T.D. Märk

Mass spectrometric determination of partial electron impact ionization cross sections of He, Ne, Ar and Kr from threshold up to 180 eV

A new approach to mass spectrometric measurement and analysis has been used to study the electron impact ionization of the rare gases in the low electron energy regime (<180 eV). Critical considerations are given to the problems of ion extraction from the ion source (Nier‐type) and the transmission of the extracted ions through the mass spectrometer system (double focusing sector field Varian MAT CH5). Accurate relative partial cross section functions were obtained for production of He+ and He2+ from He; Ne+, Ne2+, and Ne3+ from Ne; Ar+, Ar2+, Ar3+ from Ar, and Kr+, Kr2+, Kr3+ and Kr4+ from Kr. In addition, the cross section ratios for the partial ionization cross sections of each individual gas were determined. These ratios were used to sum up the relative partial cross section functions for each gas, hence obtaining relative total ionization cross section functions by weighting each partial cross section function with the respective charge number. The shape of the relative total ionization cross section function thus obtained was compared with ionization tube measurements of the total ionization cross section curve. Good agreement with the data of Rapp and Englander‐Golden was found. This comparison allows the normalization of the measured relative partial cross section functions, yielding absolute partial cross section values for all of the processes mentioned above. The experimental results presented are compared with previous determinations where available.A new approach to mass spectrometric measurement and analysis has been used to study the electron impact ionization of the rare gases in the low electron energy regime (<180 eV). Critical considerations are given to the problems of ion extraction from the ion source (Nier‐type) and the transmission of the extracted ions through the mass spectrometer system (double focusing sector field Varian MAT CH5). Accurate relative partial cross section functions were obtained for production of He+ and He2+ from He; Ne+, Ne2+, and Ne3+ from Ne; Ar+, Ar2+, Ar3+ from Ar, and Kr+, Kr2+, Kr3+ and Kr4+ from Kr. In addition, the cross section ratios for the partial ionization cross sections of each individual gas were determined. These ratios were used to sum up the relative partial cross section functions for each gas, hence obtaining relative total ionization cross section functions by weighting each partial cross section function with the respective charge number. The shape of the relative total ionization cross section...

Theoretical determination of absolute electron-impact ionization cross sections of molecules

Abstract Much emphasis has been devoted recently to the experimental determination of absolute electron-impact ionization cross sections of molecules and radicals because of the importance of these cross sections in many applications (e.g. as input parameters to modeling codes for various purposes). Supporting theoretical calculations have been lagging behind to some extent. Because of the inherent complexity of such calculations, simplistic additivity rules and semiempirical methods have often been used in place of more rigorous calculation schemes, particularly in applications where a larger number of cross section data were needed with reasonable precision. Although these methods have often proved to be quite successful as descriptive tools (i.e. reproducing existing experimental ionization cross sections reasonably well), their ability to calculate cross sections for species for which no experimental data are available (predictive capabilities) tend to be limited or questionable. This topical review describes recent progress in the development of more rigorous approaches for the calculation of absolute electron-impact molecular ionization cross sections. The main emphasis will be on the application of the semiclassical Deutsch–Mark (DM) formalism, which was originally developed for the calculation of atomic ionization cross sections, to molecular targets, and on the binary–encounter–Bethe (BEB) method of Kim and Rudd. The latter is a simpler version of the more rigorous binary–encounter–dipole (BED) theory, which was also first developed for the calculation of atomic ionization cross sections, and based on the methods developed by Khare and co-workers. Extensive comparisons between available experimental cross sections and the predictions of these theoretical models will be made for 31 molecules and free radicals (H 2 , N 2 , O 2 , S 2 , C 2 , C 3 , O 3 , H 2 O, NH 3 , CO 2 , CH 4 , CH 3 , CH 2 , CH, CF 4 , CF 3 , CF 2 , CF, NF 3 , NF 2 , NF, SiF 3 , SiF 2 , SiF, TiCl 4 , C 2 H 2 , C 2 H 6 , C 6 H 6 , SiF 6 , C 2 F 6 , CH 3 OH).

