Electron Detachment Research Articles

Electronic Structure and Anion Photoelectron Spectroscopy of Uranium-Gold Clusters UAun-, n = 3-7.

A collaborative effort between experiment and theory toward elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAun-(n = 3-7) were taken at the third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193 nm) photon energies, where the experimental adiabatic electron affinities and vertical detachment energies values were measured. Complementary first-principles calculations were subsequently carried out to corroborate experimentally determined electron detachment energies and to determine the geometry and electronic structure for each cluster. Except for the ring-like neutral isomer of UAu6 where one unpaired electron is spread over the Au atoms, all other neutral and anionic UAun clusters (n = 3-7) were calculated to possess open-shell electrons with the unpaired electrons localized on the central U atom. The smaller clusters closely resemble the analogous UFn species, but significant deviations are seen starting with UAu5 where a competition between U-Au and Au-Au bonding begins to become apparent. The UAu6 system appears to mark a transition where Au-Au interactions begin to dominate, where both a ring-like and two heavily distorted octahedral structures around the central U atom are calculated to be nearly isoenergetic. With UAu7, only ring-like structures are calculated. Overall, the calculated electron detachment energies are in good agreement with the experimental values.

Review of fragmentation of synthetic single-stranded oligonucleotides by tandem mass spectrometry from 2014 to 2022.

The fragmentation of oligonucleotides by mass spectrometry allows for the determination of their sequences. It is necessary to understand how oligonucleotides dissociate in the gas phase, which allows interpretation of data to obtain sequence information. Since 2014, a range of fragmentation mechanisms, including a novel internal rearrangement, have been proposed using different ion dissociation techniques. The recent publications have focused on the fragmentation of modified oligonucleotides such as locked nucleic acids, modified nucleobases (methylated, spacer, nebularine and aminopurine) and modification to the carbon 2'-position on the sugar ring; these modified oligonucleotides are of great interest as therapeutics. Comparisons of different dissociation techniques have been reported, including novel approaches such as plasma electron detachment dissociation and radical transfer dissociation. This review covers the period 2014-2022 and details the new knowledge gained with respect to oligonucleotide dissociation using tandem mass spectrometry (without priori sample digestion) during that time, with a specific focus on synthetic single-stranded oligonucleotides.

Beyond Duality: Rationalizing Repulsive Coulomb Barriers in Host-Guest Cyclodextrin-Dodecaborate Complexes.

The repulsive Coulomb barrier (RCB), an intrinsic potential energy barrier along electron detachment or charge-separation coordinates in multiply charged anions (MCAs), provides dynamic stability to MCAs whose electronic and thermodynamic stabilities are largely dictated by strong internal Coulomb repulsions. Spectroscopic and theoretical characterizations of the RCB have been focused on isolated MCAs. In this work, we extend the RCB investigation beyond the previous scope by including noncovalent host-guest cyclodextrin-closo-dodecaborate dianionic complexes χCD·B12X122- (χ = α, β, γ; X = H, F-I). Photodechment photoelectron spectroscopy reveals the existence of two distinctly different RCBs, derived from detaching electrons from the guest dianions (RCB1) or ionizing the host neutrals (RCB2), respectively, with the latter being substantially smaller than the former. Theoretical calculations support the duality of RCBs in these complexes and further exhibit highly anisotropic nature of the RCBs.

Photodestruction Action Spectroscopy of Silver Cluster Anions, AgN- (N = 3-19), with a Linear Ion Trap: Observation of Bound Excited States above the Photodetachment Threshold.

