Dissociative Electron Attachment Process Research Articles

Electron attachment to PSCl3.

Electron attachment to PSCl3 was studied in 133-Pa pressure of helium gas at temperatures from 298-550 K. Measurements of rate constants and branching fractions were made in a flowing-afterglow Langmuir-probe (FALP) apparatus. These experiments yielded an electron attachment rate constant of 5.1 x 10(-8) cm3 s(-1) that was found not to change significantly in the 298-550 K temperature range. This rate constant represents an attachment efficiency of about 14%. Attachment in 133 Pa of He gas yielded only the dissociative ion products PSCl2- and Cl-. The FALP data suggest that there is an activation energy of about 17 meV for production of PSCl2-. Attachment to PSCl3 was also studied at high pressure (9-93 kPa) of N2 in an ion mobility mass spectrometer, at 298 K. In contrast to the low-pressure data, the parent anion product channel (PSCl3-) was observed (along with the dissociative channels), and increased in importance with N2 pressure. Gaussian-3 (G3) calculations were carried out for PSCl3 and PSCl2 neutrals and anions to aid in interpretation of the experimental results. The calculations indicate that the electron affinity EA(PSCl2) is slightly smaller than EA(Cl), which may account for the observed branching fractions for PSCl2- and Cl- in the low-pressure experiments. A natural population analysis was performed to obtain the charges associated with each atom in the molecules in order to estimate how the attached electron is distributed. Comparison is made between the present study of electron attachment to PSCl3 and our earlier work on attachment to POCl3, and G3 calculations are reported here for neutral and anionic POCl2 and POCl3. In contrast to PSCl2, the calculations imply that EA(POCl2) is slightly greater than EA(Cl). For both PSCl3 and POCl3, the calculations show that the dissociative electron attachment process is close to thermoneutral.

Phenol, chlorobenzene and chlorophenol isomers: resonant states and dissociative electron attachment.

This paper reports a study of resonant dissociative electron attachment (DEA) to the phenol, chlorobenzene, p-, m-, and o-chlorophenol molecules. On the basis of spectroscopic and thermochemical approaches the resonant states of the molecular negative ions (NIs) and the structures of some dissociative decay products are assigned. In the electron energy range up to 3 eV, DEA processes are determined by the two 2[pi*]-shape resonances resulting mainly in formation of [M-H]- and/or Cl- ions. At higher electron energies the energy correlation between peaks in the negative ion effective yield curves and bands of UV spectra allowed identification of the core-excited resonances. The peculiarities of Cl- ion formation and the vibrational fine structure on the effective yield curves of the [M-H]- ions are discussed. The mass spectrometric procedures for measurement of relative cross sections for NI formation are described.

Temperature dependencies of negative ions formation by capture of low-energy electrons for some typical MALDI matrices

Some typical matrix assisted laser desorption/ionization (MALDI) matrices (gentisic, nicotinic, succinic, sinapinic, caffeic acids, urea and 6-aza-2-thiothymine) have been investigated by electron capture negative ions mass spectrometry (ECNI-MS) with purpose to find such decay channels of target molecule that may be important for processes of desorption and “primary” ions formation in MALDI. The temperature dependencies of the process of dissociative electron attachment to molecules of MALDI matrices were obtained for six different temperatures in the range of 70–300 °C. Also the relative dissociative attachment peak cross-sections for molecules under investigation were measured and ionization and desorption abilities of the MALDI matrices were calculated. The general conclusions are (1) a capture of low energy electrons by molecules of MALDI matrices leads to an appearance of anions CN − and [M−H] − that may take part in formation of analyte anions, (2) gas molecules in MALDI (CO 2, H 2, CH 3CN and so on) are formed by a capture of low energy electrons that may leads to desorption of the analyte in according to CO 2-MALDI model, (3) the dissociative attachment cross-section for molecules of matrix determines the ability to form both anions and gas bubbles as well as indicates how well a matrix preserves positive analyte ions from neutralization by the free low energy electrons.

Dissociative electron attachment to H 2O 2: a very effective source for OH and OH − generation

Dissociative electron attachment (DEA) to hydrogen peroxide (H 2O 2) is studied by two independent electron beam experiments using mass spectrometric detection of the product ions. The fragments OH − and O − are observed from a prominent low energy resonance peaking near 0.4 eV. The partial absolute DEA cross-sections are 6.8×10 −17 and 1.7×10 −17 cm 2 for the OH − and the O − channels, respectively. These numbers are several orders of magnitude higher than for the corresponding DEA processes in simple OH containing organic compounds. These numbers are also important from the point of view of radiation damage in biological tissues.

