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.

Effect of impurities on the matrix-assisted laser desorption/ionization mass spectra of insulin.

The effect of impurity (sodium sulfate Na2SO4; copper sulfate CuSO4; potassium ferrocyanide K4Fe(CN)6; and triammonium citrate (NH4)3C6H5O7) concentration on the positive matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) spectra of insulin is described. 2,5-Dihydroxybenzoic acid was used as a desorption matrix. The estimated maximum concentrations of tested salts for which no insulin signal was observed were determined.

High-resolution kinetic energy release distributions and dissociation energies for fullerene ions Cn+, 42 < or = n < or = 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.

The anomalous shape of the cross section for the formation of SF3+ fragment ions produced by electron impact on SF6 revisited.

The partial ionization cross section for the formation of SF(3) (+) fragment ions following electron impact on SF(6) is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF(3) (+) partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF(4) (2+) ions into two singly charged ions SF(3) (+) and F(+), with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF(3) (+) partial ionization cross section is the result of two contributions, (i) the direct formation of SF(3) (+) fragment ions via dissociative ionization of SF(6) with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF(4) (2+) ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF(4) (2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 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.

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.

Probing electronic states of Ne2+ and Ar2+ by measuring kinetic-energy-release distributions.

Dissociative decay of metastable, electronically excited neon and argon dimer ions produces fragment ions with strikingly dissimilar kinetic-energy-release distributions. The distributions have been modeled based on ab initio calculations of potential energy curves. The unusual bimodal distribution observed for dissociation of Ne2+ arises from competition between radiative and nonradiative decay of the long-lived II(1/2)(u) state. For Ar2+, however, electronic predissociation is insignificant.