RESUMEN

Cyanogen NCCN and cyanoacetylene HCCCN are isoelectronic molecules, and as such, they have many similar properties. We focus on the bond cleavage in these induced by the dissociative electron attachment. In both molecules, resonant electron attachment produces CN- with very similar energy dependence. We investigate the very different dissociation dynamics, in each of the two molecules, revealed by velocity map imaging of this common fragment. Different dynamics are manifested both in the excess energy partitioning and in the angular distributions of fragments. Based on the comparison with electron energy loss spectra, which provide information about possible parent states of the resonances (both optically allowed and forbidden excited states of the neutral target), we ascribe the observed effect to the distortion of the nuclear frame during the formation of core-excited resonance in cyanoacetylene. The proposed mechanism also explains a puzzling difference in the magnitude of the CN- cross section in the two molecules which has been so far unexplained.

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We experimentally show that N-H bond cleavage in the pyrrole molecule following resonant electron attachment is allowed and controlled by the motion of the atoms which are not dissociating, namely, of the carbon-attached hydrogen atoms. We use this fact to steer the efficiency of this bond cleavage. In order to interpret the experimental findings, we have developed a method for locating all resonant and virtual states of an electron-molecule system in the complex plane, based on all-electron R-matrix scattering calculations. Mapping these as a function of molecular geometry allows us to separate two contributing dissociation mechanisms: a π* resonance formation inducing strong bending deformations and a nonresonant σ* mechanism originating in a virtual state. The coupling between the two mechanisms is enabled by the out-of-plane motion of the C-H bonds, and we show that it must happen on an ultrafast (few fs) time scale.

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The electron-induced reactivity of 5-(4-chlorophenyl)-1H-tetrazole and 5-chloro-1-phenyl-1H-tetrazole was studied using a trochoidal electron monochromator quadrupole mass spectrometer experimental setup. 5-(4-chlorophenyl)-1H-tetrazole underwent dissociative electron attachment to form Cl-, [M-HCl]-, and [M-H]-. 5-chloro-1-phenyl-1H-tetrazole underwent associative electron attachment to form the parent anion and dissociative electron attachment to form Cl-, CN2Cl-, [M-N2-Cl]-, and [M-HCl]-. For each anion product, the ion yield was measured as a function of incident electron energy. Density functional theory calculations were performed to support the experimental results with estimates of the energetic thresholds for the different reaction pathways. While the tetrazole group is susceptible to electron-induced ring opening in both molecules, this process was only observed for 5-chloro-1-phenyl-1H-tetrazole, indicating that this process is influenced by the structure of the molecule.

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The present work combines experiment and theory to reveal the behavior of bromo-substituted-biphenyls after an electron attachment. We experimentally determine anion lifetimes using an electron attachment-magnetic sector mass spectrometer instrument. Branching ratios of dissociative electron attachment fragments on longer timescales are determined using the electron attachment-quadrupole mass spectrometer instrument. In all cases, fragmentation is low: Only the Br- and [M-Br]- ions are detected, and [M-H]- is observed only in the case of 4-Br-biphenyl and parent anion lifetimes as long as 165 µs are observed. Such lifetimes are contradictory to the dissociation rates of 2- and 4-bromobiphenyl, as measured by the pulse radiolysis method to be 3.2 × 1010 and >5 × 1010 s-1, respectively. The discrepancy is plausibly explained by our calculation of the potential energy surface of the dissociating anion. Isolated in vacuum, the bromide anion can orbit the polarized aromatic radical at a long distance. A series of local minima on the potential energy surface allows for a roaming mechanism prolonging the detection time of such weakly bound complex anions. The present results illuminate the behavior recently observed in a series of bromo-substituted compounds of biological as well as technological relevance.