Electron detachment thresholds of metal cluster anions, MN-, are a few electron volts. The excess electron is therefore detached by visible or ultraviolet light, which also creates low-lying bound electronic states, MN-*; i.e., MN-* energetically overlaps with the continuum, MN + e-. Here, we perform action spectroscopy of photodestruction, leading either to photodetachment or to photofragmentation, for size-selected silver cluster anions, AgN- (N = 3-19), to unveil such bound electronic states embedded in the continuum. The experiment takes advantage of a linear ion trap that enables us to measure photodestruction spectra with high quality at well-defined temperatures, where bound excited states, AgN-*, are clearly identified above their vertical detachment energies. Structural optimization of AgN- (N = 3-19) is conducted by using density functional theory (DFT), which is followed by calculations of vertical excitation energies by time-dependent DFT to assign the observed bound states. Spectral evolution observed as a function of cluster size is also discussed, where the optimized geometries are found to be closely related to the observed spectral profiles. A plasmonic band consisting of nearly degenerate individual excitations is observed for N = 19.

Global Minimum Search and Bonding Analysis of Tl2 Hx and Tl3 Hy (x=0-6; y=0-5) Series.

A remarkable distinction between boron and carbon hydrides lies in their extremely different bonding patterns and chemical reactivity, resulting in diverse areas of application. Particularly, carbon, characterized by classical two-center - two-electron bonds, gives rise to organic chemistry. In contrast, boron forms numerous exotic and non-intuitive compounds collectively called non-classical structures. It is reasonable to anticipate that other elements of Group 13 exhibit their own unusual bonding patterns; however, our knowledge of the hydride chemistry for other elements in Group 13 is much more limited, especially for the heaviest stable element, thallium. In this work, we performed a conformational analysis of Tl2 Hx and Tl3 Hy (x=0-6, y=0-5) series via Coalescence Kick global minimum search algorithm, DFT, and ab initio quantum chemistry methods; we investigated the bonding pattern using the AdNDP algorithm, thermodynamic stability, and stability toward electron detachment. All found global minimum structures are classified as non-classical structures featuring at least one multi-center bond.

Fragmentation of and electron detachment from hot copper and silver dimer anions: A comparison

Fragmentation of and electron detachment from hot copper and silver dimer anions: A comparison

Investigation on the correlation of stability, reactivity, and structural properties of C/C2-doped neutral and charged Aln (n = 2–7) clusters

The equilibrium geometries, electronic structures, reactivity, and stability of C/C2-doped Aln0,± (n = 2–7) clusters were investigated computationally at B3LYP-D4/def2-TZVPP and CCSD(T)/def2-TZVPP levels of theory. The lowest energy structures of AlnC1,20,± clusters were the carbon atoms inserted inside the aluminium clusters. The stability of the neutral and charged clusters were investigated under various reaction conditions, such as dissociation of the clusters via Al0,+,C0,-,C20,--elimination and electron elimination from neutral (ionization) and anionic species (electron detachment). The dominant dissociation reaction channels are the Al-elimination from the neutral and anionic clusters, Al+-elimination from cationic clusters, rather than the C/C2-elimintion reaction. On the other hand, C-elimination are were more favorable compared to C2-elimination due to the stronger CC and Al-C bonds. In addition, binding energy, second-order difference of energy, chemical hardness, and electron affinity parameters were used to determine the stability and reactivity. The results show the singlet cluster (Al2,4,6C1,2 and Al3,5,7C0-20,±) have the greater stability than the doublet clusters (Al3,5,7C0-2 and Al2,4,6C1,20,±) and triplet clusters (Al2,4,6). It has been demonstrated that anionic clusters are more stable than the neutral species, and cationic species are less stable than that of neutral species. The current study could be useful in understanding the stability and reactivity of metal clusters after carbon doping, which is important in combustion, material science, and the astrochemical community.

Evidence of 1000 eV positive oxygen ion flux generated in reactive HiPIMS plasma

The arrival of highly energetic (near 1000 eV) positive atomic oxygen ions at the substrate region has been detected in a unipolar reactive high-power impulse magnetron sputtering process operated with an uncooled copper target in argon–oxygen mixtures. Examination of the ion fluxes from discharge plasma was performed with a magnetic sector mass-spectrometer and an electrostatic energy analyzer. The energy of fast positive O+ ions is close to the value of eV d (e—elementary charge, V d—discharge voltage), which indicates their connection to the well-studied fraction of negative O− ions, which undergo acceleration in the cathode sheath. After switching the oxygen gas supply off, the flux of energetic O+ species decreases gradually as the poisoned target surface layers become depleted of oxygen due to sputtering in pure argon. Presumably, the observed energetic O+ ions originate as a result of low-angle scattering of fast negative O− ions from other charged or neutral species in the plasma followed by electron detachment, ionization, or charge exchange.

