the core. The good agreement between our calculated and measured electron affinities and vertical detachment energies confirm not only the calculated geometries of these superhalogens but also our interpretation of their electronic structure and relative stability.

Electronic states of thiophenyl and furanyl radicals and dissociation energy of thiophene via photoelectron imaging of negative ions

We report photoelectron images and spectra of deprotonated thiophene, C(4)H(3)S(-), obtained at 266, 355, and 390 nm. Photodetachment of the α isomer of the anion is observed, and the photoelectron bands are assigned to the ground X(2)A(') (σ) and excited A(2)A(") and B(2)A(") (π) states of the thiophenyl radical. The photoelectron angular distributions are consistent with photodetachment from the respective in-plane (σ) and out-of-plane (π(∗)) orbitals. The adiabatic electron affinity of α-(●)C(4)H(3)S is determined to be 2.05 ± 0.08 eV, while the B(2)A(") term energy is estimated at 1.6 ± 0.1 eV. Using the measured electron affinity and the electron affinity/acidity thermodynamic cycle, the C-H(α) bond dissociation energy of thiophene is calculated as DH(298)(H(α)-C(4)H(3)S) = 115 ± 3 kcal/mol. Comparison of this value to other, previously reported C-H bond dissociation energies, in particular for benzene and furan, sheds light of the relative thermodynamic stabilities of the corresponding radicals. In addition, the 266 nm photoelectron image and spectrum of the furanide anion, C(4)H(3)O(-), reveal a previously unobserved vibrationally resolved band, assigned to the B(2)A(") excited state of the furanyl radical, (●)C(4)H(3)O. The observed band origin corresponds to a 2.53 ± 0.01 eV B(2)A(") term energy, while the resolved vibrational progression (853 ± 42 cm(-1)) is assigned to an in-plane ring mode of α-(●)C(4)H(3)O (B(2)A(")).

O− + Acetaldehyde Reaction Products: Search for Singlet Formylmethylene, a Wolff Rearrangement Intermediate

The mass-resolved anionic products of the reaction of O(•-) with acetaldehyde, H(3)CCHO, are studied using photoelectron imaging. The primary anionic products are vinoxide, H(2)CCHO(-), formylmethylene anion, HCCHO(•-), and ketenylidene anion, CCO(•-). From photoelectron spectra of HCCHO(•-), the electron affinity of triplet (ground state) formylmethylene (1.87 ± 0.02 eV) and the vertical detachment energy corresponding to the first excited triplet state (3.05 eV) are determined, but no unambiguous assignment for singlet HCCHO could be made. The elusive singlet is a key intermediate in the Wolff rearrangement, resulting in formation of ketene. The fast rearrangement associated with a large geometry change upon photodetachment to the singlet surface may be responsible for the low intensity of the singlet compared to the triplet bands in the photoelectron spectrum. The title reaction also yields CCO(•-), whose formation from acetaldehyde is novel and intriguing, since it requires a multistep net-H(4)(+) abstraction. A possible mechanism is proposed, involving an [H(2)CCO(•-)]* intermediate. From the measured electron affinities of HCCHO (above), H(2)CCHO (1.82 ± 0.01 eV), and CCO (2.31 ± 0.01 eV), several new thermochemical properties are determined, including the C-H bond dissociation energies and heats of formation of several organic molecules and/or their anions. Overall, the reactivity of O(•-) with organic molecules demonstrates the utility of this anion in the formation of a variety of reactive intermediates via a single process.

Electronic structure and properties of isoelectronic magic clusters: Al13X (X=H,Au,Li,Na,K,Rb,Cs)

The equilibrium structure, stability, and electronic properties of the Al(13)X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable us to identify the ground state geometries of these clusters both in neutral and anionic configurations.