G. Hanel

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...

Dissociative electron attachment to N2O clusters: attachment spectra for (N2O)nO− anions (n = 0–7) from about 0 up to 25 eV

Abstract Using a recently constructed high resolution crossed beams apparatus involving a hemispherical electron monochromator we have studied from threshold up to 25 eV electron attachment to N2O clusters in the size range up to about 10. Attachment spectra obtained for the production of (N2O)nO− exhibit with increasing cluster size a gradual redshift of the maximum peak position of the major rather broad resonance located at around 2 eV, i.e. values varying from 2.58 ± 0.1 eV for O− to 1.70 ± 0.1 eV for (N2O)7O−. In addition, with increasing cluster size the anion signal is more fully developed near 0 eV, i.e., whereas for n = 0 the O− ion signal is zero at about zero energy (in accordance with expectations from thermochemical data), there exists an appreciable ion signal at zero energy for n = 7. Above the 2 eV resonance there exists a shallow minimum in the cross section at about 4 eV with a slightly increasing, probably structured, broad feature toward higher energies. The present results are compared to previous attachment studies concerning the monomer and clusters of N2O and to O− desorption studies from condensed N2O thereby elucidating the origin and evolution (under different phase conditions) of the various features of the attachment spectra.

Investigations of chemical warfare agents and toxic industrial compounds with proton-transfer-reaction mass spectrometry for a real-time threat monitoring scenario

RATIONALE Security and protection against terrorist attacks are major issues in modern society. One especially challenging task is the monitoring and protection of air conditioning and heating systems of buildings against terrorist attacks with toxic chemicals. As existing technologies have low selectivity, long response times or insufficient sensitivity, there is a need for a novel approach such as we present here. METHODS We have analyzed various chemical warfare agents (CWAs) and/or toxic industrial compounds (TICs) and related compounds, namely phosgene, diphosgene, chloroacetone, chloroacetophenone, diisopropylaminoethanol, and triethyl phosphate, utilizing a high-resolution proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOFMS) instrument with the objective of finding key product ions and their intensities, which will allow a low-resolution quadrupole mass spectrometry based PTR-MS system to be used with high confidence in the assignment of threat agents in the atmosphere. RESULTS We obtained high accuracy PTR-TOFMS mass spectra of the six compounds under study at two different values for the reduced electric field in the drift tube (E/N). From these data we have compiled a table containing product ions, and isotopic and E/N ratios for highly selective threat compound detection with a compact and cost-effective quadrupole-based PTR-MS instrument. Furthermore, using chloroacetophenone (tear gas), we demonstrated that this instruments response is highly linear in the concentration range of typical Acute Exposure Guideline Levels (AEGLs). CONCLUSIONS On the basis of the presented results it is possible to develop a compact and cost-effective PTR-QMS instrument that monitors air supply systems and triggers an alarm as soon as the presence of a threat agent is detected. We hope that this real-time surveillance device will help to seriously improve safety and security in environments vulnerable to terrorist attacks with toxic chemicals.

Distinguishing two isomeric mephedrone substitutes with selective reagent ionisation mass spectrometry (SRI-MS).

The isomers 4-methylethcathinone and N-ethylbuphedrone are substitutes for the recently banned drug mephedrone. We find that with conventional proton transfer reaction mass spectrometry (PTR-MS), it is not possible to distinguish between these two isomers, because essentially for both substances, only the protonated molecules are observed at a mass-to-charge ratio of 192 (C12 H18NO(+)). However, when utilising an advanced PTR-MS instrument that allows us to switch the reagent ions (selective reagent ionisation) from H3O(+) (which is commonly used in PTR-MS) to NO(+), O2(+) and Kr(+), characteristic product (fragment) ions are detected: C4H10N(+) (72 Da) for 4-methylethcathinone and C5 H12N(+) (86 Da) for N-ethylbuphedrone; thus, selective reagent ionisation MS proves to be a powerful tool for fast detection and identification of these compounds.

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.

High Resolution Electron Ionization of N2O Clusters: Appearance Energies

Electron ionization cross sections for (N2O)n clusters have been investigated near the threshold with a newly constructed crossed beams apparatus using a hemispherical electron monochromator (HEM) to monochromatize the primary electron beam. Exploratory test measurements including electron attachment studies to CCl4 and CO and appearance energy (AE) determinations for some rare gases (Ar, Kr, Xe) and molecules (N2, O2, N2O, O2) measured for calibration purposes showed that the accuracy of the energy scale is better than 10 meV exhibiting a good linearity and that AE´s derived for these compounds agree within 20 meV with known photoionization data when using a novel data handling procedure (involving a simultaneous non-linear weighted least-squares fit of two functions). Comparison of the presently derived appearance energies with earlier photoionization data shows a distinct difference in corresponding values attributed to different ionization mechanisms. Both data sets can be fitted successfully by a n-1/3 dependence yielding by extrapolation a value for the bulk photoelectric threshold (11.25 eV in the case of the photoionization data).

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.