RESUMEN

We probe the transient anion states (resonances) in the dielectric gas C4F7N by the electron energy loss spectroscopy and the dissociative electron attachment spectroscopy. The vibrationally inelastic electron scattering leads to two excitation types. The first is the excitation of specific vibrational modes that are assigned with the help of an infrared spectrum of this molecule and quantum chemistry calculations. In the second type of vibrational excitation, the excess energy is randomized via internal vibrational redistribution in the temporary anion, and the electrons are emitted statistically. The electron attachment proceeds in three different regimes. The first is the formation of the parent C4F7N- anion at energies close to 0 eV. The second is a statistical evaporation of the F-atom, leading to the defluorinated anion C4F6N-. Finally, the third is dissociative electron attachment proceeding via the formation of several resonances and leading to a number of fragments. The present data explain the puzzling recent results of the pulsed-Townsend experiments with this gas.

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We explore solvation of electrons in nonpolar matter, here represented by butadiene clusters. Isolated butadiene supports only the existence of transient anions (resonances). Two-dimensional electron energy loss spectroscopy shows that the resonances lead to an efficient vibrational excitation of butadiene, which can result into the almost complete loss of energy of the interacting electron. Cluster-beam experiments show that molecular clusters of butadiene form stable anions, however only at sizes of more than 9 molecular units. We have calculated the distribution of electron affinities of clusters using classical and path integral molecular dynamics simulations. There is almost a continuous transition from the resonant to the bound anions with an increase in cluster size. The comparison of the classical and quantum dynamics reveals that the electron binding is strongly supported by molecular vibrations, brought about by nuclear zero-point motion and thermal agitation. We also inspected the structure of the solvated electron, finding it well localized.

RESUMEN

We probe the low-energy electron collisions with methyl formate HCOOCH3, focusing on its resonant states. Experimentally, we (i) use two-dimensional electron energy loss spectroscopy to gain information about the vibrational excitation and (ii) report the absolute dissociative electron attachment cross sections. The electron scattering spectra reveal both the threshold effects due to the long-range electron-molecule interaction and a pronounced π* resonance centered around 2.1 eV. This resonance gives rise to dissociative electron attachment into three different anionic channels, the strongest one being the production of the formate anion. Theoretically, we characterize this resonant state using the complex absorbing potential approach combined with multistate multireference perturbation theory, which predicts its position and width in excellent agreement with the experiment.

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Fluoronitrile C3F7CN is a promising candidate for the replacement of SF6 dielectric gas in high-voltage insulation. We present a combined experimental and theoretical study on its ionization dynamics probed in the 0-100 eV energy range. We exploited the total ion collection technique to determine the absolute ionization cross section, mass spectrometry to determine the fragment branching ratios and ab initio nonadiabatic molecular dynamics to simulate the ionization process. The latter two approaches showed the dominating presence of the CF3+ cation over the whole electron energy range. The Binary-Encounter-Bethe (BEB) approximation reproduces experimental cross sections very well and reveals that the ionization from a surprisingly large number of orbitals contributes almost equally to the processes. We show that the initially populated cation excited states undergo an ultrafast internal conversion to the ground state where the dissociation into a single decay channel takes place. Implications for the use of C3F7CN as an insulating material are discussed.

RESUMEN

Electron attachment to the 4-bromobiphenyl molecule and the decay channels of its molecular anion were investigated by means of Dissociative Electron Attachment (DEA) spectroscopy with two different spectrometers. The first apparatus is equipped with a static magnet mass analyzer (Ufa group) and the second one with a quadrupole mass filter (Prague group). The dominant DEA channel at low electron energy leads to formation of Br- negative fragments. Long-lived (τa = 40 µs at the temperature of 80 °C) molecular negative ions were detected only in the Ufa experiment. We explored the involved potential energy surfaces and found that the molecular anion has two distinct structures with the C-Br distances of 1.92 Å and 2.8 Å. The statistical model based on the Arrhenius approximation fully explains the experimental observations and sheds light on the earlier anion dissociation kinetic studies in solution.

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We probe the electron attachment in hexafluoropropylene oxide (HFPO), C3F6O, a gas widely used in plasma technologies. We determine the absolute electron attachment cross section using two completely different experimental approaches: (i) a crossed-beam experiment at single collision conditions (local pressures of 5 × 10-4 mbar) and (ii) a pulsed Townsend experiment at pressures of 20-100 mbar. In the latter method, the cross sections are unfolded from the electron attachment rate coefficients. The cross sections derived independently by the two methods are in very good agreement. We additionally discuss the dissociative electron attachment fragmentation patterns and their role in the radical production in industrial HFPO plasmas.

