Detachment Channel Research Articles

Photodetachment Spectroscopy of Highly Excited \(\text{C}_{2}^{ - }\) and Their Temporal Evolution in the Ion Storage Ring RICE

We performed laser-induced electron detachment spectroscopy of \(\text{C}_{2}^{ - }\) ions, produced by a cesium-sputter ion source and stored in the electrostatic ion storage ring RICE, to study highly excited states and their temporal evolution. From the observed peaks originating in the doublet–doublet transition X \(^{2}\Sigma_{g}^{ + }\)–B \(^{2}\Sigma_{u}^{ + }\), we determined the spectroscopic constants for B \(^{2}\Sigma_{u}^{ + }\) v′ = 5–8 by applying the Δv = −1 transition. Moreover, we found that the overall spectra showed a striking difference from previously reported spectroscopic measurements. The majority of the observed peaks could not be ascribed to doublet–doublet transitions. We studied the lifetimes of these unassigned peaks by varying the ion storage time prior to pulsed laser irradiation and discovered shorter- and longer-lived components of 1.5–3 ms and more than 10 ms, respectively. The former lifetime is in good agreement with the autodetachment component observed without laser irradiation, suggesting a possible detachment channel from a metastable quartet state.

Reactions of sulfur and oxygen containing anions with nitrogen and oxygen atoms: A comparative study

Sulfur‒containing anions are expected to exist in molecular clouds and other astrochemical environments due to the relative abundance of sulfur and the predicted stability of the anions. Furthermore, nitrogen and oxygen atoms are of high abundance in the interstellar medium. Therefore, reactions of nitrogen and oxygen atoms with small sulfur‒containing anions, SCN−, CH3COS−, C6H5COS−, −SCH2COOH, C6H5S−, and related oxygen‒containing anions, OCN−, CH3COO−, C6H5COO−, HOCH2COO−, C6H5O−, have been studied both experimentally and computationally. The experimental results show that none of the studied anions react with nitrogen atoms. All of the studied anions with the exception of OCN− react with oxygen atoms, and the reaction channels are more diverse than those for the previously studied reactions with hydrogen atoms. In addition to the associative electron detachment (AED) channel, some ionic products such as CN−, CH3COO−, C6H5COO−, HO− and S− were observed. A sulfur‒oxygen exchange channel was observed in the reactions of oxygen atoms with CH3COS−, C6H5COS− and C6H5S− anions. A sulfur abstraction channel was observed in the reactions of oxygen atoms with SCN− and −SCH2COOH anions. The rate constants for reactions between sulfur‒containing anions and oxygen atoms are found to be generally higher than for the related oxygen‒containing anions. Density functional theory calculations were conducted to provide insight into the experimental results.

Evidence of Electron Capture of an Outgoing Photoelectron Wave by a Nonvalence State in (C6F6) n.

Frequency-resolved photoelectron spectra are presented for (C6F6) n- with n = 1-5 that show that C6F6- is solvated by neutral C6F6 molecules. Direct photodetachment channels of C6F6- are observed for all n, leaving the neutral in the S0 ground state or triplet states, T1 and T2. For n ≥ 2, an additional indirect electron loss channel is observed when the triplet-state channels open. This indirect emission appears to arise from the electron capture of the outgoing photoelectron s-wave by a neutral solvent molecule through an anion nonvalence state. The same process is not observed for the S0 detachment channel because the outgoing electron wave is predominantly a p-wave. Our results show that anion nonvalence states can act as electron-accepting states in cluster environments and can be viewed as precursor states for diffuse states of liquid C6F6.

Sensitivity of Photoelectron Angular Distributions to Molecular Conformations of Anions.

An anion photoelectron imaging study probing the sensitivity of the photoelectron angular distribution (PAD) to conformational changes is presented. The PADs of a series of para-substituted phenolate anions is compared with those calculated using the Dyson orbital formalization. Good agreement was attained for the two observed direct detachment channels of all anions, except for the lowest-energy detachment channel of para-ethyl phenolate for which two conformations exist that yield very different PADs. The conformational freedom leads to an observed PAD that is the incoherent sum of the PADs from all conformers populated under experimental conditions. In contrast, a second detachment channel shows no sensitivity to the conformational flexibility of para-ethyl phenolate. Our results show that PADs can provide detailed information about the electronic structure of the anion and its conformations.

3D Coincidence Imaging Disentangles Intense Field Double Detachment of SF6(–).