Above-threshold detachment by strong ultrashort two-component laser pulses

Detachment of electrons from negative ions by the ultrashort few-cycle laser pulses in the near-infrared spectral region is investigated using a theory based on the strong-field approximation. The differential detachment probability is computed by the numerical integration or by using the saddle-point method. The symmetry properties of the photoelectron momentum distribution are analyzed for the elliptically polarized as well as for the bi-elliptical driving field with the sine-square pulse envelope and compared with those obtained using a long pulse with a flat envelope. We show that the differential detachment probability depends, to a significant extent, on the polarization state of the driving light. For the direct electrons, i.e., the electrons which, after the detachment, do not interact with the parent atom, this is not the case if the process is induced by a long pulse with a flat envelope. In addition to the polarization state, the photoelectron momentum distribution also depends on the value of the absolute phase. We show that the qualitative and quantitative characteristics of the momentum distribution can accurately be controlled using this parameter as a control knob. This control is particularly successful in the regions where the rescattered electrons are dominant due to the fact that they stay longer in the applied laser field.

Accurate Vertical Electron Detachment Energies and Multiphoton Resonant Photoelectron Spectra of Biochromophore Anions in Aqueous Solution.

We introduce a new methodology for calculating vertical electron detachment energies (VDEs) of biologically relevant chromophores in their deprotonated anionic forms in aqueous solution. It combines a large-scale mixed DFT/EFP/MD approach with the high-level multireference perturbation theory XMCQDPT2 and the Effective Fragment Potential (EFP) method. The methodology includes a multiscale flexible treatment of inner (∼1000 water molecules) and outer (∼18000 water molecules) water shells around a charged solute, capturing both the effects of specific solvation and the properties of bulk water. VDEs are calculated as a function of system size for getting a converged value at the DFT/EFP level of theory. The XMCQDPT2/EFP approach, adapted for calculating VDEs, supports the DFT/EFP results. When corrected for a solvent polarization contribution, the XMCQDPT2/EFP method yields the most accurate estimate to date of the first VDE for aqueous phenolate (7.3 ± 0.1 eV), which agrees well with liquid-jet X-ray photoelectron spectroscopy data (7.1 ± 0.1 eV). We show that the geometry of the water shell and its size are essential for accurate VDE calculations of aqueous phenolate and its biologically relevant derivatives. By simulating photoelectron spectra of aqueous phenolate upon two-photon excitation at wavelengths resonant with the S0 → S1 transition, we also provide interpretation of recent multiphoton UV liquid-microjet photoelectron spectroscopy experiments. We show that its first VDE is consistent with our estimate of 7.3 eV, when experimental two-photon binding energies are corrected for the resonant contribution.

Electron Detachments of NbSin–/0 (n = 1–3) Clusters from Density Matrix Renormalization Group-CASPT2 Calculations

The electronic states of NbSin-/0/+ (n = 1-3) clusters have been explored using the state-of-the-art DMRG-CASPT2 method with relatively large active spaces. The leading configurations, bond distances, vibrational frequencies, and relative energies of the low-lying states were identified. Electron detachment energies of the anionic cluster and ionization energies of the neutral clusters were reported at the DMRG-CASPT2 level. The ground states of the NbSin-/0/+ (n = 1-3) clusters were predicted as the 3Δ, 4Π, and 5Π states of the linear NbSi-/0/+, the 3A2, 4B1, and 3B1 states of cyclic NbSi2-/0/+, and the 1A', 2A', and 3A″ states of tetrahedral NbSi3-/0/+ isomers. The first feature in the photoelectron spectrum of NbSi- was attributed to the transitions from the anionic 3Δ ground state to the neutral 4Π, 4Δ, and 4Φ states, whereas the second feature was assigned to the transitions to the neutral 2Δ, 2Σ+, and 2Φ states. The first band in the photoelectron spectrum of NbSi3- was ascribed to the transition from the anionic 1A' ground state to the neutral 12A' and 12A″ states; the second band was attributed to the transitions to 22A', 22A″, and 32A' states; and the third band was assigned to the transition to 32A' states.

