B. Gstir

Electron attachment to gas-phase uracil.

We present results about dissociative electron attachment (DEA) to gas-phase uracil (U) for incident electron energies between 0 and 14 eV using a crossed electron/molecule beam apparatus. The most abundant negative ion formed via DEA is (U-H)-, where the resonance with the highest intensity appears at 1.01 eV. The anion yield of (U-H)- shows a number of peaks, which can be explained in part as being due to the formation of different (U-H)- isomers. Our results are compared with high level ab initio calculations using the G2MP2 method. There was no measurable amount of a parent ion U-. We also report the occurrence of 12 other fragments produced by dissociative electron attachment to uracil but with lower cross sections than (U-H)-. In addition we observed a parasitic contaminating process for conditions where uracil was introduced simultaneously with calibrant gases SF6 and CCl4 that leads to a sharp peak in the (U-H)- cross section close to 0 eV. For (U-H)- and all other fragments we determined rough measures for the absolute partial cross section yielding in the case of (U-H)- a peak value of sigma (at 1.01 eV)=3 x 10(-20) m2.

Electron attachment to 5-chloro uracil

Electron attachment (EA) and dissociative electron attachment (DEA) to 5-chloro uracil (5-ClU) was studied in the gas phase using a crossed electron/molecule beams technique. Besides production of a parent anion via a zero energy resonance, ion yields of nine different negative ions were observed in the electron energy range from about 0 to 14 eV. In the electron energy range from about zero to 5 eV, the formation of a transient negative ion was induced by electron attachment to the π* resonances located at about 0.24, 1.5, and 3.6 eV leading subsequently by unimolecular decay to various negative fragment ions. Absolute partial cross sections for EA and DEA to 5-ClU were obtained from the measured ion yields using a simple calibrating method. The dominant negative ion observed in the present experiment was (C4H2N2O2)− (corresponding to 5-ClU minus HCl) with a mass to charge ratio of 110, followed by Cl− ion (mass to charge ratios 35 and 37), the partial cross sections being σ(0.23 eV)=5×10−18 m2 and σ(0.2...

Isotope effects in the electron impact ionization of H2/D2,H2O/D2O, and C6H6/C6D6 near threshold

Appearance energies of all parent ions and several fragment ions produced by electron impact ionization of the isotope systems H2/D2, H2O/D2O, and C6H6/C6D6 were determined with high precision using a dedicated high-resolution electron impact ionization mass spectrometer. The determination of the appearance energies from scans of the ion signal as a function of electron energy in the near-threshold region of each ion utilized a fitting and analysis procedure that has recently been successfully applied to the determination of appearance energies of singly and multiply charged rare-gas ions and several molecular ions and cluster ions. The experimentally determined appearance energies are in good agreement (i) with theoretical calculations that we carried out using standard quantum chemistry codes and (ii) with appearance energy values listed in standard reference data tables (to the extent that tabulated values are available). We find isotope shifts for all three systems ranging from a few meV for the paren...

Calculated electron impact cross sections for the K-shell ionization of Fe, Co, Mn, Ti, Zn, Nb, and Mo atoms using the DM formalism

Abstract We used the Deutsch-Mark (DM) formalism to calculate atomic K-shell electron impact ionization cross sections for the elements Fe, Co, Mn, Ti, Zn, Nb, and Mo. The calculated K-shell ionization cross sections are compared with recently measured K-shell ionization cross sections. Good to satisfactory agreement was found for all atoms with the exception of Ti. Moreover, when compared to other available K-shell ionization cross sections for these atoms, calculated using other theoretical methods and semiempirical formulae, the predictions of the DM formalism achieve a level of agreement with experimental data that is as good or better than the predictions from the other methods.

Electron attachment to chlorouracil: A comparison between 6-ClU and 5-ClU

Low energy electron impact to the isomers 6-chlorouracil (6-ClU) and 5-chlorouracil (5-ClU) yields a variety of negative ion fragments with surprisingly high cross sections. These ions are dominantly formed via sharply structured resonance features at energies below the threshold for electronic excitation and result from dissociative electron attachment (DEA). The most dominant DEA channel is formation of (M-HCl)-, i.e., ejection of a neutral HCl molecule with the negative charge remaining on the ring. The reaction cross section is 9 x 10(-18) m2 and 5 x 10(-18) m2 for 6-Cl and 5-ClU, respectively, and thus about two orders of magnitude higher than the geometrical cross section of the molecule. Further reactions also operative via low energy resonances (<2.5 eV) are Cl- abstraction, dehydrogenation [formation of (M-H)-, M=ClU], and DEA processes associated with a ring opening. Most of the ion yield curves exhibit remarkably sharp structures which have not been observed before in DEA to a polyatomic system. Although some possibilities on their origin are discussed, their interpretation remains a challenge for theory and further experiments. While electron attachment to both 6-ClU and 5-ClU generates fragments of the same stoichiometric composition, their ion yields and also their relative intensities show some very pronounced differences which can be explained by the different structure but also the different energetic situation in the two isomers.

