Dissociative Electron Attachment Process Research Articles

Electron-induced chemistry of cobalt tricarbonyl nitrosyl (Co(CO)3NO) in liquid helium nanodroplets

SynopsisCobalt tricarbonyl nitrosyl (Co(CO)3NO - CTCN) is a widely used source for cobalt in focused electron-beam induced deposition (FEBID) processes. By doping helium nanodroplets at a temperature of 0.37 K with several Co(CO)3NO molecules we managed to create a multitude of molecular clusters and fragments. These clusters provide a similar environment to the materials used in FEBID. Dissociative electron attachment processes are hence studied in order to provide insight into factors limiting the achievable resolution in FEBID.

Dissociative electron attachment to bromotrifluoromethane

Here we present a study on dissociative electron attachment (DEA) to bromotrifluoromethane, CF3Br, in the energy range from 0 to 10eV. The dominating process observed is the formation of Br− close to 0eV incident electron energy. The fragment ion Br− is also observed through a broad contribution from about 3 to 5eV. In this energy range, F−, FBr− and CF2Br− are also observed, and in the Br− and F− ion yield, a weak contribution is observed close to 8.1eV. For the DEA processes we calculate the thermochemical thresholds using conventional DFT methods, and the results are discussed in context to these, earlier work and the assignment of the underlying resonances.

Dissociation dynamics in the dissociative electron attachment to carbon dioxide

Dissociative electron attachment (DEA) to gas phase ${\mathrm{CO}}_{2}$ has been probed using a velocity slice imaging technique. DEA to ${\mathrm{CO}}_{2}$ produces only an ${\mathrm{O}}^{\ensuremath{-}}$ ionic fragment and shows two major resonances located at 4.4 and 8.2 eV, respectively. The kinetic energy and angular distribution of the ${\mathrm{O}}^{\ensuremath{-}}$ ions are measured around the second resonance with higher efficiency and sensitivity that provide details of the DEA dynamics. The kinetic energy distributions are in good agreement with the previous reports. However, the distinct angular distributions show substantial difference from the two recent studies within the limited electron energies. Our angular distribution results show two negative ion resonant states are involved in the underlying DEA process at the entire electron energies over the second resonance. We discussed the recent conflicting findings in the angular distribution results. The forward-backward asymmetry observed in the angular distributions is explained due to the interference effect of different partial waves associated with the attaching electron.

Ab initio molecular dynamics simulation study of dissociative electron attachment to dialanine conformers.

Dissociative electron attachment (DEA) processes of six low-lying conformers (1-6) of dialanine in the gas phase are investigated by using ab initio molecular dynamics simulations. The incoming electron is captured and primarily occupies the virtual molecular orbital π*, which is followed by the different dissociation processes. The electron attachments to conformers 1 and 2 having the stronger N-H···N and O-H···O intramolecular hydrogen bonds do not lead to fragmentations, but two different backbone bonds are broken in the DEAs to conformers 3 (or 4) and 6, respectively. It is interesting that the hydrogen abstraction of -NH from the terminal methyl group -CH3 is found in the roaming dissociation of the temporary anion of conformer 3. The present simulations enable us to have more insights into the peptide backbone bond breaks in the DEA process and demonstrate a promising way toward understanding of the radiation damages of complicated biological system.

Theory of dissociative electron attachment: Biomolecules and clusters

Very broad σ∗ resonances, which are responsible for threshold structures and dissociative attachment in electron collisions with hydrogen halides, are also important in electron-impact bond-breaking in nucleobases and amino acids. We investigate this mechanism in more detail by carrying out model calculations of the N-H bond breaking in the uracil molecule. Although the σ∗ resonance is extremely broad at the equilibrium nuclear geometry, it is stabilized fast when the N-H bond is stretched, and this produces a substantial dissociative attachment cross section. In addition, very pronounced vibrational Feshbach resonances are seen below vibrational excitation thresholds. To incorporate the effect of a cluster environment in the dissociative electron attachment process, we develop further the multiple scattering theory for this process and calculate the dissociative attachment cross section for the CF3Cl molecule embedded in the (H2O)6 cluster.

Fabrication of carbon nanomembranes by helium ion beam lithography.

