Anion Yields Research Articles

Formation of Negative Ions upon Dissociative Electron Attachment to the Astrochemically Relevant Molecule Aminoacetonitrile.

Aminoacetonitrile (NH2CH2CN, AAN) is a molecule relevant for interstellar chemistry and the chemical evolution of life. It is a very important molecule in the Strecker diagram explaining the formation of amino acids. In the present investigation, dissociative electron attachment to NH2CN was studied in a crossed electron-molecular beams experiment in the electron energy range from about 0 to 17 eV. In this electron energy range, the following six anionic species were detected: C2H3N2(-), C2H2N2(-), C2H2N(-), C2HN(-), CN(-), and NH2(-). Possible reaction channels for all the measured negative ions are discussed, and the experimental results are compared with calculated thermochemical thresholds of the observed anions. Similar to other nitrile and aminonitrile compounds, the main anions detected were the negatively charged nitrile group, the dehydrogenated parent molecule, and the amino group. No parent anion was observed. Low anion yields were observed indicating that AAN is less prone to electron capture. Therefore, AAN can be considered to exhibit a relatively long lifetime under typical conditions in outer space.

Electron attachment to chlorinated alcohols

We have studied low energy electron attachment to a series of alcohols with one/two chlorine atoms. We have applied two experimental methods: the electron swarm (SWARM) and negative ion mass spectrometry (NIMS). The electron attachment processes in the mixtures of chloroalcohols with carbon dioxide have been investigated using an electron swarm technique. We obtained rate coefficients (k's) and the activation energies (Ea's) for CH2ClCH2OH, CH2ClCH2CH2OH, CH2ClCH(OH)CH2Cl, CH2ClCHClCH2OH and CH2ClCH(OH)CH2OH in the temperature range 298–378K.The negative ion mass spectra of CH2ClCH2OH, CH2ClCH2CH2OH, CH2ClCH(OH)CH2Cl and CH2ClCH(OH)CH2OH has been investigated. The anion yields were measured in the electron energy range 0–15eV.

Dissociative electron attachment to the gas-phase nucleobase hypoxanthine.

We present high-resolution measurements of the dissociative electron attachment (DEA) to isolated gas-phase hypoxanthine (C5H4N4O, Hyp), a tRNA purine base. The anion mass spectra and individual ion efficiency curves from Hyp were measured as a function of electron energy below 9 eV. The mass spectra at 1 and 6 eV exhibit the highest anion yields, indicating possible common precursor ions that decay into the detectable anionic fragments. The (Hyp - H) anion (C5H3N4O(-)) exhibits a sharp resonant peak at 1 eV, which we tentatively assign to a dipole-bound state of the keto-N1H,N9H tautomer in which dehydrogenation occurs at either the N1 or N9 position based upon our quantum chemical computations (B3LYP/6-311+G(d,p) and U(MP2-aug-cc-pVDZ+)) and prior studies with adenine. This closed-shell dehydrogenated anion is the dominant fragment formed upon electron attachment, as with other nucleobases. Seven other anions were also observed including (Hyp - NH)(-), C4H3N4 (-)/C4HN3O(-), C4H2N3 (-), C3NO(-)/HC(HCN)CN(-), OCN(-), CN(-), and O(-). Most of these anions exhibit broad but weak resonances between 4 and 8 eV similar to many analogous anions from adenine. The DEA to Hyp involves significant fragmentation, which is relevant to understanding radiation damage of biomolecules.

Electron impact induced anion production in acetylene

A detailed experimental investigation of electron induced anion production in acetylene, C2H2, in the energy range between 1 and 90 eV is presented. The anions are formed by two processes in this energy range: dissociative electron attachment (DEA) and dipolar dissociation (DD). DEA in C2H2 is found to lead to the formation of H(-) and C2(-)/C2H(-) through excitation of resonances in the electron energy range 1-15 eV. These anionic fragments are formed with super thermal kinetic energy and reveal no anisotropy in the angular distributions. DD in C2H2 leads to the formation of H(-), C(-)/CH(-) and C2(-)/C2H(-) with threshold energies of 15.7, 20.0 and 16.5 eV respectively. The measured anion yields have been used to calculate anion production rates for H(-), C(-)/CH(-) and C2(-)/C2H(-) in Titan's ionosphere.