Electron impact multiple ionization of neon, argon and xenon atoms close to threshold: appearance energies and Wannier exponents

We report the results of the experimental determination of the appearance energy values AE(Xn + /X) for the formation of multiply charged Ne, Ar and Xe ions up to n = 4 (Ne), n = 6 (Ar) and n = 8 (Xe) following electron impact on Ne, Ar and Xe atoms using a dedicated high-resolution electron impact ionization mass spectrometer. The data analysis uses the Marquart-Levenberg algorithm, which is an iterative, nonlinear least-squares-fitting routine, in conjunction with either a two-function or a three-function fit based on a power threshold law. This allows us to extract the relevant AEs and corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and spectroscopic values of the AEs and the extracted exponents are compared with other available experimental data and with the predictions of the various Wannier-type power law models.

Electron impact ionization studies for SF5CF3

Electron impact ionization cross sections measured close to threshold are reported for the newly discovered greenhouse gas SF5CF3. No stable parent ion is detected but several cations are detected and their appearance energies?(AE) have been measured. The AE (CF3+/SF5CF3) = 12.87?0.10?eV is in excellent agreement with a recently reported value derived from a photoelectron-photoion coincidence experiment. Other ion thresholds are: AE (CF+/SF5CF3) = 24.44?0.10?eV, AE (CF2+/SF5CF3) = 17.80?0.50?eV; AE (SF+/SF5CF3) = 12.66?0.20?eV; AE (SF3+/SF5CF3) = 14.48?0.10?eV; AE (SF5+/SF5CF3) = 13.16?0.60?eV. Two thresholds are suggested for AE (SF4+/SF5CF3) = 16.10?0.30 and 12.10?0.30?eV. A tentative assignment of an AE for SF2+/SF5CF3 of 15.6?1.0 is also given. These results confirm that SF5CF3 is unlikely to be photolysed in the lower stratosphere and therefore will have a long lifetime against solar photodissociation.

Ionization energy studies for Cl2O monomers and dimers

Electron ionization cross sections measured close to threshold are reported for the Cl2O monomer and dimer using a high resolution electron impact apparatus. Besides measuring the appearance energies (AEs) (Cl2O+/Cl2O) = 11.04±0.06 eV, (ClO+/Cl2O) = 12.29 ± 0.14 eV, (Cl+/Cl2O) = 16.11 ± 0.35 eV, (O+/Cl2O) = 15.15 ± 0.14 eV we have also determined, for the first time, the dimer AE (Cl2O)2+/(Cl2O)2 = 10.47±0.2 eV. From the data we derive a lower bound of the bond energy of 0.71±0.3 eV for (Cl2O)2+ which is similar to that of other dimer ions. Quantum chemical calculations carried out to complement the present study support the experimental data.

High resolution electron interaction studies with atoms, molecules, biomolecules and clusters

Ionisation and attachment by electrons are two of the most fundamental inelastic electron collision processes. Electron-impact ionisation/attachment processes are also important in many practical applications such as low-temperature plasma processing, fusion edge plasmas, planetary atmospheres, radiation chemistry and chemical analysis. Considerable progress in the experimental and theoretical description of electron-driven ionisation and attachment processes involving atomic and molecular targets has been achieved in the past decade, for instance concerning the quantitative determination of total and partial electron impact ionisation cross sections. Nevertheless, with respect to information about the finer details of this interaction, which has to come primarily from experimental studies, little is known due to the fact that experiments require the availability of electron beams of high quality in terms of electron energy resolution and accuracy.

High resolution multiple electron impact ionisation of He, Ne, Ar, Kr and Xe atoms close to threshold: Appearance energies and Wannier exponents

Abstract We have determined appearance energies AE(X n + /X) for the formation of multiply charged He, Ne, Ar, Kr and Xe ions up to charge state n =2 (He), n =4 (Ne), n =6 (Ar), n =6 (Kr) and n =8 (Xe) using a recently commissioned high-resolution electron impact ionization mass spectrometer. The data analysis is based on the Marquart–Levenberg algorithm, involving an iterative, non-linear least-squares fitting of the threshold data assuming a 2-function or a 3-function fit based on a Wannier-type power law. This allows us to extract the relevant AEs and corresponding Wannier exponents.