The efficient intense field double detachment of molecular anions observed in SF6(–) is studied by 3D coincidence imaging of the dissociation products. The dissociation anisotropy and kinetic energy release distributions are determined for the energetically lowest double detachment channel by virtue of disentangling the SF5(+) + F fragmentation products. The observed nearly isotropic dissociation with respect to the linear laser polarization and surprisingly high kinetic energy release events suggest that the dissociation occurs on a highly excited state. Rydberg (SF6(+))* states composed of a highly repulsive dication core and a Rydberg electron are proposed to explain the observed kinetic energy release, accounting also for the efficient production of all possible cationic fragments at equivalent laser intensities.

Formation and electronic structures of organoeuropium sandwich nanowires.

Organoeuropium sandwich clusters, comprising europium (Eu) and 1,3,5,7-cyclooctatetraene (COT) (Eu(n)(COT)(m)), were produced in the gas phase using a laser vaporization synthesis method. Photoionization mass spectra revealed an exclusive Eu(n)(COT)(m) formation with three compositions: m = n + 1, m = n, and m = n - 1, which, we propose, correspond to full-sandwich, half-sandwich, and inverted-sandwich structures, respectively. The charge distributions, metal-ligand bonding characteristics, and electronic structures of the clusters were comprehensively investigated by photoionization measurements of Eu(n)(COT)(m) neutrals and by photoelectron spectroscopy of Eu(n)(COT)(m)(-) and isoelectronic Ba(n)(COT)(m)(-) anions. The results confirmed that (1) highly ionic metal-ligand bonding is formed between Eu(2+) and COT(2-) within the sandwich structure (at the termini, ionic forms are Eu(+) and COT(-)) and (2) size dependence of orbital energy can be explained by the Coulombic interaction of simple point charge models between the detaching electrons and dipoles/quadrupoles. When the terminus of the sandwich clusters is Eu(2+), COT(2-), or Eu(0), the orbital energy of the electron detachment channel at the opposite terminus strongly depends on the cluster size. In this case, the molecular stack behaves as a one-dimensionally aligned dipole; otherwise, it behaves as a quadrupole, and the relationship between cluster size and electron detachment energy is much weaker. The study also reports on the 4f orbital energy in Eu ions and the formation mechanism of organoeuropium sandwich nanowires up to 12 nm in length. The nanowires are formed by successive charge transfer at the terminal part, Eu(+) and COT(-), which reduces the ionization energy and increases the electron affinity, respectively.

Photoelectron Angular Distributions as Probes of Cluster Anion Structure: I–·(H2O)2 and I–·(CH3CN)2

The use of photoelectron angular distributions to provide structural details of cluster environments is investigated. Photoelectron spectra and angular distributions of I(-)·(H2O)2 and I(-)·(CH3CN)2 cluster anions are recorded over a range of photon energies. The anisotropy parameter (β) for electrons undergoes a sharp change (Δβmax) at photon energies close to a detachment channel threshold. I(-)·(H2O)2 results show the relationship between dipole moment and Δβmax to be similar to that observed in monosolvated I(-) detachment. The Δβmax of the 4.0 eV band in the I(-)·(CH3CN)2 photoelectron spectrum suggests a dipole moment of 5-6 D. This is consistent with predictions of a hydrogen bonded conformer of the I(-)·(CH3CN)2 cluster anion [Timerghazin, Q. K.; Nguyen, T. N.; Peslherbe, G. H. J. Chem. Phys. 2002, 116, 6867-6870].

Experimental observations ofs−pcoupling in copper dimer anions

The photoelectron velocity-map image technique was employed to examine the one-photon detachment of Cu monomer and dimer anions using a linearly polarized infrared femtosecond laser. While the photoelectron angular distribution (PAD) in a single-photon detachment experiment of Cu${}^{\ensuremath{-}}$ shows no dependence on photon energy, the dimer anion PAD does show a strong dependence on photon energy. As the laser wavelength increases from 532 to 817 nm, the photoelectron angular distributions of the main one-photon detachment channel of the prominent detachment channel of Cu${}_{2}$${}^{\ensuremath{-}}$ changes its direction from parallel to the direction of polarization for the 532-nm laser to perpendicular to the direction of polarization for the 817-nm laser. Based upon a numerical analysis of the anisotropy parameters, the relative contributions of the $s$, $p$, and $d$ orbitals are obtained. The fully occupied $d$ orbitals contribute less to the detachment amplitude. The contribution of the $p$ waves to the detachment amplitude increases with the photon energy, while that of $s$ waves decreases, which is in agreement with the prediction of the Wigner threshold law. The relative contributions of the different partial waves, $s$ and $p$, to the detachment amplitude are altered by the excess energy of the detached electron, resulting in the determined shape of the photoelectron angular distributions. Our one-photon photodetachment experiments on the energy-dependent angular distributions of the copper anionic dimers indicate that $s$-$p$ coupling is important to the photodetachment of copper dimer anions, especially at increased photon energies.

