P. Cicman

Dissociative electron attachment to gas phase glycine: Exploring the decomposition pathways by mass separation of isobaric fragment anions

Dissociative electron attachment to gas phase glycine generates a number of fragment ions, among them ions observed at the mass numbers 15, 16 and 26 amu. From stoichiometry they can be assigned to the chemically rather different species NH(-)/CH(3)(-)(15 amu), O(-)/NH(2)(-)(16 amu) and CN(-)/C(2)H(2)(-)(26 amu). Here we use a high resolution double focusing two sector mass spectrometer to separate these isobaric ions. It is thereby possible to unravel the decomposition reactions of the different transient negative ions formed upon resonant electron attachment to neutral glycine in the energy range 0-15 eV. We find that within the isobaric ion pairs, the individual components generally arise from resonances located at substantial different energies. The corresponding unimolecular decompositions involve complex reaction sequences including multiple bond cleavages and substantial rearrangement in the precursor ion. To support the interpretation and assignments we also use (13)C labelling of glycine at the carboxylic group.

Dissociative electron attachment to CF2Cl2

Using a recently constructed high resolution crossed electron/molecular beam apparatus consisting of a hemispherical electron monochromator and a quadrupole mass spectrometer we have measured the relative production cross sections for CI– and F– via electron attachment to CF2Cl2. The relative Cl– cross section is placed on an absolute scale by reference to an absolute rate coefficient using a calibration method involving integration of the measured anion signal. The most efficient Cl– production process is at about zero energy and its magnitude is resolution limited. The present high resolution value of 6 × 10−16 cm2 compares well with an earlier value reported by Chen and Chantry. A second peak is detected at around 0.8 eV in accordance with some of the earlier beam and swarm measurements. The observed production of F– has an appearance energy of 1.9 eV and the energy of maximum cross section is 3.36 eV, the latter value comparing well with several previous studies.

Low-energy electron attachment to mixed ozone/oxygen clusters

Abstract Electron attachment to a cluster beam formed by adiabatic expansion of a mixture of O 3 (1%) and O 2 (99%) is studied in the energy range 0–4 eV. Despite the initial large excess of oxygen molecules, the dominant attachment products are undissociated cluster ions (O 3 ) m − including the monomer O 3 − , while oxygen cluster ions (O 2 ) n − appear with comparatively low intensity. This behaviour is explained by an enrichment of ozone in the cluster formation process and the preferential formation of O 3 − from mixed clusters. The structured energy dependence of the cross section of O 3 − formation is interpreted in terms of three different mechanisms, in the low-energy region by s-wave capture, around 1 eV via Feshbach resonances, and above 1.5 eV by self-scavenging, i.e. inelastic scattering of the primary electron involving low-lying electronic states of neutral ozone and subsequent attachment of the slowed-down electron to another ozone molecule in the same cluster.

The ion extraction efficiency in a crossed beams experiment using a trochoidal electron monochromator

Abstract The ion extraction efficiency from a trochoidal electron monochromator was investigated for the same set-up as used in previous experiments. Using the Simion program the ion trajectories in the monochromator were simulated and the ratio of the extracted ions to all ions produced was estimated. The extraction efficiency was found to change very dramatically even at low ion energies, thus, deforming measured attachment cross sections. The results of the simulation were then applied to previously measured cross-section data for dissociative electron attachment to ozone. The corrected data showed very good agreement with the experiments of other authors.

Electron attachment to molecules and clusters of atmospheric relevance: oxygen and ozone

Highly monochromatized electrons are used in a crossed beams experiment to investigate electron attachment to oxygen clusters at electron energies from approximately zero up to 2 eV. At energies close to zero the attachment cross section for the reaction varies inversely with the electron energy, indicative of s-wave electron capture to . Peaks in the attachment cross section present at higher energies can be ascribed to vibrational levels of the oxygen anion. The vibrational spacings observed can be quantitatively accounted for. In addition, electron attachment to ozone and mixed oxygen/ozone clusters has been studied in the energy range up to 4 eV. Absolute attachment cross sections for both fragment ions, and , from ozone could be deduced. Moreover, despite the initially large excess of oxygen molecules in the neutral oxygen/ozone clusters the dominant attachment products are undissociated clusters ions including the monomer while oxygen ions appear with comparatively low intensity.

