Dynamics of low-energy electron attachment to formic acid

Low-energy electrons (<2 eV) can fragment gas phase formic acid (HCOOH) molecules through resonant dissociative attachment processes. Recent experiments have shown that the principal reaction products of such collisions are formate ions (HCOO-) and hydrogen atoms. Using first-principles electron scattering calculations, we have identified the responsible negative ion state as a transient pi* anion. Symmetry considerations dictate that the associated dissociation dynamics are intrinsically polyatomic: a second anion surface, connected to the first by a conical intersection, is involved in the dynamics and the transient anion must necessarily deform to nonplanar geometries before it can dissociate to the observed stable products.     

Broadening of energy spectra of molecular ion fragments produced by surface small angle scattering

Relative half-widths of energy spectra of molecular hydrogen ions and their fragments scattered by a tungsten surface have been experimentally studied for various primary molecules and their isotopic analogues in the energy range of 100–4000 eV per nucleon. The spectra of fragments turned out to be broader than the spectra of the same particles that are not fragmentation products. The broadening is interpreted within the “explosion” model, i.e., dissociation of a molecular ion due to the change in the shape of the interaction potential of the fragments. The model of “explosive” dissociation of molecules agrees with the data obtained assuming an effective repulsion energy of 1.1 ± 0.1 eV for all molecules.     

Role of secondary low-energy electrons in the concomitant chemoradiation therapy of cancer

Solid films of DNA with and without the chemotherapeutic agent cisplatin bonded to guanine were bombarded with electrons of 1, 10, 100, and 60,000 eV causing single and double strand breaks. In the presence of cisplatin this damage was increased by factors varying from 1.3 to 4.4 owing to an increase in bond dissociation triggered by the formation of transient anions. This mechanism may lie at the basis of the efficiency of concomitant cisplatin-radiation therapy.     

Stabilizing dicyanoacetylene anions in planetary atmospheres: quantum dynamics of its transient negative ions

Quantum computations that follow the electron-attachment process at low energies (<10 eV) to the NCCCCN gaseous molecule are carried out in order to understand the role of transient negative ions (TNIs) which act as “doorway states" to molecular stabilization and/or fragmentation after resonant attachment of an environmental electron. The computed behaviour of the found TNIs suggests that an NCCCCN^*- intermediate could be formed under conditions which justify and explain the existence of stable carbonaceous anions in the interstellar medium, while further anionic fragments of the title molecule, already seen in laboratory experiments, are also identified by the present calculations.     

A detailed study on the decomposition pathways of the amino acid valine upon dissociative electron attachment

In the present study we investigate free electron attachment to the amino acid valine. Mass spectra and anion efficiency curves are measured in the electron energy range from about zero eV to about 15 eV and the anionic fragments are analyzed with a double focusing mass spectrometer. The high sensitivity of the present setup allows the detection of 10 fragment anions that have not been reported before and the high mass resolution of our sector field mass spectrometer allows us the separation and identification of isobaric anions. Thus the isobaric ion pairs, CN^-/C₂H₂^-_2^-, and O^-/NH₂^-_2^-, can be identified and assigned to individual resonances. For some of the heavier fragment anions formed we have studied collision induced dissociation to collect more information on the structures of these anions.     

Ionisation energy,electron affinity,and mass spectral decomposition mechanisms of RDX isomers upon electron attachment and electron ionisation

ABSTRACT

The structures, ionisation energies (IE), and electron affinities (EA) of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) isomers upon loss and gain of an electron were calculated using density functional theory (DFT) methods. The adiabatic electron affinities (EA_ad) range from 1 to 2 eV. The vertical detachment energies are between 1.3 and 4.0 eV. The adiabatic ionisation energies (IE_ad) are in the 9.9–10.2 eV range. The vertical ionisation energies are in the 10.4–10.9 eV range. It is shown that NO₂^?/NO₂ loss would be common in anions and cations, respectively. Isomerisation and N—N bond dissociation accompany cation and anion formation, respectively. The suggested mass spectral fragmentation products for the cations along the S₀ surface are 84, 130, and 176 amu, in agreement with earlier mass spectrometry studies.     