Cluster ions: Production, detection and stability

Abstract Todays knowledge of free gas-phase cluster ions originates mostly from very recent studies. This review first summarizes exciting recent discoveries, the potential role of clusters in many applied fields, various classification schemes, and the unique position of clusters between the gaseous and solid state of matter. Next, the most important production techniques are reviewed, including, besides other techniques, adiabatic gas expansion, ion swarm methods, and laser evaporation. Finally, this review is particularly dedicated to the discussion of the mass spectrometry of van der Waals clusters including (i) ionization processes, mechanisms, and efficiency for electron-impact ionization of van der Waals clusters, (ii) the stability of singly and multiply charged cluster ions (metastable dissociations), and (iii) the timely subject of magic numbers in cluster mass spectra. Current understanding of these phenomena is discussed and recent work undertaken to improve that understanding is surveyed.

On the unimolecular fragmentation of C60+ fullerene ions: The comparison of measured and calculated breakdown patterns

The stability of singly charged C60+ fullerene ions, produced by electron impact ionization of C60, has been studied as a function of the electron energy and the time elapsed from ionization in a Nier‐type ion source/double‐focusing, sector‐field mass spectrometer system. A huge kinetic shift of more than 34 eV (dependent on the observation time) was observed for the dissociation process C60+→C58++C2. The ionization efficiency curves for C58+, C56+, and C54+ fragment ions have been recorded with an energy resolution of approximately 0.5 eV. This allowed us to construct a time‐resolved breakdown graph of the decaying C60+ fullerene ion. Two different methods, i.e., the finite heat bath model of Klots and the Rice–Ramsperger–Kassel–Marcus (RRKM) expression, have been used to calculate the decay rates and the breakdown graph of the C60+ ion, and the results of the calculation have been compared with the experimentally obtained breakdown graph. The best fit leads to a dissociation energy (C58+−C2) of 7.1±0.4 ...

Experimental studies on cluster ions

Publisher Summary This chapter discusses several experimental studies conducted on cluster ions. Cluster ions comprise a nonrigid assembly of components having properties between those of large gas phase molecules and the bulk condensed state. Research on the properties of cluster ions is valuable in obtaining a complete understanding of the forces between ions and neutral atoms or molecules, where, for example, data on energetics provide a direct measure of the depth of the potential well of interaction between the ion and the collection of neutral molecules. There are three types of experimental approaches to produce and study these cluster ions depending on different degrees of physical isolation, that is, clusters moving freely in space (gas phase studies), clusters supported upon a substrate surface, and clusters existing in solids and liquids. Moreover, the use of cluster ions for fuel injection into fusion reactors and cluster ions are also present in other high-temperature systems such as those designed as sources of energy employing magnets and hydrodynamics.

On quantitative determination of volatile organic compound concentrations using Proton Transfer Reaction Time-of-Flight Mass Spectrometry

Proton transfer reaction - mass spectrometry (PTR-MS) has become a reference technique in environmental science allowing for VOC monitoring with low detection limits. The recent introduction of time-of-flight mass analyzer (PTR-ToF-MS) opens new horizons in terms of mass resolution, acquisition time, and mass range. A standard procedure to perform quantitative VOC measurements with PTR-ToF-MS is to calibrate the instrument using a standard gas. However, given the number of compounds that can be simultaneously monitored by PTR-ToF-MS, such a procedure could become impractical, especially when standards are not readily available. In the present work we show that, under particular conditions, VOC concentration determinations based only on theoretical predictions yield good accuracy. We investigate a range of humidity and operating conditions and show that theoretical VOC concentration estimations are accurate when the effect of water cluster ions is negligible. We also show that PTR-ToF-MS can successfully be used to estimate reaction rate coefficients between H(3)O(+) and VOC at PTR-MS working conditions and find good agreement with the corresponding nonthermal theoretical predictions. We provide a tabulation of theoretical rate coefficients for a number of relevant volatile organic compounds at various energetic conditions and test the approach in a laboratory study investigating the oxidation of alpha-pinene.