Monte Carlo simulation of ion kinetics in nitrogen and oxygen plasmas under non-uniform electric field conditions

The kinetics of N4+ and O− ions was numerically studied in nitrogen and oxygen plasmas in a highly non-uniform electric field. Mean ion energy and reaction rate coefficients in a background gas at pressures from 1 to 10 Torr were calculated through a Monte Carlo simulation. The ion characteristics followed the local reduced electric field at high pressures, whereas nonhydrodynamic effects leading to a nonlocal dependence of the mean ion energy and rate coefficients on the field were obtained at low pressures. As a result, the rates of N4+ ion dissociation, electron detachment from O− ions, and charge exchange in collisions between O− and O2 lagged the local field value. The non-local effect on the ion rate coefficients was more profound when the field decreased in space. We suggested a simplified method of describing ion rates in spatially varying electric fields on the basis of the Monte Carlo simulation of these rates for uniform electric field conditions and mean ion energy calculations in non-uniform fields. This method is similar to the local-mean-energy approximation utilized for describing electron swarm parameters in varying electric fields. The results of the simplified method were compared with the results of the direct Monte Carlo simulation.

Predicted Negative Ion Photoelectron Spectra of 1-, 2-, and 9-Cyanoanthracene Radical Anions and Computed Thermochemical Values of the Three Cyanoanthracene Isomers.

In this work, the negative ion photoelectron spectra of 1-, 2-, and 9-cyanoanthracene (anthracenecarbonitrile, ACN) radical anions, obtained via the calculation of Franck-Condon (FC) factors based on a harmonic oscillator model, are reported. The FC calculations utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory using the B3LYP/6-311++G (2d,2p) basis set. The removal of an electron from the doublet anion allows for the computation of the negative ion photoelectron spectra that represents the neutral ground singlet state (So) and the lowest triplet state (T1) in each of the three ACN molecules. The respective adiabatic electron affinity (EA) values for the So state in 1-, 2-, and 9-ACN isomers are calculated to be 1.353, 1.360, and 1.423 eV. The calculated EA of the 9-cyanoanthracene singlet isomer is in close agreement with the previously reported experimental value of 1.27 ± 0.1 eV. Calculations show that the T1 states in 1-, 2-, and 9-ACN are located 1.656, 1.663, and 1.599 eV above the So state. The calculated T1 negative ion spectra exhibit intense vibrational origins and weak FC activity beyond the origins, indicating little change in geometry following electron detachment from the doublet anionic state. Upon deprotonation, the EA values of the radical isomers increase by ∼400-700 meV, resulting in neutral deprotonated radicals with EAs between 1.740 and 2.220 eV. The calculated site-specific gas-phase acidity values of ACN isomers indicate that ACN molecules are more acidic than benzonitrile.

The stability and reactivity of neutral and charged aluminium doped carbon clusters (Al1,2C2-70,±)

