S. Matt-Leubner

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.

High-resolution kinetic energy release distributions and dissociation energies for fullerene ions Cn+,42⩽n⩽90

We have measured the kinetic energy released in the unimolecular dissociation of fullerene ions, Cn+ --> C(n-2)+ + C2, for sizes 42 < or = n < or = 90. A three-sector-field mass spectrometer equipped with two electric sectors has been used in order to ensure that contributions from isotopomers of different masses do not distort the experimental kinetic energy release distributions. We apply the concept of microcanonical temperature to derive from these data the dissociation energies of fullerene cations. They are converted to dissociation energies of neutral fullerenes with help of published adiabatic ionization energies. The results are compared with literature values.

High resolution measurements of kinetic energy release distributions of neon, argon, and krypton cluster ions using a three sector field mass spectrometer

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg(n) (n> or =10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below n approximately 10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations.

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.

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.

Mechanisms and dynamics of the metastable decay in Ar2

A detailed experimental as well as theoretical investigation of the properties of the metastable dissociation Ar2+ --> Ar+ + Ar is presented. The mass-analyzed ion kinetic energy (MIKE) scan technique has been performed using a three sector field mass spectrometer. The possible mechanisms of the metastability of Ar2+ have been examined and the observed decay process is assigned to the II(1/2)(u)-->I(1/2)(g) bound to continuum radiative transition, in agreement with earlier work. The calculation of the theoretical shape of the kinetic energy release distribution of fragment ions allowed us to construct the theoretical MIKE peak and compare it with the raw experimental data. The accuracy of various sets of potential energy curves for Ar2+ is discussed, as well as the way of production of the metastable Ar2+[II(1/2)(u)] electronic state by electron impact. Excellent agreement between the experimental data and theoretical model has been observed.

The role of internal energy of polyatomic projectile ions in surface-induced dissociation

Abstract The effect of initial internal energy on the extent of surface-induced fragmentation was investigated in a tandem mass spectrometer for CH 3 + and CH 4 + ions prepared in different ion sources. The different initial internal energy content had a considerable effect on the extent of fragmentation of the surface-excited projectile ions: ions from a Nier source with a large amount of initial internal energy fragmented at much lower collision energies than internally relaxed projectile ions from a Colutron source. A quantitative estimation of this effect showed that the initial internal energy content of the projectile ions was entirely preserved in the projectile ion during the ion/surface collision.

Ne2+[II (1/2)u]: radiative decay and electronic predissociation

Metastable fragmentation of neon dimer ions has been investigated by measuring and analyzing high resolution kinetic energy release distributions. Data were obtained by a modified MIKE (mass-analyzed ion kinetic energy) scan technique. Due to the high energy resolution it was possible to distinguish two different reaction mechanisms in the micros time regime which produce Ne+ ions with different kinetic energy distributions. Theoretical studies based on ab initio calculations of potential energy curves allowed the assignment of the reactions to specific electronic transitions in the excited Ne2+ ion. The unusual bimodal kinetic energy release distribution arises from competition between radiative and non radiative decay of the long-lived Ne2+ II(1/2)u state.

Electron Impact Ionization/Dissociation of Molecules: Production of Energetic Radical Ions and Anions

In order to provide quantitative information on electron collision processes involving various plasma constituents (in particular hydrocarbons) and to elucidate the properties of cations and anions produced we have carried out the past years a series of studies with a variety of techniques in our laboratory in Innsbruck. In the present review we will present some recent results on electron impact ionization and attachment in order to illustrate recent progress in this field in particular concerning the production of energetic fragment cations for hydrocarbons and differences in the attachment of isomers for nitro-organics. Using a Nier type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene, propene and other hydrocarbons have been measured for electron energies up to 1000 eV. Discrimination factors for ions have been determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover charge separation reactions (for instance in case of acetylene the Coulomb explosion of the doubly-charged parent ions C2H2++ into the fragment ions C2H+ and H+) are investigated by means of a number of metastable mass spectrometry methods and the associated mean kinetic energy release is deduced. Free electron attachment to the three different isomers of mono-nitrotoluene molecules in the gas phase is studied using two different crossed electron-molecule beams technique. In contrast to previous studies for a large number of negative ions in nitro-organic compounds the presently measured relative cross section curves are recorded with an electron energy resolution of better than 100 meV. For several product anions including the nitro anion NO2− remarkable differences for the three isomers are observed. For almost all fragment anion efficiency curves the 2-nitrotoluene exhibits pronounced differences compared to the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other.

Energetics, kinetics and dynamics of decaying metastable ions studied with a high-resolution three-sector field mass spectrometer

Mass spectrometric analysis of metastable decay reactions is devoted to the measurements of the kinetic energy release distribution (KERD) for the decay of singly charged rare gas dimer ions and , the doubly charged acetylene parent ion and the singly and doubly charged SF6 fragment ions, like for example , and . The KERDs are obtained either from high-resolution mass analysed ion kinetic energy spectra or the measurement of ion beam profiles using a specially improved mass spectrometric system. Due to the high energy resolution measurements and theoretical studies based on ab initio calculations of potential energy curves it is possible to assign the reaction products of the rare gas dimer decays to electronic transitions in the excited parent ion. The and also the ions are investigated because of obscurities in the production of their fragment ions. The unusual shape of the ionization cross section indicates that at sufficiently high electron energies the fragmentation channel of doubly charged contributes significantly to the ion yield. Additional measurements of the corresponding appearance energies confirm the existence of this second production channel.