A. Stamatovic

Dissociative electron attachment cross section to CHCl3 using a high resolution crossed beams technique

Using a crossed electron-molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CHCl3+e→Cl− at electron energies from about 0 to 2 eV and in a target gas temperature range of about 300–430 K. The energy resolution and working conditions of this newly constructed crossed beams machine have been characterized using CCl4 as a test and calbrant gas. Utilizing the improved energy resolution of the present experimental setup (which allows measurements with FWHM energy spreads down to below 5 meV) it was possible to determine the accurate shape and magnitude of the cross section function in the low-energy range. This leads to the conclusion that between an electron energy of about 20 and 130 meV the reaction proceeds via deBroglie s-wave capture, whereas at higher energy (above about 0.4 eV) autodetachment plays a significant role. Moreover, the present measurements allow us to clarify previously reported d...

Partial cross sections for positive and negative ion formation following electron impact on uracil

We report absolute partial cross sections for the formation of selected positive and negative ions resulting from electron interactions with uracil. Absolute calibration of the measured partial cross sections for the formation of the three most intense positive ions, the parent C4H4N2O+2 ion and the C3H3NO+ and OCN+ fragment ions, was achieved by normalization of the total single uracil ionization cross section (obtained as the sum of all measured partial single ionization cross sections) to a calculated cross section based on the semi-classical Deutsch–Mark formalism at 100 eV. Subsequently, we used the OCN+ cross section in conjunction with the known sensitivity ratio for positive and negative ion detection in our apparatus (obtained from the well-known cross sections for SF+4 and SF−4 formation from SF6) to determine the dissociative attachment cross section for OCN− formation from uracil. This cross section was found to be roughly an order of magnitude smaller, about 5 × 10−22 m2 at 6.5 eV, compared to our previously reported preliminary value. We attribute this discrepancy to the difficult determination of the uracil target density in the earlier work. Using a reliably calculated cross section for normalization purposes avoids this complication.

Absolute partial and total cross‐section functions for the electron impact ionization of C60 and C70

Electron impact ionization of C60 and C70 was studied using a molecular/electron beam ion source in combination with a two sector field mass spectrometer operated in the ion beam deflection mode. Relative partial ionization cross sections for the production of singly and multiply charged parent ions (up to charge state z=4) and fragment ions (down to C+44 in the case of C60 and down to C2+50 in the case of C70) were determined from threshold up to 1000 eV electron energy. Absolute partial and total ionization cross sections are obtained using a novel approach for the absolute calibration involving an intercomparison of the cation with the anion yield. The results obtained reveal not only an anomalous large parent ion cross section as compared to other ionization channels [e.g., σ(C+60/C60) is more than a factor of 30 larger than σ(C+58/C60)] but also anomalies for the production of multiply charged parent and fragment ions. For instance, the maximum cross section for the formation of C2+60 amounts to 30% ...

Electron-impact ionization of helium clusters close to the threshold: appearance energies.

We have investigated the ionization threshold behavior of small helium cluster ions (cluster size n=2-10) formed via electron-impact ionization of neutral helium droplets and derive appearance energies for mass-selected cluster ions using a nonlinear least-square-fitting procedure. Moreover, we report magic numbers in the mass spectrum observed at the electron energy of 70 eV. The apparatus used for the present measurements is a hemispherical electron monochromator combined with a quadrupole mass spectrometer. Our experiment demonstrates that helium clusters are not only exclusively formed via direct ionization above the atomic ionization potential but also indirectly via autoionizing Rydberg states. The present results are compared with previous electron-impact and photoionization results.

A crossed beam high resolution study of dissociative electron attachment to CCl4

Abstract Using a crossed electron/molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CCl4 + e → Cl− + CCl3 (at electron energies between threshold and 2 eV) in a target gas temperature range of 300–500 K. With the available high energy resolution of about 11 meV FWHM from an improved trochoidal electron monochromator it was possible to investigate in a single collision crossed beam experiment the true electron energy dependence of the cross-section of this s-wave electron capture process down to the limiting E−1/2 behavior. At energies above approximately 20 meV the cross-section decreases more strongly towards higher energies, exhibiting around 100 meV the typical E−1 behavior predicted for the s-wave de Broglie cross-section. Additional structure above 200 meV is interpreted in terms of autodetachment and a peak at around 0.8 eV (with a peak height of approximately 0.3% of that of the zero energy peak) is attributed to the triply degenerate 2T2 excited state resonance. No temperature dependence of the measured cross-section function could be observed in the investigated temperature range of 300–500 K.