Surface versus Sub-Surface D? Electron Stimulated Desorption from Diamond Surfaces

In this work we report on a study of the low energy electron stimulated desorption (ESD) of D— from in-situ hot filament deuterated surfaces of diamond films. For incident electron energies in the 5–15 eV range a dissociative electron attachment (DEA) processes occur. The cross section for D— ESD obtains a maximum value at ∼9 eV. Ion kinetic energy distribution (KED) measurements show, that in the DEA regime desorption results in a narrow peak, whose energy position increases with incident electron energy at an energy which corresponds – minus a multiphonon excitation factor – to the thermodynamic limit in agreement with gas phase considerations. To study the effect of inelastic interactions between the desorbing D— ions and the surface KED measurements were performed as function of desorbing angle with respect to the surface normal. It was found that with increasing angle from the surface normal the D— KED broadens and its lower energy component increases in intensity. These results clearly show that inelastic interactions between the outgoing D— and the solid surface take place. Our results show that ESD may be used to determine the presence of surface versus sub-surface hydrogen (deuterium) adsorbed on diamond.

Synthesis of Cl2 induced by low energy (0–18 eV) electron impact to condensed 1,2-C2F4Cl2 molecules

The interaction of low energy electrons (0–18 eV) with C2F4Cl2 molecules condensed in multilayer amounts on a cryogenically cooled Au substrate generates Cl2, as can be seen from the energy and temperature dependence of the Cl− desorption signal. The cross section for Cl2 formation exhibits two pronounced resonant features with maxima near 0 and 10 eV, dropping to essentially zero in the energy range between the resonances (near 3 eV). The energy dependence of the reaction cross section qualitatively follows that of Cl− desorption, which itself can be correlated to dissociative electron attachment (DA) processes in the gas phase. While at 10 eV excited negative ion resonances and electronically excited states of neutral C2F4Cl2 may be involved in the process, in the low energy region (<2 eV) the reaction can only be initiated by dissociative electron attachment. Possible reaction pathways are discussed.

Mechanisms and dynamics of electron-stimulated desorption ofD−from deuterated diamond surfaces: Surface versus subsurface stimulated desorption

In this work we report on a study of low-energy electron-stimulated desorption (ESD) of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ from in situ hot-filament-deuterated surfaces of diamond films. This deuteration procedure ensures that deuterium is predominantly adsorbed on the diamond surface and that no significant diffusion underneath the surface takes place. For incident electron energies in the 5--35-eV range, dissociative electron attachment (DEA) and dipolar dissociation (DD) processes occur. The cross section for ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ ESD obtains a maximum value at \ensuremath{\sim}8 eV, whereas the DD process displays a threshold at \ensuremath{\sim}14 eV. Ion kinetic-energy distribution (KED) measurements show that in the DEA regime desorption results in a narrow peak whose energy position increases with the incident electron energy to a value that corresponds, minus a multiphonon excitation factor, to the thermodynamic limit, in agreement with gas-phase considerations. In the DD regime the ion KED displays a peak at \ensuremath{\sim}2 eV which does not depend on the incident electron kinetic energy. To study the effect of inelastic interactions between the desorbing ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ ions and the surface, in the DEA regime, KED measurements were performed as a function of desorbing angle with respect to the surface normal. It was found that with an increasing angle from the surface normal the ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ KED broadens, and its lower-energy component increases in intensity. These results clearly show that inelastic interaction between the outgoing ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ and the solid surface takes place, and determines the KED of desorbing ions. The ESD results obtained for the in situ deuterated surface are compared with those previously obtained for deuterated diamond films that contain some subsurface deuterium for which broad KED's were measured. A difference in the ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ KED in the DD regime is also measured. The indirect DEA process observed in the case of hydrogenated (deuterated) diamond films is strongly reduced in the present case of an on-top deuterated diamond surface. Our results show that ESD may be used to determine the presence of surface versus subsurface hydrogen (deuterium) adsorbed on diamond.