The irradiation-induced cross-linking of aromatic self-assembled monolayers (SAMs) is a universal method for the fabrication of ultrathin carbon nanomembranes (CNMs). Here we demonstrate the cross-linking of aromatic SAMs due to exposure to helium ions. The distinction of cross-linked from non-cross-linked regions in the SAM was facilitated by transferring the irradiated SAM to a new substrate, which allowed for an ex situ observation of the cross-linking process by helium ion microscopy (HIM). In this way, three growth regimes of cross-linked areas were identified: formation of nuclei, one-dimensional (1D) and two-dimensional (2D) growth. The evaluation of the corresponding HIM images revealed the dose-dependent coverage, i.e., the relative monolayer area, whose density of cross-links surpassed a certain threshold value, as a function of the exposure dose. A complete cross-linking of aromatic SAMs by He+ ion irradiation requires an exposure dose of about 850 µC/cm2, which is roughly 60 times smaller than the corresponding electron irradiation dose. Most likely, this is due to the energy distribution of secondary electrons shifted to lower energies, which results in a more efficient dissociative electron attachment (DEA) process.

Electron attachment to molecules studied by electron beam and electron swarm experiments

Relative cross sections obtained in dissociative electron attachment (DEA) reactions from crossed beams experiment are often normalised using absolute rate coefficients obtained from electron swarm experiments. Both methods, however, are carried out under rather different pressure conditions, namely single collision conditions (beam experiments) and in a higher pressure environment (electron swarm experiments). The peculiarities of the DEA process are considered with respect to these different conditions. Results from both methods are compared by means of two case studies, namely electron attachment to CF2Cl2 (swarm, effusive beam, supersonic beam containing clusters) and CClF2COOH (swarm, effusive beam).

Electron scattering studies of nitrogen dioxide

Present work reports total cross sections for e-NO2 scattering over an extensive range of impact energy from 0.5eV to 2000eV, through a composite formalism. The ab initio R-matrix method is employed for calculations up to 15eV while the spherical complex optical potential formalism is used beyond that energy. The electronic excitation cross sections are computed from ground state X2A1 to seven electronically allowed excited states 2B1, 2B2, 2A2, 4A2, 4B2, 4A1, 4A1 and 4B1. The eigenphase diagrams presented here reproduces the resonances identified earlier. The structures detected at 1.33, 3 and 8.41eV can be associated with the observed O− anion formation through dissociative electron attachment process.

Electron-induced chemistry of methyl chloride caged within amorphous solid water

The interaction of low energy electrons (1.0-25 eV) with methyl-chloride (CD3Cl) molecules, caged within Amorphous Solid Water (ASW) films, 10-120 monolayer (ML) thick, has been studied on top of a Ru(0001) substrate under Ultra High Vacuum (UHV) conditions. While exposing the ASW film to 3 eV electrons a static electric field up to 8 × 10(8) V∕m is developed inside the ASW film due to the accumulation of trapped electrons that produce a plate capacitor voltage of exactly 3 V. At the same time while the electrons continuously strike the ASW surface, they are transmitted through the ASW film at currents of ca. 3 × 10(-7) A. These electrons transiently attach to the caged CD3Cl molecules leading to C-Cl bond scission via Dissociative Electron Attachment (DEA) process. The electron induced dissociation cross sections and product formation rate constants at 3.0 eV incident electrons at ASW film thicknesses of 10 ML and 40 ML were derived from model simulations supported by Thermal Programmed Desorption (TPD) experimental data. For 3.0 eV electrons the CD3Cl dissociation cross section is 3.5 × 10(-16) cm(2), regardless of ASW film thickness. TPD measurements reveal that the primary product is deuterated methane (D3CH) and the minor one is deuterated ethane (C2D6).

Dissociative electron attachment to acetaldehyde, CH3CHO. A laboratory study using the velocity map imaging technique

A detailed experimental investigation of the dissociative electron attachment (DEA) process to acetaldehyde, CH(3)CHO is presented. To investigate this process we use a time of flight spectrometer coupled with the velocity slice imaging technique. DEA in CH(3)CHO is found to lead to the formation of CH(3)(-), O(-), OH(-), C(2)H(-), C(2)HO(-) and CH(3)CO(-) anionic products produced through scattering resonances in the electron energy range of 6 to 13 eV. Of these product ions only O(-) is formed with any measurable kinetic energy distribution indicating a two-body dissociation process. CH(3)CO(-), although formed with very low kinetic energy, shows anisotropy in the velocity slice image, indicating ejection of the H atom in the 180° direction with respect to the electron beam. The low kinetic energy distributions and absence of any anisotropy in the angular distributions of the other product ions indicate that they are formed through multiple fragmentation of the transient molecular negative ion. The angular distribution of O(-) is analysed in terms of the various partial waves.