Mass spectrometry of anions and cations produced in 1–4 keV H −, O −, and OH − collisions with nitromethane, water, ethanol, and methanol

Interactions between 1 and 4 keV anions (H −, O −, and OH −) and gas-phase molecules (nitromethane, water, ethanol, and methanol) have been studied using quadrupole mass spectrometry of the product anions and cations. The low collision velocities (0.07–0.40 v Bohr) provide favourable conditions for electron transfer from the anion projectile to the neutral target molecule yielding negative ion formation, while strong competition with cation formation is also observed. Relative production of fragment cations increases with H − impact energy and with projectile mass when energy is constant. Considered together, these results suggest a momentum dependence on collisional energy deposition. As far as negative ion production is concerned, comparisons with previous free electron attachment studies are drawn as a starting point for the interpretation of the anion fragmentation channels. For nitromethane and water, the present anion fragmentation patterns are substantially different to the free electron attachment data. Conversely the fragmentation channels of ethanol and methanol anions only show clear dependence on the electron attachment/transfer process in terms of the relative anion yields.

Cluster size and velocity dependences of sputtering and secondary ion emission under gold cluster impact

Sputtering and ion emission rates have been measured from CsI and gold targets under the impact of Au n + ( n = 1–9) clusters at energies between 30 and 350 keV/atom. The two materials have similar behaviors in regard to the variations of the sputtering and anion emission yields with energy and cluster size. The sputtering and anion emission yields increase nonlinearly with the projectile size. The maximum anion yields are found at lower energies than the maximum sputtering yields which themselves occur substantially below the maximum energy losses. The variations with energy of the atomic ion yields differ from those of the cluster ion yields. The experimental results are in agreement with an ion emission from linear collision cascades and spike collisions, the relative contribution of these two processes depending on the size of the cluster projectile and of the emitted ion. In addition they show that the ion emission yield enhancements under cluster impact result from a more effective sputtering mechanism and not from an enhanced ionization of the ejected species.

High-resolution electron attachment to the molecules CCl4 and SF6 over extended energy ranges with the (EX)LPA method

Using a variant of the laser photoelectron attachment (LPA) method with an extended energy range (EXLPA), we have studied low-energy electron attachment to the molecules CCl4 (Cl− and Cl−2 formation) and SF6 (SF−6 and SF−5 formation) in a diffuse gas target (TG = 300 K) from 0 eV up to 2 eV at energy widths down to 14 meV. In the EXLPA method, pulses of near-zero energy photoelectrons are produced in a guiding magnetic field, accelerated by a weak electric field, brought to the energy of interest prior to their traversal through the target region and subsequently accelerated and deflected onto a detecting plate. Anions due to electron attachment are extracted by a pulsed electric field, during which the photoelectron current is interrupted, and detected by a quadrupole mass spectrometer. The EXLPA anion yields are combined with absolute cross sections, obtained at very high resolution (≈1 meV) with the LPA method over the range 0–0.17 eV, to yield new recommended absolute partial and total attachment cross sections over the range 0–2 eV at the well-defined gas temperature TG = 300 K. Our cross sections show characteristic deviations from previously reported results. At least in part, these differences can be attributed to the fact that in the earlier electron beam experiments the gas temperature was higher than 300 K. For SF6, the branching fractions for SF−5 formation at electron energies 0.002–0.43 eV and for different initial rovibrational distributions are compared with those recently predicted from kinetic modelling within the framework of statistical unimolecular rate theory. Satisfactory agreement is observed, but our data provide evidence that an additional path for producing SF−6 and SF−5 ions is available at electron energies above about 0.3 eV.