Single and double electron photodetachment from the oxygen anion at 41.7 nm

Single and double photodetachment of the oxygen anion O${}^{\ensuremath{-}}$ have been investigated at 41.7 nm (29.8 eV) in a crossed beams experiment using intense photon pulses from a free-electron laser. The ratio of single (O${}^{0}+{e}^{\ensuremath{-}}$) and double (O${}^{+}+2{e}^{\ensuremath{-}}$) detachment was determined to be ${\ensuremath{\sigma}}_{{\mathrm{O}}^{0}}/{\ensuremath{\sigma}}_{{\mathrm{O}}^{+}}=4.12\ifmmode\pm\else\textpm\fi{}$ 0.17 as identified directly from the yield of O${}^{0}$ and O${}^{+}$ fragments after irradiation. The absolute cross section for the dominating single detachment channel was measured to ${\ensuremath{\sigma}}_{{\mathrm{O}}^{0}}=(2.1\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ cm${}^{2}$. Analysis of photoelectrons detected in coincidence with neutral fragments (O${}^{0}$) suggests that single photodetachment primarily happens via the ground (${}^{3}\phantom{\rule{-0.16em}{0ex}}P$) or lowest excited (${}^{1}\phantom{\rule{-0.16em}{0ex}}D$) state of oxygen. The results demonstrate the feasibility and advantage of crossed beams experiments for complete studies of photodetachment reactions.

Spectroscopically resolved competition between dissociation and detachment from nitrobenzene radical anion

We report the vibrational spectrum of the gas-phase isolated nitrobenzene radical anion. The spectrum has been acquired by infrared multiple-photon absorption induced dissociation and electron detachment using the FT mass spectrometer coupled to the infrared free-electron laser FELIX. Upon wavelength-dependent multiple-photon absorption of intense IR irradiation, the vibrational spectrum acquired by on-resonance dissociation to NO 2 − was shown to correlate with the more sensitive electron detachment channel which is indirectly observed by using SF 6 as electron scavenger. The spectrum is compared to previous spectroscopic studies and novel DFT calculations. The frequency and intensity changes of the vibrational bands for the radical anion with respect to the neutral are interpreted with the aid of molecular orbital calculations and mode projection analysis. The vibrations of the neutral and the anion are interpreted in terms of the component benzene modes. The anion shows a reversal of the familiar strongly deactivating meta-directing electrophilic aromatic substitution effect of the neutral due to a resonance effect placing electron density at the ortho- and para-positions, resulting in a structure of distonic character. The greater abundance of the electron detachment channel over the NO 2 − loss dissociation channel is interpreted in terms of statistical models of the energy-dependent unimolecular rates. The anion and neutral vibrational frequencies employed in a quasi-equilibrium theory (QET) model of electron detachment compare favorably to previous experimental results of metastable anion autodetachment lifetimes. The ratio of dissociation to detachment is investigated as a function of FEL power and the competition between these channels is in agreement with a statistical model.

Induction of paranodal myelin detachment and sodium channel loss in vivo by Campylobacter jejuni DNA-binding protein from starved cells (C-Dps) in myelinated nerve fibers

In an axonal variant of Guillain–Barré syndrome (GBS) associated with Campylobacter jejuni ( C. jejuni) enteritis, the mechanism underlying axonal damage is obscure. We purified and characterized a DNA-binding protein from starved cells derived from C. jejuni (C-Dps). This C-Dps protein has significant homology with Helicobacter pylori neutrophil-activating protein (HP-NAP), which is chemotactic for human neutrophils through binding to sulfatide. Because sulfatide is essential for paranodal junction formation and for the maintenance of ion channels on myelinated axons, we examined the in vivo effects of C-Dps. First, we found that C-Dps specifically binds to sulfatide by ELISA and immunostaining of thin-layer chromatograms loaded with various glycolipids. Double immunostaining of peripheral nerves exposed to C-Dps with anti-sulfatide antibody and anti-C-Dps antibody revealed co-localization of them. When C-Dps was injected into rat sciatic nerves, it densely bound to the outermost parts of the myelin sheath and nodes of Ranvier. Injection of C-Dps rapidly induced paranodal myelin detachment and axonal degeneration; this was not seen following injection of PBS or heat-denatured C-Dps. Electron microscopically, C-Dps-injected nerves showed vesiculation of the myelin sheath at the nodes of Ranvier. Nerve conduction studies disclosed a significant reduction in compound muscle action potential amplitudes in C-Dps-injected nerves compared with pre-injection values, but not in PBS-, heat-denatured C-Dps-, or BSA-injected nerves. However, C-Dps did not directly affect Na + currents in dissociated hippocampal neurons. Finally, when C-Dps was intrathecally infused into rats, it was deposited in a scattered pattern in the cauda equina, especially in the outer part of the myelin sheath and the nodal region. In C-Dps-infused rats, but not in BSA-infused ones, a decrease in the number of sodium channels, vesiculation of the myelin sheath, axonal degeneration and infiltration of Iba-1-positive macrophages were observed. Thus, we consider that C-Dps damages myelinated nerve fibers, possibly through interference with paranodal sulfatide function, and may contribute to the axonal pathology seen in C. jejuni-related GBS.