The stability and reactivity of Al-doped linear and quasi-linear carbon chains (Al1,2C2-70,±) were computationally investigated using various reaction and binding energy parameters. The dissociation of the clusters was studied via the elimination of Al, C from the neutral species, Al0,+, C0,+ the cationic species, and Al0,-, C0,- from the anionic species. The stability was further examined based on electron elimination reactions, such as electron detachment from Al1,2C2-7- and ionization of Al1,2C2-7. The study proceeded by formulating a novel relative energy parameter, which was generated from the binding energy values. The lowest energy dissociation channels of Al1,2C1-70,± were the Al and Al+ elimination from the neutral/anionic and cationic clusters, respectively. Under the dissociation conditions, the even-carbon species (m = 2,4,6) AlCm (doublet), AlCm+ (triplet), AlCm- (singlet), Al2Cm (singlet), Al2Cm+ (doublet) were found to be more stable than that of the odd-carbon clusters (m = 3,5,7), irrespective of the spin states. In the case of Al2Cm-, the odd carbon clusters were found to be more stable. The stability orders based on the widely used highly endothermic C-elimination reaction were found to be significantly different. We have systematically addressed the above disparities and have commented on how to accurately predict the abundance of the species under extreme experimental conditions. The reactivity of Al1,2-doped clusters was found to be higher than the bare carbon chains. Based on the bond-strength parameters calculated using AIM analysis, the Al1,2-doping led to an activated Al-C bond rather than the carbon chains, eventually resulting in reactive meal-carbon clusters. The Al-C bonding was discussed using electron density at the Al-C bond critical points (ρAl-C), the vibrational frequency of the Al-C bond (νAl-C), bond length (rAl-C), the NBO charge on the Al atoms (qAl) and with the molecular orbital diagrams. The results of the current investigations can be highly relevant to astrochemistry, spectroscopy, material, and organometallic chemistry communities.

Quasidegeneracy of (H2C = B = CH2)– borata-alkene synthon and (H3B–C≡CH)– boryl-substituted acetylene anion induced by an excess electron attachment

Using ab initio methods we examined neutral and anionic BC2H4 systems. The most stable neutral isomer was found to correspond to H2CBCH2 structure. Among other neutral isomers, H3B–CCH system was identified as the highest energy isomer. The energetic order of anionic isomers was found to be different as (H2CBCH2)– and (H3B–CCH)– species were identified as nearly isoenergetic and the most stable. The energetic effect caused by an excess electron attachment was found responsible for rendering (H3B–CCH)– isomer the most stable on the anionic potential energy surface. Vertical electron detachment energies (1.71–4.32 eV) characterizing (BC2H4)– anions were predicted and discussed.

Detection of 135Cs &137Cs in environmental samples by AMS

The detection of low abundances of 135Cs in environmental samples is of significant interest in different fields of environmental sciences, especially in combination with its shorter-lived sister isotope 137Cs. The method of Ion–Laser InterAction Mass Spectrometry (ILIAMS) for barium separation at the Vienna Environmental Research Accelerator (VERA) was investigated and further improved for low abundance cesium detection. The difluorides BaF2− and CsF2− differ in their electron detachment energies and make isobar suppression with ILIAMS by more than 7 orders of magnitude possible. By this method, samples with ratios down to the order of 135,137Cs/133Cs ≈10−11 are measurable and the 135Cs/137Cs ratios of first environmental samples were determined by AMS.

Lightning Initiation as a Consequence of Natural Thundercloud Evolution. Part 3. Streamers and Transition from Streamers to a Leader Channel

This article presents the final part of the study devoted to description of the authors’ scenario of lightning initiation in a thundercloud, the first two parts of which are presented in [1, 2]. The first part shows that consideration of the electron detachment from negative ions results in that the air breakdown field reduces by 15–30 %. This partially simplifies but does not solve the problem of lightning initiation in a thundercloud, the maximum electric fields in which are approximately an order of magnitude lower than the dielectric strength of air. The second part describes the transition from millimeter corona discharges to decimeter-scale regions of elevated ionic conductivity, which becomes possible if the spatiotemporal frequency of corona discharges arising due to collisions (nearly collisions) of hydrometeors exceeds a quite moderate value of 0,1 m-3s-1. In turn, the regions of elevated ionic conductivity give rise to positive streamers. The article presents the lightning initiation process final stage, at which streamers oriented by the large-scale electric field are combined into a single plasma network, inside which a hot leader channel is generated. It is shown that, for the transition from streamers to a lightning leader seed to occur, a potential difference between the boundaries of the zone of a strong intracloud field is to exceed 3 MV. The article ends with a general conclusion summarizing the trilogy results.