Metal-Enhanced Fluorescence (MEF)

In recent years our laboratories have described the favorable interactions and outcomes of both plasmon supporting particles (Ag, Au, Cu, Zn, Ni, Cr) and substrates with electronically excited states. These favorable effects have included enhanced fluorescence emission from singlet states, S1 and S2, as well as enhanced phosphorescence yields from triplet, T1, states (MEP). In addition, we have observed and described plasmon enhanced chemiluminescence intensities (MEC), as well as highly directional emission. As a result of enhanced triplet yields, we have also observed both enhanced singlet oxygen and superoxide anion yields. These favorable influences on the photophysical properties of close proximity excited states to plasmon supporting substrates/particles has led to wealth of biochemical applications, such as the high sensitivity and ultra fast detection of proteins, DNA and ultra bright and photostable metal-enhanced fluorescence based particles for downstream cellular imaging applications. In addition, there are a lot downstream applications of MEP such as in photodynamic therapy by surface plasmon controlled single oxygen generation. Current thinking, describes Metal-Enhanced Fluorescence as the near-field coupling of electronic excited states to surface plasmons (a surface mirror dipole), the particle subsequently radiating the photophysical characteristics of the coupled excited state in the far-field, remarkably, even vibronic structure. In this paper, we communicate our recent findings for metal-fluorophore interactions and our current thinking and progress towards developing a unified metal-fluorophore description.

Investigations of electron attachment to the perfluorocarbon molecules c-C4F8, 2-C4F8, 1,3 C4F6, and c-C5F8

Non-dissociative and dissociative electron attachment to a series of gas-phase perfluorocarbons (PFCs), namely octafluorocyclobutane, c-C4F8, octafluorobut-2-ene (perfluoro-2-butene), 2-C4F8, hexafluorobuta-1,3-diene (1,3 perfluorobutadiene), 1,3 C4F6, and octafluorocyclopentene (perfluorocyclopentene), c-C5F8, of importance to technological plasmas, have been investigated using two different, but complimentary, instruments available in Innsbruck over the electron energy range 0–20eV. Anion yields as a function of electron energy have been recorded, with the positions and intensities of the electron attachment resonances being determined. One of these instruments is a double focusing sector field mass spectrometer (VG-ZAB-2SEQ), which has been used for measurements requiring high sensitivity and for obtaining accurate relative anion yields. It has also been used to determine the electron detachment lifetimes of the parent anions under various accelerating voltages, and these results are also presented. The second instrument (CELIA) is a trochoidal electron monochromator coupled to a quadrupole mass filter with a pulse counting system for detecting product anionic species. This provides a much higher energy resolution than the VG-ZAB, which makes it a better instrument to investigate narrow energy resonances close to 0eV. The results of anion yields, peak positions and the relative intensities presented in this paper are compared with previous data of electron attachment to the above PFCs, including investigations by Professor Eugen Illenberger.

Negative ion formation in dissociative electron attachment to selected halogen derivatives of propane

Dissociative electron attachment (DEA) to halogenated derivatives of propane: 1-bromo-3-chloropropane, 2-bromo-1-chloropropane, 3-bromo-1,1,1-trichloropropane and 1,3-dibromo-1,1-difluoropropane was studied in the gas phase at ambient temperature using a high resolution crossed electron/molecule beams technique. The negative ions formed via DEA reaction were identified using mass spectrometric technique and the anion yields were measured in the electron energy range from 0 to 10eV. The absolute partial cross sections for DEA to the molecules were estimated using the relative flow technique.

Isotope effects in dissociative electron attachment to the DNA base thymine

Dissociative electron attachment to thymine and two partially deuterated derivatives are studied with high electron energy resolution. For all fragment anions involving hydrogen loss, a dramatically reduced anion yield is measured for loss of D compared to loss of H. For the low-energy vibrational Feshbach resonances this isotope effect reaches a value of 40. Narrow features in the anion yields that previously were assigned to H loss from the N1 site of thymine and concomitant excitation of specific stretching modes are shifted to lower electron energies when deuterium is involved.

Positive and negative ion formation via slow electron collisions with 5-bromouridine

Cation and anion formation is studied experimentally upon inelastic low energy electron interactions with 5-bromouridine (BrUrd) using a crossed electron/neutral beams set-up combined with a quadrupole mass spectrometer. The BrUrd molecule belongs to the class of radiosensitizers that increase the sensitivity of DNA (or RNA) to ionizing radiation. In the case of positive ion formation the ionization efficiency curves are measured near the threshold and the corresponding appearance energies are determined using a non-linear least square fitting procedure. The anion yields are investigated in the electron energy range from about 0 to 14 eV. From the comparison of the present results for negative and positive ions information concerning the underlying mechanism of radiosensitizers is deduced.