DNA damage induced by low-energy electrons: electron transfer and diffraction

Thin films of the short single strand of DNA, GCAT, in which guanine (G) or adenine (A) have been removed, were bombarded under vacuum by 4 to 15 eV electrons. The fragments corresponding to base release and strand breaks (SB) were analyzed by high performance liquid chromatography and their yields compared with those obtained from unmodified GCAT. From such a comparison, it is shown that, using GCAT as a model system, (1) most SB result from electron capture by DNA bases followed by electron transfer to the phosphate group and (2) the initial capture probability depends on the coherence of the electron wave within the tetramer.     

Modeling the dissociation and ionization of a sputtered organic molecule

The evolution of an organic molecule after sputtering from a gold surface has been analyzed by classical molecular dynamics and ab initio calculations to gain insight into the ionization and fragmentation processes occurring in SIMS. The calculated ionization potential (6.2 eV) of the tetraphenylnaphthalene (TPN) molecule has been found to be close to the unimolecular dissociation energy (5.4 eV) of the most favorable reaction channel involving the loss of a phenyl ring. On the other hand, our calculations show that the internal energies of sputtered TPN molecules can be significantly larger than 5-6 eV. Therefore, it appears energetically possible to relax such excited molecules via both fragmentation and ionization.We propose to virtually decompose the TPN molecule into its basic fragments. The rationale is that, if the molecule is very excited, then separate parts (e.g. pendant phenyl rings) can interact with each other almost independently. The analysis of the molecular motion after emission shows that the oscillations along the phenyl-naphthalene bond direction, expected to induce the molecule fragmentation by the loss of a phenyl ring, are relatively small (they store only about 0.2 eV). On the other hand, the relative energy stored in the inter-phenyl interactions, modulated by their bending and responsible for ionization according to our hypothesis, oscillates over a range of 6-7 eV and favors ionization.     

Resonance capture of electrons by electroactive organic molecules

Bound states of electrons with electroactive aromatic molecules, promising for use in molecular electronics, are studied employing electron attachment spectroscopy Anions are produced in the gas phase through the capture of electrons with energies of up to 15 eV by molecules via the resonance mechanism. The possible pathways of fragmentation of molecular anions and the times of electron retention by the test molecules at various incident electron energies are measured. The resonance states identified are interpreted with the help of quantum-chemical calculations. The possibility of applying the results obtained to molecular electronics is discussed.     

紫外光诱导光电子发射负离子源的阈值光电子成像仪设计 (cited: 2)

设计了一套紧凑的光电子成像装置,它包括解离式光电子贴附负离子源、垂直安装的高分辨阈值光电子速度成像装置和线性飞行时间质谱仪．紫外光辐射金属表面诱导低能光电子发射,再通过低能电子贴附超声分子束产生高强度和冷的负离子源．结合这种负离子源和飞行时间质谱-光电子成像仪装置,仪器的质量分辨能达到200左右,能量分辨优于3%(即对1 eV动能的电子,分辨达到30 meV)．此外,使用该实验装置获得了CH₃S^-和S₂^-在611.46 nm下的低能阈值光电子成像结果．同时得到了CH₃S和S₂的更精确的电子亲和势分别为1.8626±0.0020和1.6744±0.0035 eV．初步的结果证明了该装置对研究阈值光电子成像精确测量光电子亲和势非常有效     

Dissociative electron attachment to DNA

Electron-stimulated desorption of anions from thin films of linear and supercoiled DNA is investigated in the range 3-20 eV. Resonant structures are observed with maxima at 9.4+/-0.3, 9.2+/-0.3, and 9.2+/-0.3 eV, respectively, in the yield dependence of H-, O-, and OH- on the incident electron energy. Their formation is attributed to dissociative electron attachment.     