Generalized diatomics-in-molecule method for polyatomics

A new derivation of the generalized diatomics-in-molecule (GDIM) method for the description of electronically stationary and quasi-stationary states of a polyatomic molecule is given in the partial-wave representation of the electron continuum. The method is based on the combination of the projection-operator approach of scattering theory with the diatomics-in-molecule fragmentation of the fixed-nuclei Hamiltonian, resulting in the construction of diabatic discrete states, electron-molecule scattering continua and discrete–continuum couplings from the corresponding data of the fragments. The GDIM method is applied to the H3− anion. The ground-state local-complex-potential surface (both the real and the imaginary parts) is obtained. Classical trajectories of H−+H2 collisions with inclusion of the electron detachment channel are calculated.

Influence of Reagent Rotation on (H-, D2) and (D-, H2) Collisions: A Quantum Mechanical Study

Time-independent quantum mechanical (TIQM) approach (helicity basis truncated at k = 2) has been used for computing differential and integral cross sections for the exchange reaction H- + D2 (v = 0, j = 0-4) --> HD + D- and D- + H2 (v = 0, j = 0-3) --> HD + H- in three dimensions on an accurate ab initio potential energy surface. It is shown that the j-weighted differential reaction cross section values are in good agreement with the experimental results reported by Zimmer and Linder at four different relative translational energies (Etrans = 0.55, 0.93, 1.16 and 1.48 eV) for (H-, D2) and at one relative translational energy (Etrans = 0.6 eV) by Haufler et al. for both (H-, D2) and (D-, H2) collisions. The j-weighted integral reaction cross section values are in good agreement with the crossed beam measurements by Zimmer and Linder in the Etrans range 0.5-1.5 eV and close to the guided ion beam results by Haufler et al. for (H-, D2) in the range 0.8-1.2 eV. Time-dependent quantum mechanical (TDQM) results obtained using centrifugal sudden approximation are reported in the form of integral reaction cross section values as a function of Etrans in the range 0.3-3.0 eV for both reactions in three dimensions on the same potential energy surface. The TDQM reaction cross section values decline more sharply than the TIQM results with increase in the initial rotational quantum number (j) for the D2 molecules in their ground vibrational state (v = 0) for (H-, D2) collisions. The computed j-weighted reaction cross section values are in good agreement with the experimental results reported by Zimmer and Linder for (H-, D2) collisions and guided ion beam results by Haufler et al. for both (H-, D2) and (D-, H2) collisions for energies below the threshold for electron detachment channel.

Collision-induced dissociation of [4Fe-4S] cubane cluster complexes: [Fe 4S 4Cl 4 − x(SC 2H 5) x] 2−/1− ( x = 0–4)

Collision-induced dissociation (CID) experiments on a series of [4Fe-4S] cluster ions, [Fe 4S 4Cl 4 − x (SC 2H 5) x ] 2−/1− ( x = 0–4), revealed that their fragmentation channels change with the coordination environment. Among the three Coulomb repulsion related channels for the doubly charged species, the collision induced electron detachment channel was found to become more significant from x = 0 to 4 due to the decreasing electron binding energies and the magnitude of repulsion Coulomb barrier, while both the ligand detachment of Cl − and the fission of the [Fe 4S 4] 2+ core became more and more significant with the increase of the Cl − coordination, and eventually became the dominant channel at x = 0. From the parents containing the SC 2H 5 ligand, neutral losses of HSC 2H 5 (62 u) and/or HSCH CH 2 (60 u) were observed. It was proposed that inter- and intra-ligand proton transfer could happen during the CID process, resulting in hydrogen coordination to the [4Fe-4S] cluster. In the presence of O 2, [Fe 4S 4Cl 3(SC 2H 5)] 2− and [Fe 4S 4Cl 4] 2− can form the O 2-substituted products [Fe 4S 4Cl 2(SC 2H 5)O 2] − and [Fe 4S 4Cl 3O 2] −, respectively. It was shown that the O 2 complexation occurs by coordination to the empty iron site of the [4Fe-4S] cubane core after dissociation of one Cl − ligand.

