T. Fiegele

Surface-induced reactions of acetone cluster cations

The occurrence of two different chemical reactions initiated by the surface impact of acetone dimer, trimer, and tetramer cations (energy 20–70 eV) on a stainless-steel surface (covered with hydrocarbons) was observed. The reaction product is the protonated acetone ion, formed in (i) an intracluster ion–molecule reaction, and in (ii) a hydrogen pickup reaction of the cluster ion with the surface material. Only the monomer product ions (and small amounts of their dissociation products) could be observed; the spectra did not show any presence of clustered product ions. A simple model based on the Brauman double-well potential is suggested to explain the formation of the two product ions. In accordance with predictions from molecular dynamics simulations, this appears to be the first observation of competitive chemical reactions of a cluster ion driven by energy transfer in a surface collision.

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...

Ionization energy studies for ozone and OClO monomers and dimers

Electron impact ionization cross sections measured close to threshold are reported for both the monomers and dimers of ozone and OClO using a new high resolution electron impact apparatus. The present appearance energies AE(O3+/O3)=12.70±0.02 eV, AE (OClO+/OClO)=10.55±0.02 and AE(ClO+/OClO=13.37±0.03 eV derived from the measured ionization cross sections are in excellent agreement with the vertical threshold values determined for these ions by high resolution PES and PIMS photoionization studies. The corresponding appearance energies determined for the dimer ions, 10.10±0.3 eV for (O3)2+ and 9.87±0.2 eV for (OClO)2+, are both red shifted with respect to the monomer case. The bond energy (0.70–0.3+0.5) eV of (OClO)2+ estimated from these data is similar to that of other dimer ions, whereas the bond energy of (O3–O3+) with (2.55−0.4+0.6) eV is rather large suggesting an unusual structure for the cationic ozone dimer ion. Based on quantum chemical calculations on various levels we are led to the conclusion t...

Surface-induced dissociation of singly and multiply charged fullerene ions

Collisions of singly and multiply charged ions C60z+ (z=1,2,3,4,5) with a hydrocarbon-covered stainless steel surface have been investigated; product ions of fragmentation and pickup reactions were determined as a function of the collision energy (100–500 eV) and the projectile charge z. All ions scattered off the surface are singly charged. The extent of fragmentation increases with the collision energy and the projectile charge. However, the increase of fragmentation with the charge of the projectile is less pronounced than expected from a full conversion of electronic energy, gained in the neutralization process, into internal energy of the ion.

Low-energy collisions of CF3+ ions with a hydrocarbon covered surface: scattering, chemical sputtering, and reactive collisions

Abstract The interaction of CF 3 + ions with a stainless steel surface covered with hydrocarbons has been studied with a recently constructed BESTOF tandem mass spectrometer as a function of the collision energy from about 0 up to 60 eV. Whereas the efficient reflection of projectile ions at low collision energy and the efficient ejection of chemically sputtered ions at higher collision energies found here are in semiquantitative agreement with a previous report, results obtained here also demonstrate that reactive ion surface collisions are an important feature of CF 3 + impact on the hydrocarbon-covered surface leading to the formation of various fluorohydrocarbon ions such as CH 2 F + , CH 2 CF + , and CHF 2 + .

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.

Dissociative electron attachment to hydrogen

Using two different crossed-beams high-resolution electron attachment instruments (employing either a trochoidal electron monochromator or a hemispherical electron monochromator) we have determined the cross section curve for H- production from H2 via the 4 eV resonance at two different temperatures. These relative partial cross sections have been calibrated by comparing present values for the 14 eV resonance with absolute total cross sections determined previously. Taking into account the experimental energy distribution and the rotational excitation and its influence on the cross section shape we obtain very good agreement with theoretical predictions in terms of both the shape and magnitude of this resonance peak.

High-resolution analysis of the kinetic energy distribution of fragment ions produced by dissociative ionization of propane

We report measurements of the kinetic energy distributions of fragment ions produced by electron impact dissociative ionization of propane using a two-sector-field mass spectrometer in conjunction with the retarding field method. This technique achieves a higher energy resolution, albeit at much reduced ion signals and at a significant loss of energetic fragment ions compared to the conventional ion deflection technique, which has been used extensively by other groups in the past. The higher energy resolution used in the present experiments results in a complete separation of the thermal/quasi-thermal fragment ions from the energetic fragment ions. This, in turn, reveals that the energetic fragment ions have a much narrower kinetic energy distribution than previously assumed. Because of the comparatively weak ion signals, further studies such as separate threshold cross-section measurements and appearance energy determinations for, respectively, the “slow” and “fast” fragment ions could only be carried out for the CH3+ and C2H4+ fragment ions.

Ionization energies of argon clusters: A combined experimental and theoretical study

We have measured appearance energies of Ar(n)+, n<or=30, by electron impact of gas phase clusters. Quantum-chemical calculations have been performed to determine the adiabatic and vertical ionization energies of argon clusters up to n=4 and 6, respectively. The experimental appearance energy of the dimer ion approaches, under suitable cluster source conditions, the adiabatic ionization energy. The agreement with values obtained by photoionization and threshold photoelectron-photoion coincidence (TPEPICO) spectra demonstrates that autoionizing Rydberg states are accessible by electron impact. Appearance energies of larger clusters, though, exceed the TPEPICO values by about 0.5 eV.

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).