The role of excitons and substrate temperature in low-energy (5–50 eV) electron-stimulated dissociation of amorphous D2O ice

We have studied the interaction of low-energy (5–50 eV) electrons with nanoscale (10 ML) ice films by probing the yields and quantum-state distributions of the neutral dissociation products using laser resonance-enhanced multiphoton ionization spectroscopy. In particular, we have observed the electron-stimulated desorption (ESD) of D (²S), O (³P₂) and O (¹D₂) from amorphous D₂O films. These products are observed at threshold energies (relative to the vacuum level) between 6.5–7 eV and desorb with low kinetic energies (60–85 meV) which are independent of the incident electron energy. We associate the ESD of atomic fragments from ice with dissociation of Frenkel-type excitons of 4a₁ character which are near the bottom of the ice conduction band. These excitons are created either directly or via electron-ion recombination. Changing the surface temperature from 88 to 145 K results in an increase in the thermal component of the time-of-flight (kinetic energy) distributions and an overall increase in the neutral fragment yield. We suggest that the change in neutral yield with substrate temperature results from a combination of: (1) increased electron-ion recombination; (2) exciton transport to the near-surface region; and (3) dissociation followed by inelastic scattering and desorption.     

Ionization of nitrogen,oxygen, water,and carbon dioxide molecules by near-threshold electron impact

An experimental technique for measuring the cross sections of direct and dissociative ionization of N₂, O₂, H₂O, and CO₂ molecules by electron impact in the near-threshold energy range is described. The setup used in the experiments allows mass separation of ions with a monopole mass spectrometer. It is shown that such a setup can be used to advantage in separation experiments. For incident electron energies between 7 and 35 eV, the energy dependences of the cross sections of generation of parent ions and ion fragments due to parent molecule dissociation are obtained.     

Dissociative electron attachment to the explosive detection taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB)

A detailed study on dissociative electron attachment (DEA) to 2,3-dimethyl-2,3-dinitrobutane (DMNB) in the gas phase is presented. Ion yields as a function of the incident electron energy from about 0 to 14 eV have been measured for the most dominant fragments including anions such as NO₂ ⁻, [M-NO₂]⁻ or N₂O₄ ⁻. To help identifying which anion and neutral fragments are formed upon electron attachment we calculated the thermodynamic thresholds using the G4(MP2) method.     

Collision-induced dissociation of oligonucleotide anions fully modified at the 2'-position of the ribose: 2'-F/-H and 2'-F/-H/-OMe mix-mers

Gas-phase dissociation of various 2'-position modified oligonucleotide anions has been studied as a function of precursor ion charge state using ion trap and low energy beam-type collision-induced dissociation (CID). For a completely 2'-O-methyl modified 6-mer, all possible dissociation channels along the phosphodiester linkage, generating complementary (a-B)/w-, b/x-, c/y-, d/z-ion series, were observed with no single dominant type of dissociation pathway. Full sequence information was generated from each charge state via ion trap CID. More sequential fragmentation was noted under beam-type CID conditions. Comparison with model DNA, in which all 2'-OH groups are converted to 2'-H, and RNA anions suggests that the 2'-OMe substitution stabilizes the phosphodiester linkage with respect to fragmentation relative to both DNA and RNA oligomers. For modified mix-mer anions, comprised of DNA nucleotides and 2'-F substituted nucleotides or a mixture of DNA nucleotides and 2'-O-methyl (2'-OMe) and 2'-F substituted nucleotides, 3'-side backbone cleavage was found to be inhibited by the 2'-OMe or 2'-F modification on the nucleotides under ion trap CID conditions. Thus, the sequence information was limited to the a-Base/w-fragments from the cleavage of the 3' C-O bond of the 2'-H (DNA) nucleotides. Under beam-type CID conditions, limited additional cleavage adjacent to 2'-OMe substituted nucleotides was noted but 2'-F modified residues remained resistant to cleavage.     

Negative ion photoelectron spectroscopy of 2,2’-bithiophene cluster anions, (2T)n - (n = 1–100)

Cluster anions of 2,2’-bithiophene, (2T)_n^-, were produced up to n ∼500 in the gas-phase. The energetics of the excess electron in the (2T)_n^- clusters with n =1-100 were explored by negative ion photoelectron spectroscopy. When the vertical detachment energies (VDEs) obtained from the photoelectron spectra were analyzed by a plot against n^-1/3, it has been revealed that the excess electron trapping level thus extrapolated is located at ∼0.8 eV below the conduction band minimum (i.e. LUMO) of the 2T thin film. The large slope of the VDEs vs. n^-1/3 plot suggests that the neutral 2T molecules surrounding the anion core take non-planar twisted conformations with permanent dipole moments, resulting in the exceedingly deep trapping of the excess electron in the 2T cluster anions.     