Jacobian-free Newton–Krylov method for the simulation of non-thermal plasma discharges with high-order time integration and physics-based preconditioning

A preconditioning framework for the numerical simulation of non-thermal streamer discharges is developed using the Jacobian-free Newton-Krylov (JFNK) method. A reduced plasma fluid model is considered, consisting of electrons, one positive ion, one negative ion, and the electrostatic potential. The plasma kinetics model includes ionization, electron-ion recombination, electron attachment, electron detachment, and ion-ion recombination. The governing equations are made dimensionless, discretized in space with finite differences, and integrated in time with a fully implicit method based on high-order backward differentiation formulas. The preconditioning framework is based on a linearized form of the governing equations and physics-based operator splitting. The efficiency of the preconditioning strategy is assessed through two test cases: streamer propagation between parallel plates and an axisymmetric pin-to-pin discharge. The fully implicit approach overcomes traditional restrictions in the time step size due to processes such as electron drift, electron diffusion, and dielectric relaxation. Excellent performance is observed through relevant statistics of the JFNK solver, although the number of linear iterations increases for the pin-to-pin discharge when nonlinear numerical boundary conditions are imposed at the electrodes. Performance studies show scalability with O(100-1000) processors for O(10M) unknowns with ample room for optimization.

Non-oxidative photoreaction of 1,3,5-triaryl-2-pyrazoline-4-ol (4-methoxy) derivatives

• A suitable procedure for the synthesis of 2-pyrazoline-4-ol (-methoxy) derivatives is proposed. • Non-oxidative photochemical reaction of 2-pyrazoline-4-ol (-methoxy) derivatives in CHCl 3 is occurred. • The electronic nature of the substituent on the 4- and 5-positions on the photochemical behavior is studied. • An efficient mechanism for the electron-transfer induced photoelimination reaction in chloroform is proposed. • DFT computational studies (optimization, orbital contour maps, MEP, NBO) at the B3LYP/6-31++(d,p) level of theory on these compoundsare reported. A series of 4-hydroxy- or 4-methoxy-substituted 1,3,5-triaryl-2-pyrazolines were synthesized and their structures, especially trans -orientation of 4- and 5-hydrogens of the 2-pyrazoline ring were characterized by analysis of IR, 1 H NMR, 13 C NMR spectra as well as the single-crystal structure study. In order to elucidate the effect of 4-hydroxy- or 4-methoxy-substitution on the photochemical behavior of these compounds, they were exposed to the UV light. The results explain the occurrence of a non-oxidative photoreaction, by the expulsion of 4-substitution and elimination of a molecule of water or methanol under the formation of the aromatized pyrazole ring. Density functional theory (DFT) calculations were carried out to support the proposed electron-transfer induced photoreaction mechanism. Natural bond orbital (NBO) analysis explains the preferred electron detachment process from the N 1 -atom rather than the oxygen atom of the hydroxy or the methoxy groups. Proton detachment from the involved radical cation species leads to the formation of the radical intermediate centered at the C 5 -position of the heterocyclic ring. Elimination of the hydroxy or the methoxy group accomplished the photoreaction under the formation of the C 4 -unsubstituted pyrazole molecule.

Charge Transfer Reactions from I- to Polar Protic Solvents Studied Using Ultrafast Extreme Ultraviolet Photoelectron Spectroscopy.

Charge transfer reactions from I- to solvent water, methanol, and ethanol were studied using extreme ultraviolet time-resolved photoelectron spectroscopy (EUV-TRPES). This technique eliminates spectral broadening, previously seen in UV-TRPES, caused by electron inelastic scattering in liquids, and enables clear observation of the temporal evolution of the spectral shape. The peak position, width, and intensity of the electron binding energy distribution indicate electron detachment and subsequent solvation and thermalization processes. Geminate recombination between detached electrons and iodine atoms is discussed using a diffusion equation and a global fitting analysis based on a kinetics model.