Formation of long-lived CO−2, N2O−, and their dimer anions, by electron attachment to van der waals clusters

Abstract CO2− and N2O− anions, which cannot be formed by electron attachment to the corresponding molecules in the gas phase, are formed by dissociative attachment to clusters of CO2 and N2O, respectively. The relative yields of the monomer and dimer anions are presented and discussed.

Rapid Detection of Meat Spoilage by Measuring Volatile Organic Compounds by Using Proton Transfer Reaction Mass Spectrometry

ABSTRACT The evolution of the microbial spoilage population for air- and vacuum-packaged meat (beef and pork) stored at 4°C was investigated over 11 days. We monitored the viable counts (mesophilic total aerobic bacteria, Pseudomonas spp., Enterobacteriaceae, lactic acid bacteria, and Enterococcus spp.) by the microbiological standard technique and by measuring the emission of volatile organic compounds (VOCs) with the recently developed proton transfer reaction mass spectrometry system. Storage time, packaging type, and meat type had statistically significant (P < 0.05) effects on the development of the bacterial numbers. The concentrations of many of the measured VOCs, e.g., sulfur compounds, largely increased over the storage time. We also observed a large difference in the emissions between vacuum- and air-packaged meat. We found statistically significant strong correlations (up to 99%) between some of the VOCs and the bacterial contamination. The concentrations of these VOCs increased linearly with the bacterial numbers. This study is a first step toward replacing the time-consuming plate counting by fast headspace air measurements, where the bacterial spoilage can be determined within minutes instead of days.

Vibrational Feshbach resonances in uracil and thymine

Sharp peaks in the dissociative electron attachment (DEA) cross sections of uracil and thymine at energies below 3 eV are assigned to vibrational Feshbach resonances (VFRs) arising from coupling between the dipole bound state and the temporary anion state associated with occupation of the lowest sigma* orbital. Three distinct vibrational modes are identified, and their presence as VFRs is consistent with the amplitudes and bonding characteristics of the sigma* orbital wave function. A deconvolution method is also employed to yield higher effective energy resolution in the DEA spectra. The site dependence of DEA cross sections is evaluated using methyl substituted uracil and thymine to block H atom loss selectively. Implications for the broader issue of DNA damage are briefly discussed.

Absolute partial and total electron impact ionization cross sections for CF4 from threshold up to 180 eV

Electron impact ionization of carbon tetrafluoride was studied as a function of electron energy from threshold up to 180 eV. A double focusing mass spectrometer system with an improved electron impact ion source was used, alleviating the problems of ion extraction from the source and the transmission of the extracted ions through the mass spectrometer system. Absolute partial ionization cross section functions for the production of CF/sup +//sub 3/, CF/sup +//sub 2/, CF/sup +/, C/sup +/, F/sup +/, CF/sup 2 +//sub 3/, and CF/sup 2 +//sub 2/ in CF/sub 4/ have been determined. In addition, the total (and the counting) ionization cross section function of CF/sub 4/ has been determined (summation method) and is compared with calculations based on classical and semiclassical binary encounter approximations. Using nth root extrapolation ionization energies of the following doubly charged fragment ions have been derived: AE (CF/sup 2 +//sub 3/) = 41.8 +- 0.3 eV, AE (CF/sup 2 +//sub 2/) = 42.9 +- 0.3 eV, and AE (CF/sup 2 +/) = 52.1 +- 0.5 eV. In accordance with previous results no stable CF/sup +//sub 4/ parent ion has been detected, however, a metastable dissociation process CF/sup +asterisk//sub 4/..-->.. CF/sup +//sub 3/+F has been observed.