Experiments on clusters

In this contribution we discuss four different types of experiments that have been conducted at molecular beams of neutral clusters. The size of particularly stable sodium chloride clusters and their corresponding geometrical structure is inferred from intensity anomalies in mass spectra. This information is obtained either for charged or for neutral clusters depending on whether the clusters are ionized by electron impact or by multiphoton absorption. The important role of fragmentation in mass spectrometry of xenon clusters is revealed by multiphoton ionization; dissociative reactions occurring on the time scale of 10⁻⁷ s with respect to the ionizing event can be analyzed. The solvation energy of negatively charged carbon dioxide clusters as a function of cluster size is obtained from electron attachment spectra. A resonance in the ion yield close to zero eV electron energy signifies that all clusters except for the monomer feature a positive electron affinity. An analysis of the kinetic energy of fragment ions, originating from delayed dissociation of triply charged carbon dioxide clusters, reveals that the size distribution of their fission fragments is extremely symmetric.     

Reactions and anion desorption induced by low-energy electron exposure of condensed acetonitrile

Thermal desorption spectrometry (TDS) and electron stimulated desorption (ESD) are employed to investigate mechanisms responsible for the formation of C₂H₆ in electron irradiated multilayer films of acetonitrile (CH₃CN) at 30 K. Using a high sensitivity time-of-flight mass spectrometer, we observe the ESD of anionic fragments H⁻, CH₂ ⁻, CH₃ ⁻ and CN⁻. Desorption occurs following dissociative electron attachment (DEA) via several negative ion resonances in the 6 to 14 eV energy range and correlates well with a “resonant” structure seen in the TDS yield of C₂H₆ (i.e., at mass 30 amu). It is proposed that C₂H₆ is formed by the reactions of CH₃ radicals generated following DEA to CH₃CN which also yields CN⁻. Between 2 and 5 eV, a second resonant feature is seen in the C₂H₆ signal. While DEA is observed in the gas phase at these energies, no anion desorption occurs since anionic fragments likely have insufficient kinetic energy to desorb. Since the CH₂ ⁻ ion has not been observed in gas-phase measurements, we propose that it is formed, along with HCN (that is detected in TDS) when dissociation into CH₃ ⁻ and CN is hindered by adjacent molecules.     

Spectroscopic states of PTCDA negative ions and their relation to the maxima of unoccupied state density in the conduction band

Electron attachment spectroscopy is employed to demonstrate that the scattering of slow (0–15 eV) electrons from perylenetetracarboxilic dianhydride (PTCDA) molecules in the gas phase leads to the resonant formation of molecular and fragment negative ions detected in the mass-spectrometric experiment. Depending on the electron energy, currents of anions have clearly manifested peaks at 0.14, 1.9, 3.0, 4.8, and 5.7 eV. In addition, resonant states are also detected at thermal energy (0 eV) of scattered electrons, as well as at 0.4 and 1.0 eV, as shoulders on experimental curves. The spectroscopic states of PTCDA anions at energies exceeding 0 eV are interpreted in terms of the formation of shape resonances on the basis of calculated values of energies of π*-type unoccupied molecular orbitals. It is found that the positions of unoccupied orbitals of an isolated PTCDA molecule correspond to the peaks in the density of states of the conduction band of PTCDA films provided that the energies of the orbitals are shifted by 1.4 eV. The latter value can be interpreted as the binding energy of a molecule in the film due to the polarization interaction with the surroundings.     

Multiple ionization and fragmentation of the DNA base thymine by interaction with C q+ ions

Ionization and fragmentation of the DNA base thymine upon interaction with keV C^q+ ions (q = 1 - 6) has been studied. By means of time-of-flight spectrometry of two or more thymine fragments in coincidence with an ejected electron we could investigate particular dissociation channels by means of their associated kinetic-energy-release. The fragmentation dynamics are strongly influenced by the C^q+ charge state: for low q values mainly fragmentation due to direct collisions is observed. With increasing q, electron capture becomes more important. For larger q we could identify several Coulomb explosion channels, leading to very energetic fragments.