RESUMEN

In a combined experimental and theoretical study, we probe the dissociative electron attachment in isocyanic acid HNCO. The experimental absolute cross section for the NCO^{-} fragment shows a sharp onset and fine structures near the threshold. The autoionizing state responsible for the dissociative attachment is found in both the R-matrix calculation and using analytic continuation in the coupling constant. The involved A^{'} resonance has a mixed π^{*}/σ^{*} character along the dissociating bond and thus combines the effects of nonzero electron angular momentum and dipole-supported states. This leads to unusual behavior of its width at various geometries. Because the potential energy gradient of the autoionizing state points essentially in the direction of the N─H bond, nuclear dynamics can be described by a one-dimensional nonlocal model. The results agree with the experiment both quantitatively and qualitatively. The present system may be a prototype for interpretation of the dissociative electron attachment process in a number of other polyatomic systems.

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In a combined experimental and theoretical study we characterize dissociative electron attachment (DEA) to, and electronically excited states of, Fe(CO)5. Both are relevant for electron-induced degradation of Fe(CO)5. The strongest DEA channel is cleavage of one metal-ligand bond that leads to production of Fe(CO)4-. High-resolution spectra of Fe(CO)4- reveal fine structures at the onset of vibrational excitation channels. Effective range R-matrix theory successfully reproduces these structures as well as the dramatic rise of the cross section at very low energies and reveals that virtual state scattering dominates low-energy DEA in Fe(CO)5 and that intramolecular vibrational redistribution (IVR) plays an essential role. The virtual state hypothesis receives further experimental support from the rapid rise of the elastic cross section at very low energies and intense threshold peaks in vibrational excitation cross sections. The IVR hypothesis is confirmed by our measurements of kinetic energy distributions of the fragment ions, which are narrow (∼0.06 eV) and peak at low energies (∼0.025 eV), indicating substantial vibrational excitation in the Fe(CO)4- fragment. Rapid IVR is also revealed by the yield of thermal electrons, observed in two-dimensional (2D) electron energy loss spectroscopy. We further measured mass-resolved DEA spectra at higher energies, up to 12 eV, and compared the bands observed there to resonances revealed by the spectra of vibrational excitation cross sections. Dipole-allowed and dipole/spin forbidden electronic transitions in Fe(CO)5-relevant for neutral dissociation by electron impact-are probed using electron energy loss spectroscopy and time-dependent density functional theory calculations. Very good agreement between theory and experiment is obtained, permitting assignment of the observed bands.

RESUMEN

We report partial cross sections for electron attachment to c-C4F8O, a gas with promising technological applications in free-electron-rich environments. The dissociative electron attachment leads to a number of anionic fragments resulting from complex bond-breaking and bond-forming processes. However, the anion with the highest abundance is the non-dissociated (transient) parent anion which is formed around 0.9 eV electron energy. Its lifetime reaches tens of microseconds. We discuss the origin of this long lifetime, the anion's strong interactions with other molecules, and the consequences for electron-scavenging properties of c-C4F8O in denser environments, in particular for its use in mixtures with CO2 and N2.

RESUMEN

We report partial cross sections for the dissociative electron attachment to pyruvic acid. A rich fragmentation dynamics is observed. Electronic structure calculations facilitate the identification of complex rearrangement reactions that occur during the dissociation. Furthermore, a number of fragment anions produced at electron energies close to 0 eV are observed, that cannot originate from single electron-molecule collisions. We ascribe their production to secondary reactions of the transient anions with neutral molecules. Such reactions turn out to be unusually efficient; the most probable reason for this is that they proceed via the formation of a double-hydrogen-bonded complex followed by an ultrafast proton transfer between the reaction partners.

