Ionization Efficiency Curves Research Articles

Positive and negative ion formation via slow electron collisions with 5-bromouridine

Cation and anion formation is studied experimentally upon inelastic low energy electron interactions with 5-bromouridine (BrUrd) using a crossed electron/neutral beams set-up combined with a quadrupole mass spectrometer. The BrUrd molecule belongs to the class of radiosensitizers that increase the sensitivity of DNA (or RNA) to ionizing radiation. In the case of positive ion formation the ionization efficiency curves are measured near the threshold and the corresponding appearance energies are determined using a non-linear least square fitting procedure. The anion yields are investigated in the electron energy range from about 0 to 14 eV. From the comparison of the present results for negative and positive ions information concerning the underlying mechanism of radiosensitizers is deduced.

Near threshold electron impact ionization cross-section for tellurium atoms

A crossed electron and atomic beams scattering geometry was employed to measure the ionization efficiency curve for tellurium atoms up to 18 eV. Six distinct resonance features are revealed in this curve at the energies 9.68 eV, 11.35 eV, 12.05 eV, 13.18 eV, 14.18 eV and 15.29 eV, respectively.

Electron impact ionization of glycolaldehyde

Positive ion formation upon electron impact ionization of the monomeric and dimeric form of glycolaldehyde is studied with high electron energy resolution. In the effusive neutral beam of evaporated monomeric glycolaldehyde some ions with a mass larger than the monomer indicate the presence of weakly bound neutral dimers. The yield of all ions that originate from the electron impact ionization of these neutral dimers exhibit a strong temperature dependence that can be interpreted as being due to the formation of dimers via three body collisions and thermal decomposition of the dimeric form back into monomers at higher temperatures. Ion efficiency curves are measured and analyzed for the 10 most abundant product cations of monomeric glycolaldehyde. The appearance energies of the parent ion signals of the monomer and dimer of glycolaldehyde (10.2 and 9.51 eV, respectively) are lower than the appearance energy of the parent cation of the more complex sugar deoxyribose that was recently determined to be 10.51 eV.

A mass spectrometric and density functional study of the intermetallic molecules AuBe, AuMg, and AuCa.

The intermetallic molecules AuBe and AuCa were identified by means of the Knudsen-Effusion Mass Spectrometry technique in the high-temperature vapors produced by vaporizing Au-Be-Ca alloys of proper composition. The gaseous equilibria AuBe(g)+Au(g)=Au(2)(g)+Be(g) and AuCa(g)+Au(g)=Au(2)(g)+Ca(g) were studied in the temperature ranges 1720-1841 K and 1669-1841 K, respectively, by monitoring the partial pressures of all the species involved. The equilibrium data were analyzed by the third-law method, obtaining for the first time the dissociation energy D(0) ( composite function) of the two intermetallic species: D(0) ( composite function)(AuBe)=234.0+/-4.0 kJ/mol; D(0) ( composite function)(AuCa)=246.7+/-4.0 kJ/mol. These values are significantly higher than the recently published D(0) ( composite function) of the species AuMg (175.4+/-2.7 kJ/mol). Furthermore, the ionization energies (IE) of AuBe, AuMg, and AuCa were obtained by measuring the electron impact ionization efficiency curves, IE(AuBe)=7.5+/-0.3 eV, IE(AuMg)=6.7+/-0.3 eV, and IE(AuCa)=5.5+/-0.3 eV. Theoretical calculations were also carried out for these species by density functional theory methods (PW91 and BP86) used in conjunction with Stuttgart relativistic effective core potentials. Both functionals were found to perform very well in reproducing experimental D(0) ( composite function), IE, and molecular parameters.

Threshold electron impact ionization studies of uracil

Electron impact ionization near the threshold of the DNA base uracil (U) was investigated using a high resolution hemispherical electron monochromator combined with a quadrupole mass spectrometer. The mass spectrum measured at the electron energy of 70 eV reveals a rich fragmentation pattern. For the parent ion and some of the most intense fragment ions we measured the ion efficiency curves near the threshold and determined the corresponding appearance energies (AEs) using a nonlinear least square fitting routine based on a Wannier type power law. The present AE(U +/U)= 9.59 ± 0.08 eV is in good agreement with vertical ionization energies obtained in previous photoelectron spectroscopy experiments. The appearance energies for the two most abundant fragment ions are AE(C 3H 3NO +/U)= 10.89 ± 0.07 eV and AE(OCN +/U)= 13.41 ± 0.10 eV.