Electron impact ionization of CHF2Cl: Unusual ordering of ionization energies for parent and fragment ions

Electron impact ionization of the chlorodifluoromethane molecule is studied using crossed beams of high-resolution electrons and an effusive molecular beam of CHF2Cl. Ionization energies (IEs) for many positive ions from CHF2Cl (CHF2Cl+,CF2Cl+,CHFCl+,CFCl+,CHF2+,CF2+,HCl+,Cl+,CF+,CH+,F+ C+) are determined from a careful examination of the threshold behavior of the ionization cross sections. Reaction pathways for the dissociative ionization products are suggested using known thermodynamic quantities. Surprisingly, it is observed that the ionization threshold for the parent positive ion IE(CHF2Cl+/CHF2Cl)=12.50(±0.05) eV lies above that for the fragment ions CHF2+, CHFCl+, and CF+ [IE(CHF2+/CHF2Cl)=12.24(±0.03) eV, IE(CHFCl+/CHF2Cl)=12.3(±0.05) eV, and IE(CF+/CHF2Cl)=11.5(±0.1) eV]. Experiments using a three sector field BEE mass spectrometer provide evidence for the existence of two states of the parent CHF2Cl+ ion, one exhibiting a short lifetime of about 2 μs and another parent ion state which appears to...

Dissociative electron attachment to CO2

The 4.4 eV dissociative electron attachment peak in CO2 was reinvestigated paying particular attention to (i) its structure associated with vibrational excitation and to (ii) the temperature dependence of the onset. For this purpose we have used two specially designed crossed beams machines, one having a trochoidal electron analyzer (TEM) and one with a hemispherical electron analyzer (HEM) for the production of the highly monochromatized electron beams (with FWHM’s down to 5 and 50 meV, respectively). The present results confirm the earlier findings in (i) interpreting the dominant structures of the 4.4 eV peak as being due to vibrationally excited states of CO in the reaction CO2+e → O−+CO and (ii) assigning the much weaker and narrower structures to the intermediate CO2−. In the temperature range between 300 and 245 K almost no temperature dependence of the onset can be seen in the present study. In comparison to NO and CO where the onset is vertical the CO2 threshold behavior is less steep indicating that in the CO2 case a different type of transition must be responsible for the onset of the O− production. Besides DA to CO2 we have for comparison and calibration purposes investigated also DA to CO and NO.

The Rate Constant for Dissociative Electron Attachment to Ozone Revisited

The rate constant for dissociative electron attachment to ozone has been derived over the energy range from about 0 to 10 eV using recently measured and also corrected cross section data. The new rate constant data sets for two partial dissociative channels, as well as for the total dissociative electron attachment, are compared with previously reported values, and existing discrepancies are discussed.

Dissociative electron attachment to dinitrogen pentoxide, N2O5

Electron attachment was studied in gaseous dinitrogen pentoxide, N(2)O(5), for incident electron energies between a few meV and 10 eV. No stable parent anion N(2)O(5) (-) was observed but several anionic fragments (NO(3) (-), NO(2) (-), NO(-), O(-), and O(2) (-)) were detected using quadrupole mass spectrometry. Many of these dissociative pathways were found to be coupled and provide detailed information on the dynamics of N(2)O(5) fragmentation. Estimates of the cross sections for production of each of the anionic fragments were made and suggest that electron attachment to N(2)O(5) is amongst the most efficient attachment reactions recorded for nonhalogenated polyatomic systems.

Dissociative electron attachment to CHF2Cl

Abstract We present results of a crossed electron/molecule beams study concerning dissociative electron attachment (DEA) to CHF 2 Cl. This study was performed in the electron energy range from about 0–10 eV and at a gas temperature of 300 K. Four fragment negative ions (Cl − , F − , CClF − and CF 2 − ) were observed. The following partial absolute cross-sections for DEA to CHF 2 Cl were obtained, 2×10 −19 cm 2 at the 1.1 eV resonance for Cl − , 3×10 −20 cm 2 at the 3.4 eV resonance for F − , ≈2×10 −21 at the 3.0 eV resonance for CClF − and ≈3×10 −21 at the 8.6 eV resonance for CF 2 − . Using the cross-section functions obtained, total rate coefficients for DEA to CHF 2 Cl were calculated giving a value of 9.0×10 −13 cm 3 s −1 at room temperature. Present results are compared with previous data allowing thus to clarify existing discrepancies.