Another look at the electron attachment to nitrous oxide

Ab initio molecular orbital calculations up to the coupled-cluster level with the aug-cc-pVQZ basis set allowed us to have a new look at the electron affinity of nitrous oxide (N2O) resulting in a detection of a new N2O− entity, and thereby a novel mechanism for the dissociative electron attachment process, N2O+e−→N2+O−. Addition of an electron to the linear N2O ground state (X 1Σ+) leads first to an open-chain bound anion which lies 25 kJ/mol above the neutral. Upon a cyclization of the open anion with an additional energy barrier of 25 kJ/mol, a cyclic anionic species is formed which is more stable than the open isomer and lies now, at most, 3 kJ/mol above the neutral ground state (the transition structure for cyclization being 50 kJ/mol above neutral N2O). The cyclic anionic species constitutes a weak complex between N2 and O− characterized by a binding energy of only 16 kJ/mol. The electronic structure of the anion complex is analyzed, a number of earlier experimental results are clarified and a resolution for the long-standing disagreement between experiment and theory around the electron affinity of N2O is proposed.

Low-energy electron-induced processes in condensed CF2Cl2 films

We report measurements of electron stimulated desorption yields and kinetic energy distributions for F− and Cl− anions from thin films of CF2Cl2 condensed on polycrystalline platinum at 20 K. Dissociative electron attachment processes cause a peak in the F− yield at an electron energy of ∼4 eV and a peak in the Cl− yield at ∼7 eV. After electron irradiation, a new dominant structure appears in the Cl− yield at 5 eV incident electron energy, and Cl− ions having high kinetic energy (&amp;gt;1 eV) are observed in the kinetic energy distributions. These changes are attributed to the synthesis of Cl2 in the condensed phase induced by electron irradiation of the CF2Cl2 film. This Cl2 formation is demonstrated by the observation in the Cl− yield of the Cl2−/Cl2 2Πu resonance state, and by Cl− kinetic energy distributions that are similar to those obtained from condensed pure Cl2 films.

Mechanism of negative ion formation from phenol and para-chlorophenol by interaction with free electrons

Dissociative electron attachment (DEA) to phenol and para-chlorophenol in the energy range 0-12 eV is studied. Analogies in formation of the resonance states in an ionic benzene and its derivatives are found to arise from the similarity of the aromatic base of the molecules. Differences in DEA processes are defined mainly by the influence of the functional OH-group and, to a lesser degree, by the presence of a chlorine atom. A correlation between the energies of the resonance states and ionization energies of p-chlorophenol and phenol, analogous to that found previously for phenol, is proved. On this basis it is established that the dominating mechanism for formation of molecular negative ions at energies above 5 eV is Feshbach resonance.Copyright 2000 John Wiley & Sons, Ltd.

Photoabsorption and near-threshold electron energy-loss spectroscopy of OClO

The spectroscopy of chlorine dioxide, OClO, has been investigated using the techniques of dissociative electron attachment (DEA), near-threshold electron energy loss (EEL, 0-10 eV) and optical absorption (2.5-10.8 eV). Mass and energy analyses of the ions formed in the DEA processes show that at energies above about 3 eV, the product ions are predominantly near-thermal , suggesting that the accompanying oxygen atoms are excited electronically. All features observed in the EEL experiments are in regions of optical absorption, indicating that the optically dark doublet and quartet states have energies which overlap optically allowed transitions. The optical spectrum itself is in good qualitative agreement with previous measurements, but there are differences in relative absorption cross sections. Alternative interpretations of some spectral features are proposed.

Dissociative electron attachment negative ion mass spectrometry: a chlorine-specific detector for gas chromatography

This work describes the application of negative ion chemical ionization, optimized for dissociative electron attachment (DEA), to location of unknown trace chlorinated compounds in complex gas chromatograms by selected ion recording (SIR) of m / z 35 and 37. The DEA-SIR technique is compared with other GC detectors, including the electron capture detector, electrolytic conductivity detector, the atomic emission detector and the chemical reaction interface mass spectrometry method, with respect to selectivity for chlorine, sensitivity, linear dynamic range, and general robustness and ease of use. When applied to quantitative analysis of target analytes such as polychlorobiphenyls, the DEA-SIR method has potential problems arising from the possibility of suppression effects due to abundant co-eluting components, and possible alleviating measures are discussed. In addition to these practical investigations, literature information on the fundamental physical and chemical phenomena underlying the DEA process is summarized in order to guide future work on extension to other compound types and on general improvements to the technique.

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP H 2/P Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar+ and H2 which lead to ArH+ and H 2 + ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H+, H 2 + , and H 3 + ) or excited species H(n=2) with H2 producing H atoms.