Grazing incidence scattering of hyperthermal C60+, C76+, and C84+ from graphite: Electron transfer dynamics

Hyperthermal grazing incidence scattering (Ecoll=500–700 eV, θin=80°) of fullerenes C60+, C76+, and C84+ from highly oriented pyrolytic graphite surfaces is associated with efficient one and two electron transfer to yield parent neutrals and anions—in addition to parent cations. We report lower limits to nascent anion yields under conditions for which collisional heating is kept relatively small. Largest anion yields were observed for C84, which also has the largest electron affinity of the projectiles probed. Here >3% of the particles scattered upon 11 500 ms−1 collision survive detection as anions. This can be qualitatively understood in terms of a resonant electron transfer mechanism and a simple kinetic scheme.

Anionic photofragmentation of CO: a selective probe of core-level resonances.

Anion-yield spectroscopy using x rays is shown to be a selective probe of molecular core-level processes, providing unique experimental verification of shape resonances. For CO, partial anion and cation yields are presented for photon energies near the C K edge. The O- yield exhibits features above threshold related only to doubly excited states, in contrast to cation yields which also exhibit pronounced structure due to the well-known sigma* shape resonance. Because the shape resonance is completely suppressed for O-, anion spectroscopy thus constitutes a highly selective probe, yielding information unobtainable with absorption or electron spectroscopy.

Halogen Anion Formation in 5-Halouracil Films: X Rays Compared to Subionization Electrons

The radiosensitization properties of 5-halouracils (5-FU, 5-BrU and 5-IU), i.e. the enhanced sensitivity of biological media containing these compounds to ionizing radiation, have been studied using surface science methods. We show that soft X rays and near 0 eV electrons both induce dissociation of 5-halouracils into a halogen anion and a uracilyl radical. The yield of anions from 5-FU is much smaller than that from the bromo- and iodo-analogs. We explain the high anion yields in 5-BrU and 5-IU with dissociative electron attachment (DEA) of near 0 eV electrons. The thermodynamic threshold for DEA to 5-FU is near 2 eV and therefore prohibits dissociation by near 0 eV electrons.

Estimation of the electron affinities of C 60, corannulene, and coronene by using the kinetic method

Novel anions that contain one molecule each of C60 and the polycyclic aromatic hydrocarbon coronene are generated in the gas phase by electron attachment desorption chemical ionization. Collision-induced dissociation reveals that these cluster ions are loosely bonded. Fragmentation of the mass-selected cluster anion yields, as the only products, the intact radical anions of the constituent molecules, namely, the C60 radical anion and the coronene radical anion, in almost identical relative abundances. This result is interpreted as evidence that the cluster ion can be considered as the anion radical of one molecule solvated by the other molecule. The known very high electron affinity of C60 (2.66 eV) and the comparable degree to which C60 and the PAH compete for the electron suggests that dissociation may be controlled by the electron affinity of a portion of the C60 surface, that is, in this case the kinetic method yields information on the local electron affinity of C60. The electron affinity of the bowl-shaped compound corannulene is estimated for the first time to be 0.50 ± 0.10 eV by the kinetic method by using a variety of reference compounds. Unlike coronene, corannulene reacts with C -• (60) in the gas phase to form a covalently bonded, denydrogenated cluster ion. Support for the concept of "local" electron affinity of C60 comes from a theoretical calculation on the electronic structure of C60 anions, which shows evidence for localization of the charge in the C60 molecule. The possibility of electron tunneling in the C60-coronene system is discussed as an alternative explanation for the unusual observation of equal abundances of C60 anions and coronene anions upon dissociation of the corresponding cluster ion.