Dissociative electron attachment to using a high-resolution crossed-beams technique

Using a crossed electron - molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process (at electron energies from about 0 - 2 eV and with an energy resolution of about 60 meV) in a target gas range of about 300 - 420 K. Utilizing the improved energy resolution of the present experimental set-up it was possible to determine the accurate shape and magnitude of the cross section function in this low-energy range. This leads to the conclusion that close to 0 eV energy (below about 0.15 eV) the reaction proceeds via s-wave capture, whereas at higher energy (above 1 eV) autodetachment significantly determines the shape of the cross section. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, in the number of peaks and in the energy positions of these peaks. Finally, by analysing the measured strong temperature dependence of the cross section peak at about zero electron energy the activation barrier for this dissociative attachment was determined to be in good agreement with thermochemical data deduced from swarm experiments.

Formation of SF5− in electron attachment to SF6; swarm and beam results reconciled

Abstract Based on the results of recent swarm experiments, it has been proposed that the increase in the cross section for SF 5 − formation observed at an electron energy, E e of about 0.3 eV in electron beam studies of electron attachment to SF 6 is due to the combined (opposing) effects of the vibrational heating of the molecule by the attached electron, which enhances the dissociation of the nascent (SF 6 − ) ∗ ion, and the reduction of the cross section for capture (s-wave) of the electron by SF 6 with increasing E e . Further, it has been shown that the dissociation reaction is endothermic by 0.12 eV, and that, contrary to previous suggestions, there is no potential barrier to this dissociation reaction. Now we have carried out electron beam studies of the SF 6 attachment reaction in Berlin at gas temperatures, T g , over the range 300 to 920 K and in Innsbruck at T g below 300 K. These studies have provided support for the above proposals concerning the appearance of the SF 5 − peak and for a reaction endothermicity of 0.12 eV. Thus these studies have clarified the doubts about the products of the SF 6 attachment reaction at low electron energy.

Dynamics and kinetics of the metastable decay series: Ar3+*→Ar2+*→Ar+

Metastable decay of Ar+*3 produced by electron impact ionization of a supersonic Ar cluster beam has been investigated qualitatively and quantitatively with a double focusing sector field mass spectrometer. We present direct experimental proof that the observed metastable decay of Ar+*3 into Ar+ in the microsecond time regime proceeds predominantly via a sequential decay series Ar+*3→k3,2Ar+*2→ k2,1Ar+ involving the evaporation of a single monomer in each of these successive decays. The metastable decay rates deduced are very different, i.e., k3,2 =45 s−1 and k2,1 =680 s−1 at an ion acceleration voltage of 3 kV. Moreover, the metastable decay rates of Ar+*3 exhibit a strong dependence on time elapsed since electron impact production of the Ar+3, and on ion production parameters (i.e., size and internal excitation energy of the neutral precursor). For instance photodissociation (with visible laser light) of the neutral cluster beam leads to a dramatic increase of the observed metastable reaction rules yiel...

Formation of SO−2, SO2·O− and SO2·SO− by electron attachment to van der waals SO2 clusters

Abstract SO − 2 , SO 2 ·O − and SO 2 ·SO − are produced by electron attachment to SO 2 clusters under single collision conditions. SO − 2 , which is not produced at all in low-pressure ion sources, is more abundant than SO 2 ·O − and SO 2 ·SO − . Measured relative attachment cross sections for these ions show significant differences when compared to O − and SO − produced by attachment to SO 2 , i.e. the first resonance is very efficiently quenched in favor of SO − 2 production. This is in contrast to previous findings in O 2 , CO 2 and N 2 O.

Negative ion formation from low energy (0–15 eV) electron impact to CF2Cl2 under different phase conditions

Negative ion formation following low energy (0–10 eV) electron attachment to free and bound CF2Cl2 molecules is studied in (1) a molecular beam experiment (single molecules, homogeneous clusters, and mixed CF2Cl2/NH3 clusters) and (2) a UHV surface experiment where desorption of negative ions from condensed CF2Cl2 is observed. From single gas phase CF2Cl2 molecules we observe Cl− and F− generated via dissociative electron attachment from a resonance near 0 eV and 3 eV, respectively, as the most abundant ions. From homogeneous clusters (CF2Cl2)n, we additionally detect undissociated complexes of the form (M)n−(M=CF2Cl2) including the stabilized monomer CF2Cl2− and also “solvated fragment ions” of the form Mn⋅X−(X=Cl, F). Their relative abundance vs size (n) of the final product varies in a significant different way between (M)n− and Mn⋅X− reflecting the different relaxation probabilities in the initial cluster. In the desorption spectra, the dominant low energy Cl− gas phase resonance is strongly suppresse...