Three Dimensional Quantum Dynamics of (H-, H2) and Its Isotopic Variants

We present the results of a time-dependent quantum mechanical investigation using centrifugal sudden approximation in the form of reaction probability as a function of collision energy (E(trans)) in the range 0.3-3.0 eV for a range of total angular momentum (J) values and the excitation function sigma(E(trans)) for the exchange reaction H(-) + H(2) (v = 0, j = 0) --> H(2) + H(-) and its isotopic variants in three dimensions on an accurate ab initio potential energy surface published recently (J. Chem. Phys. 2004, 121, 9343). The excitation function results are shown to be in excellent agreement with those obtained from crossed beam measurements by Zimmer and Linder for H(-) + D(2) collisions for energies below the threshold for electron detachment channel and somewhat larger than the most recent results of Haufler et al. for (H(-), D(2)) and (D(-), H(2)) collisions.

Observation of an excited C2−4 ion

This paper reports an experimental investigation of the electron impact detachment of C−4. We observe structure in the electron impact cross section for detaching a single electron from a C−4 cluster anion, which we attribute to the formation and decay of the C2−4 dianion. The system is energetically unstable and very rapidly decays via double autodetachment. The energy and width of the resonance were determined to be 8.8(5) eV and 1.4(5) eV, respectively, and the resonance lies 1.5(5) eV above the ground state of the neutral system. The experiment was conducted by merging monoenergetic electron and ion beams in the heavy ion storage ring CRYRING. The detachment channel was monitored by detecting neutral C4 fragments.

Hepta- and octacoordinate boron in molecular wheels of eight- and nine-atom boron clusters: observation and confirmation.

Clusters of atoms can adopt different atomic arrangements from bulk materials. They often exhibit novel structures and properties, which provide opportunities for a new types of chemical bonding and stoichiometry. Herein we report experimental and theoretical evidence that 8- and 9-atom boron clusters are perfectly planar molecular wheels, with a hepta- or octacoordinated central boron atom, respectively, despite the predominance of three-dimensional structures normally found in bulk boron and its compounds.

Detailed comparison of theory and experiment of strong-field photodetachment of the negative hydrogen ion

We study the strong-field interaction of a negative hydrogen ion in a short infrared laser pulse. An imaging technique is employed in a fast ion beam to record angular-resolved energy distributions of the photodetached electrons. The spectrum exhibits four excess photon channels. A detailed analysis of the energy and angular distribution is presented, including a comparison with recent theories. From our data we determine elastic-scattering phase shifts of the partial continuum waves in the lowest-order detachment channel. Quantitative agreement with scattering theory is obtained. A quantum interference effect predicted in a recent Keldysh-like theory is observed at energies close to threshold. We propose an analogy to a double-slit model which explains the interference features in a simple and intuitive way. We also verify the applicability of the Wigner threshold law in the strong-field regime and discuss its role in the observed quantum interference.

Electron-impact detachment and dissociation of C4− ions

CRYRING was used to study collision processes between an electron and a negative ion cluster C4−. The total detachment cross sections for the production of the neutral 4C, 3C, 2C, and C fragments were measured. The cross sections for pure detachment, and for detachment plus dissociation leading to the production of C3+C, 2C2, and C2+2C were extracted using a grid. It was found that the pure detachment process overwhelmingly dominates all other fragmentation processes. The threshold location for the detachment channel is found to be around 6.0 eV. Although the doubly charged negative ion C42− has received little previous attention, a defined near-threshold resonance observed in the detachment cross section curve, has been associated with the short-lived state C42− (0.7 fs lifetime).

Photodetachment of H- in a strong infrared laser field.

Negative hydrogen ions are exposed to a short infrared laser pulse of 2.15 microm wavelength and 250 fs duration. An imaging technique is used to record the energy resolved angular distribution of photoelectrons. The energy spectrum reveals at least three excess photon detachment channels. A quantum interference effect, resulting in an unusual dependence of the angular distribution on the electron kinetic energy, is most prominent in the lowest two-photon detachment channel. The appearance of this effect is discussed in terms of the threshold law of photodetachment.