A practical approach to temperature effects in dissociative electron attachment cross sections using local complex potential theory

We propose a practical computational scheme to obtain temperature dependence of dissociative electron attachment cross sections to polyatomic molecules within a local complex potential theory formalism. First we perform quantum path-integral molecular dynamics simulations on the potential energy surface for the neutral molecule in order to sample initial nuclear configurations as well as momenta. Classical trajectories are subsequently integrated on the potential energy surface for the anionic state and survival probabilities are simultaneously calculated along the obtained trajectories. We have applied this simple scheme to dissociative electron attachment processes to H2O and CF3Cl, for which several previous studies are available from both the experimental and theoretical sides.

On the N1-H and N3-H Bond Dissociation in Uracil by Low Energy Electrons: A CASSCF/CASPT2 Study.

The dissociative electron-attachment (DEA) phenomena at the N1-H and N3-H bonds observed experimentally at low energies (<3 eV) in uracil are studied with the CASSCF/CASPT2 methodology. Two valence-bound π(-) and two dissociative σ(-) states of the uracil anionic species, together with the ground state of the neutral molecule, are proven to contribute to the shapes appearing in the experimental DEA cross sections. Conical intersections (CI) between the π(-) and σ(-) are established as the structures which activate the DEA processes. The N1-H and N3-H DEA mechanisms in uracil are described, and experimental observations are interpreted on the basis of two factors: (1) the relative energy of the (U-H)(-) + H fragments obtained after DEA with respect to the ground-state equilibrium structure (S0) of the neutral molecule (threshold for DEA) and (2) the relative energy of the CIs also with respect to S0 (band maxima). The π1(-) state is found to be mainly responsible for the N1-H bond breaking, whereas the π2(-) state is proved to be involved in the cleavage of the N3-H bond.

Chemical control through dissociative electron attachment – A study on pentafluorotoluene, pentafluoroaniline and pentafluorophenol

In a combined experimental and theoretical study on dissociative electron attachment (DEA) to pentafluorotoluene, pentafluoroaniline and pentafluorophenol in the energy range 0–3eV we reveal the role of rearrangement and hydrogen bonded intermediates in the DEA process and show that HF formation can be used to enable otherwise inaccessible, efficient low energy DEA processes.

Gas phase low energy electron induced decomposition of the focused electron beam induced deposition (FEBID) precursor trimethyl (methylcyclopentadienyl) platinum(iv) (MeCpPtMe3)

Relative cross sections for dissociative electron attachment (DEA) and dissociative ionization (DI) of the FEBID precursor, trimethyl (methylcyclopentadienyl) platinum(iv), MeCpPtMe(3), are presented. The most pronounced DEA process is the loss of one methyl radical, while the loss of two or three methyl groups along with hydrogen is the main pathway in DI. Further fragments are formed in DEA and through DI by more complex rearrangement reactions but complete dissociation to bare Pt(-) in DEA or Pt(+) in DI is minor. The transient negative ion (TNI) formation in DEA is discussed and fragmentation mechanisms are proposed for individual processes. From the thermodynamics of the DEA processes we derive a lower limit for the electron affinity of the MeCpPtMe(2) radical (1.7 eV). Appearance energies (AE) of MeCpPtMe(3)(+) (7.7 eV) and Pt(+) (18.6 eV) formation through electron impact ionisation (EI) and through DI, respectively, are determined. Finally, the current DEA and DI results are compared and brought into context with earlier surface science studies on electron-induced decomposition of adsorbed MeCpPtMe(3) as well as gas phase and surface science studies on the FEBID precursors [Co(CO)(3)NO] and [Pt(PF(3))(4)]. These comparisons strongly indicate that DEA is an important process in the electron-induced decomposition of these molecules in FEBID.

Dynamics of the dissociative electron attachment in H2O and D2O: The A1 resonance and axial recoil approximation#

The dynamics of the formation and decay of negative ion resonance of A1 symmetry at 8.5 eV electron energy in the dissociative electron attachment (DEA) process in H2O and D2O are investigated using the velocity slice imaging technique. While the highest energy hydride ions formed by DEA show angular distributions characteristic to the A1 symmetry, those formed with low-kinetic energy show considerably different angular distributions indicating changes in the orientation of the dissociating bond due to bending mode vibrations. Our observations are quite different from the recently reported measurements, but consistent with the fully quantum calculations. Dynamics of the 2A1 negative ion resonance in water is probed using velocity slice imaging of H– from H2O and D– from D2O. While the momentum distributions of the highest energy ions agree with quantum calculations, those of low energy ions indicate the role of bending mode excitations before dissociation.