RESUMEN

Experiment and theory are combined to study the interaction of low energy electrons with microhydrated uracil and its halogenated analogues 5-fluorouracil and 5-bromouracil. We report electron ionization (EI) and electron attachment (EA) mass spectra for the uracils with different degrees of hydration. Both EI and EA lead to evaporation of water molecules. The number of evaporated molecules serves as a measure of the energy transferred to the solvent. Upon EI, the amount of energy transferred to neighboring water molecules is similar for all three studied species. On the other hand, the energy transferred upon EA rises significantly from uracil to 5-fluorouracil and 5-bromouracil. 5-Bromouracil is the only studied molecule that undergoes dissociative electron attachment after hydration at the studied energy of 1.2 eV. Theoretical modeling of the energetics for the electron attachment process allows for setting the energy transferred to the solvent on the absolute scale. We discuss the importance of this energy for the radiosensitization.

RESUMEN

When ionizing radiation passes biological matter, a large number of secondary electrons with very low energies (<3 eV) is produced. It is known that such electrons cause an efficient fragmentation of isolated nucleobases via dissociative electron attachment. We present an experimental study of the electron attachment to microhydrated nucleobases. Our novel approach allows significant control over the hydration of molecules studied in the molecular beam. We directly show for the first time that the presence of a few water molecules suppresses the dissociative channel and leads exclusively to formation of intact molecular and hydrated anions. The suppression of fragmentation is ascribed to caging-like effects and fast energy transfer to the solvent. This is in contrast with theoretical prediction that microhydration strongly enhances the fragmentation of nucleobases. The current observation impacts mechanisms of reductive DNA strand breaks proposed to date on the basis of gas-phase experiments.

Asunto(s)

RESUMEN

In magnetically coupled, planar ferromagnet-superconductor (F/S) hybrid structures, magnetic domain walls can be used to spatially confine the superconductivity. In contrast to a superconductor in a uniform applied magnetic field, the nucleation of the superconducting order parameter in F/S structures is governed by the inhomogeneous magnetic field distribution. The interplay between the superconductivity localized at the domain walls and far from the walls leads to effects such as re-entrant superconductivity and reverse domain superconductivity with the critical temperature depending upon the location. Here we use scanning tunnelling spectroscopy to directly image the nucleation of superconductivity at the domain wall in F/S structures realized with Co-Pd multilayers and Pb thin films. Our results demonstrate that such F/S structures are attractive model systems that offer the possibility to control the strength and the location of the superconducting nucleus by applying an external magnetic field, potentially useful to guide vortices for computing application.

RESUMEN

We report cross sections for pickup of guest molecules on neutral argon and water clusters with the mean sizes in the range from N = 50 to 600. The experiments are supported by molecular dynamics simulations and analytical models based on the interaction potentials. The cross sections for argon clusters are consistent with their assumed spherical shape and follow approximately the theoretically justified N(1/3) dependence. On the other hand, the cross sections of water clusters depart from this dependence and are considerably larger starting from N ≥ 300. We interpret this increase of cross section by the occurrence of highly irregular shapes of water clusters produced in the supersonic expansion of water vapor under the conditions of the large cluster generation.

Asunto(s)

RESUMEN

Two mechanisms for dissociative electron attachment in HCOOH, the formation of HCOO(-)+H, were proposed in the literature: (i) via a direct electron attachment into a σ* resonance, augmented by dipole binding of the incident electron [G. A. Gallup et al., Phys. Rev. A 79, 042701 (2009)], and (ii) with the 1.8 eV π* resonance as a doorway state, linked to the products by symmetry lowering-distortion of the temporary anion, primarily the C-H bond, from the planar symmetry [T. N. Rescigno et al., Phys. Rev. Lett. 96, 213201 (2006)]. The later mechanism implies a reduction of the cross section upon deuteration of the hydrogen bonded to the C atom, whereas the former mechanism would leave the cross section unaffected. Our experimental absolute cross sections for the four isotopomers of formic acid show that deuteration on the C atom reduces the cross section value only marginally (by 12%) compared to deuteration on the O atom (reduction by a factor of 16), and thus favor mechanism (i).