Electron stimulated desorption from PF3 adsorbed on Pt. II. Negative ions

We have studied electron stimulated desorption (ESD) of negative ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). ESD from adsorbed PF3 gives rise to several negative ion fragments: F− (predominantly), F−2, P−, and PF−. The F− yield produced in the electron energy range 0–15 eV proceeds via dissociative electron attachment (DEA); the F− yield exhibits a peak around 11.5 eV with an onset around 7.5 eV. At electron energies above ∼15 eV, F− ions are produced via dipolar dissociation (DD). We have found that the F− ions produced from a 1 ML PF3/Pt surface via a DEA process with 11.5 eV electron impact desorb with a peak kinetic energy of ∼0.7 eV, while the F− ions generated via DD by 175 eV electron impact desorb with a peak kinetic energy of ∼1.2 eV. The F−2 yield curve also shows a peak at ∼11.5 eV; the onset of the F−2 yield from adsorbed PF3 is ∼9 eV. The F−2 yield in the electron range 9–15 eV is initiated via DEA. The P− signal from PF3 adsorbed on Pt has an onset at ∼16 eV. We have identified some possible DEA and DD processes leading to desorption of negative ions from adsorbed PF3. We suggest that Rydberg core-excited (1-hole, 2-electron) transient anion states of PF3, formed by capture of low energy electrons, dissociate to produce the F− and F−2 ions for E(e)&amp;lt;15 eV.

Resonant vibrational excitation and electron induced dissociation processes in HREELS studies of trimethylindium chemisorbed on GaAs(100)

Vibrational high resolution electron energy loss spectroscopy (HREELS) has been used to monitor the wide variety of electron induced processes involving excited electronic states for trimethylindium ((CH 3 ) 3 In) chemisorbed on GaAs(100). The vibrational modes of the adsorbed CH 3 groups undergo strong resonant vibrational excitation at 10 eV. HREEL spectra of the adsorbed layer, recorded with beam energies greater than 8 eV, indicate the formation of a surface CH 2 species characterised by a single deformation mode (δ(CH 2 )) at 1340 cm −1 . The conversion of CH 3 to CH 2 rises dramatically at the resonance energy of 10 eV and reaches a broad maximum between 20 and 30 eV. The implication is that the incident HREELS beam leads to decomposition of adsorbed CH 3 groups. It is suggested that resonant dissociative electron attachment and dipolar dissociation processes are responsible for the conversion of CH 3 to CH 2 at the surface.

Resonant vibrational excitation and electron induced dissociation processes in HREELS studies of trimethylindium chemisorbed on GaAs(100)

Vibrational high resolution electron energy loss spectroscopy (HREELS) has been used to monitor the wide variety of electron induced processes involving excited electronic states for trimethylindium ((CH 3) 3In) chemisorbed on GaAs(100). The vibrational modes of the adsorbed CH 3 groups undergo strong resonant vibrational excitation at 10 eV. HREEL spectra of the adsorbed layer, recorded with beam energies greater than 8 eV, indicate the formation of a surface CH 2 species characterised by a single deformation mode (δ(CH 2)) at 1340 cm −1. The conversion of CH 3 to CH 2 rises dramatically at the resonance energy of 10 eV and reaches a broad maximum between 20 and 30 eV. The implication is that the incident HREELS beam leads to decomposition of adsorbed CH 3 groups. It is suggested that resonant dissociative electron attachment and dipolar dissociation processes are responsible for the conversion of CH 3 to CH 2 at the surface.

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.

Molecular dynamics of dissociative electron attachment in water

Classical trajectory calculations have been used to model dissociative electron attachment (DEA) processes in water. In previously reported experiments, DEA cross-sections show maxima at three different energies between ca. 6 and 12 eV; these have been ascribed to the formation and dissociative decay of three different resonant (anion) states of symmetry 2B1, 2A1 and 2B2 whose parent states are the Ã1 B1, B 1A1 and C 1B2 states, respectively, of neutral water. The present model calculations show that the essential features of the experimental findings can be reproduced using only two anionic potential surfaces; furthermore, most of the observed product ions follow from initial excitation to one (the lower-lying in symmetric geometries) of these. If the two surfaces are related to resonances of symmetry 2B1 and 2B2, then all of the observed features associated with resonant electron scattering from water can be rationalised.

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).

The energetics of resonant dissociative electron attachment to molecules of five-membered heterocyclic compounds

The thermochemistry of resonant dissociative electron attachment processes for furan, thiophene, selenophene, and pyrrole molecules has been studied. The structures of the dissociation products originating from negative molecular ions at energies ranging from 2 to 6 eV have been established using the measured appearance energies of fragment ions and the known thermodynamic functions of radical and molecular dissociation products. Heats of formation and electron affinities for some radicals and molecules have been assessed by calculations and estimated experimentally. It has been concluded that the majority of the fragment ions are formedvia rearrangement processes in molecular or fragment ions.