Complex configuration interaction calculations of the cross section for the dissociative electron attachment process e− + F2 → F 2− → F + F− using the complex basis function method

The F(2)(-) molecule and the corresponding dynamic processes dealing with electron scattering on the neutral F(2) species have been the subject of many theoretical and experimental investigations in the past. In the context of the Born-Oppenheimer approximation, one of the best theoretical descriptions of the electronic states involves the use of complex basis functions together with configuration interaction (CI) methods. In this work, multireference CI calculations using the complex basis function method have been carried out for the autoionizing ground state of the F(2)(-) molecule. Potential curves and vibrational levels have been obtained for the ground and various excited states of both F(2) and F(2)(-), as well as the variation of the line width of the anionic ground state for the bond distance region in which it is metastable. Cross sections for the dissociative electron attachment process e(-) + F(2) → F(2)(-) → F + F(-) have also been computed within the framework of the boomerang model, and good agreement with available experimental data has been found. In addition, some calculations for the process of vibrational excitation are included which also give good agreement with experiment.

Dissociative electron attachment and vibrational excitation of CF3Cl: Effect of two vibrational modes revisited

We present a study of dissociative electron attachment and vibrational excitation processes in electron collisions with the ${\mathrm{CF}}_{3}\mathrm{Cl}$ molecule. The calculations are based on the two-dimensional nuclear dynamics including the C-Cl symmetric stretch coordinate and the ${\mathrm{CF}}_{3}$ symmetric deformation (umbrella) coordinate. The complex potential energy surfaces are calculated using the ab initio $R$-matrix method. The results for dissociative attachment and vibrational excitation of the umbrella mode agree quite well with experiment whereas the cross section for excitation of the C-Cl symmetric stretch vibrations is about a factor-of-three too low in comparison with experimental data.

Observation of the dynamics leading to a conical intersection in dissociative electron attachment to water

Following prior work on the lower-energy resonances, we apply techniques of momentum imaging and ab initio scattering calculations to the process of dissociative electron attachment to water via the highest-energy ${}^{2}{B}_{2}$ resonance. We focus on the H${}^{\ensuremath{-}}$ anion fragment, which is produced via dynamics passing through and avoiding the conical intersection with the lower ${A}_{1}$ state, leading to OH (${}^{2}\ensuremath{\Pi}$) and OH (${}^{2}\ensuremath{\Sigma}$), respectively. The momentum imaging technique, when combined with theoretical calculations on the attachment amplitude and dissociation dynamics, demonstrates that the angular distributions provide a signature of the location of the conical intersection in the space of nuclear configurations.

Low-energy electron collisions with thioformaldehyde

The elastic integral, differential, and momentum transfer cross sections and the excitation cross sections from the ground state to the first three low-lying electron excited states for low-energy (0--10 eV) electron collisions with thioformaldehyde (H${}_{2}$CS) molecule are calculated using the $R$-matrix method. Eighteen target states are included in the close-coupling formalism. Three core-excited shape resonant states (${}^{2}$${B}_{2}$,${}^{2}$${B}_{1}$,${}^{2}$${A}_{1}$) and three Feshbach resonant states (${}^{2}$${B}_{2}$,${}^{2}$${A}_{1}$,${}^{2}$${B}_{1}$) are determined for the ($N$ + 1)-electron H${}_{2}$CS${}^{\ensuremath{-}}$ system in the low-energy range. The dissociative electron attachment processes at the first two low-lying core-excited shape resonant states are proven by performing a series of scattering calculations, indicating the S${}^{\ensuremath{-}}$(${}^{2}P$) yields together with the CH${}_{2}$ fragment at three different states.

Negative ion formation through dissociative electron attachment to GeH4: Comparative studies with CH4 and SiH4

Negative ion formation by low-energy electron impact to germane (GeH4) has been performed in an electron energy region from 6 to 11eV with an energy resolution of ∼500meV. Anion efficiency curves of four anions have been measured. Product anions are observed mainly in the 6–11eV energy region, yielding GeHx− (x=0–3). Comparative studies with methane (CH4) and silane (SiH4) are also presented with the most intense signals observed at 14amu (CH2−), 31amu (SiH3−) and 75amu (GeH3−) from CH4, SiH4 and GeH4, respectively. Fragmentation into these negative ions is attributed to resonant dissociative electron attachment processes.