Threefold Differential Electron-Stimulated Desorption Yields of D- Anions from Multilayer Films of D2O and ND3 Condensed on Platinum

Dissociative electron attachment (DEA) processes have been observed for multilayer films of D2O and ND3 condensed on polycrystalline platinum at 20 K. Kinetic energy distributions, fixed kinetic energy anion yields, and angular repartitions for D- anions are reported for primary electron energies from 5 to 15 eV. No trace of any other fragment anion was detected. Comparison with gas phase D- differential cross sections shows that the dynamics of dissociation and desorption processes is strongly perturbed in the condensed phase both by the polarizability of the adsorbate and the metallic substrate and by the transfer of excess energy to the lattice acting as a cooling bath. Moreover, the low-energy desorbing anion fragments are very sensitive to charging effects at the dielectric surface.

Dissociative electron attachment to condensed and adsorbed halomethanes

Dissociative electron attachment (DEA) processes have been observed and characterized for submonolayer and multilayer quantities of CCl4, CDCl3, CD2Cl2, CH3Cl, and CH3Br adsorbed on metallic substrates and submonolayer quantities adsorbed onto Kr spacer layers. Anion yields (Br−, Cl−, H−, D−) are reported for incident electron energies from 0 to 12 eV; no desorbing polyatomic anion fragments were detected. The results are compared to the gas-phase DEA analogs and demonstrate the perturbations on the DEA process that are introduced by the presence of the highly polarizable environment (molecular solid+metal substrate). We also report the first observation of H−(D−) produced by DEA of halomethanes containing hydrogen (deuterium).

Influence of solvent and cation on the properties of oxygen-containing organic anions. Part 4. Mechanism and reactivity of tetraaryloxirane cleavage with alkali metals

Six tetraaryloxiranes 1a–f(Scheme 4) were reduced (Schemes 1–3) with alkali metals (M = Li, Na, K, Cs) in eight polar aprotic solvents under an inert atmosphere. The organometallic solutions thus obtained were hydrolysed and the reaction products analysed. Similar experiments were carried out where the same solutions were quenched with D2O or Mel. In some cases the same solutions were studied by NMR and ESR spectroscopy before quenching. A stepwise reduction mechanism was established where the transfer of a first electron produces CO-bond scission in the oxirane ring, yielding a short-lived radical anion 4 or 5(Scheme 1), i.e. a tetraaryl-β-oxidoethyl radical. This intermediate can either eliminate oxygen as metal oxide (MO) to produce a tetraarylethylene 24(Scheme 2) or be further reduced to a dianion 8 or 9(Scheme 1). This anion yields, upon hydrolysis, low yields, if any, of the corresponding tetraphenylethanol 15 or 16(Z = H). The larger proportion of the dianion, after the first protonation step, yielding anion 11 or 12, undergoes CC-bond scission which leads eventually to the corresponding ketone and diarylmethane 19+ 20 or 21+23(Z = H)(Scheme 2). Other possible pathways were excluded through experiments where other possible intermediates were generated. These led to different end products. A triparametric linear correlation as a function of solvent parameters ETN and DN, as well as the cationic radius, was established for the influence of the nature of the solvent and counter-ion on the ratio between the rates of formation of products stemming from metal oxide (MO) elimination by the ring-opened radical anion 4 or 5(Schemes 1 and 2) and rates of formation of products stemming from further reduction of the same radical anion to the dianion 8 or 9, thus confirming the mechanism established.

Electrons, ions and excited states in the pulse radiolysis of tetrahydrofuran

Pulse radiolysis of tetrahydrofuran (THF) gives a transient absorption at long wave-lengths which is attributed to the solvated electron. When pyrene or anthracene is present their anions are formed with immediate yields of 0.66 and 0.70 when measured on microsecond time scales. These are considered to give the “free ion” yield. On nanosecond time scales, immediate yields are much higher, and there is an initial rapid decay of the anion which is attributed to ion recombination in the spurs. Lithium aluminium hydride, even on microsecond time scales, gives increased initial anion yields and longer anion lifetimes. At high concentrations G(anion)= 2.6 is attained. This is considered to be the primary ion pair yield in liquid THF and agrees with that derived from γ-radiolysis measurements. Anthracene gives G(AS)= 0.1 and G(AT)= 0.04, values which do not vary with anthracene concentrations over the range 5 × 10–4–10–2 M.