Electron Attachment to the Gas-Phase DNA Bases Cytosine and Thymine

We present a detailed study on dissociative electron attachment (DEA) to isolated gas-phase cytosine (C) and thymine (T). The experimental setup used for these measurements is a crossed electron/neutral beam instrument combined with a quadrupole mass spectrometer. Electron attachment to these biomolecules leads to dissociation into various fragments without a hint of any measurable amount of stable C or T parent anions. The fragment anions with highest abundance are (C−H)- and (T−H)-, respectively. Quantum chemical calculations were performed to calculate the electron affinities and binding energies of the different isomers of the (T−H) fragment. Besides (C−H)- and (T−H)-, we observed five other fragment anions formed by DEA to cytosine and eight additional product anions were detected in the case of thymine. Ion efficiency curves were measured for all fragment anions in the electron energy range from about 0 to 14 eV. For mixtures of T or C with SF6 or CCl4 in the collision chamber, additional resonances...

Ionization energies of the C60 fullerene and its hydrogenated derivatives C60H18 and C60H36 determined by electron ionization

Modification of a method for processing positive ion ionization efficiency curves of polyatomic molecules, as developed earlier by Märk and coworkers, has been described. Based on the method, the first, second, and third ionization energies of C60 have been re-examined and found to be in excellent agreement with the most reliable literature data. The ionization energies for C60H18 and C60H36 have been measured for the first time and were established as 7.3±0.3eV and 7.01±0.25eV, respectively. Although the appearance energy of C60H362+ (18.7±0.2eV) has been determined to be lower than that of C602+ (18.98±0.35eV), the reverse scenario is true for the second ionization energies of C60H36 and C60.

Electron impact ionization of 5- and 6-chlorouracil: appearance energies

Electron impact ionization of the gas phase modified DNA/RNA bases 5- and 6-ClU was studied using a crossed electron/neutral beams technique in combination with a quadrupole mass spectrometer. 5- and 6-ClU belong to the class of halouracils which are used in radiation therapy to increase the effect of ionizing radiation to tumours, when they are incorporated into cancer tissue. Besides determining the mass spectra for both molecules at the electron energy of 70 eV, the ionization efficiency curves for each parent ion and the most abundant fragment ions were measured near the threshold and the corresponding appearance energies (AEs) were derived using an iterative, non-linear least square fitting procedure using the Marquart–Levenberg algorithm based on the Wannier threshold law. The most abundant cations observed in mass spectra have a threshold value of AE ((C 3H 2ClNO) +/5-ClU)=11.12±0.03 eV and AE ((C 3H 2NO) +/6-ClU)=12.06±0.03 eV. The present AE value for the parent ion of 5-ClU AE((5-ClU) +/5-ClU)=9.38±0.05 eV is in fair agreement with previous calculations at the B3LYP level of theory. The AE((6-ClU) +/6-ClU)=9.71±0.05 eV is 0.33 eV higher than that for 5-ClU.

Gas phase contributions to the catalytic formation of HCN from CH4and NH3over Pt: An in situ study by molecular beam mass spectrometry with threshold ionization

Molecular beam mass spectrometry has been used for an in situ study of the Pt-catalyzed formation of hydrocyanic acid from methane and ammonia. The goal was to identify transient gas phase intermediates which would indicate homogeneous contributions to the reaction mechanism. A catalytic wall reactor operated at 1300 °C, 1013 mbar, and 74% HCN yield was connected via a molecular beam interface with a quadrupole mass spectrometer, which allowed the measurement of ionization- and appearance potentials by electron impact. Shape and width of the electron energy spread function were determined by analyzing the ionization efficiency curve of helium; the experimental uncertainty of the measured threshold values was found to be 0.6 eV. By use of the threshold ionization technique it could be shown that methylamine (CH3NH2) and methylenimine (CH2NH) are present in the gas phase under reaction conditions. The measured threshold potentials at m/z = 30 u (9.9 ± 0.6 eV) and m/z = 29 u (10.6 ± 0.6 eV) were unambiguously assigned to the appearance potential of CNH4+/CH3NH2 and the ionization potential of CNH3+/CH2NH, respectively. Both molecules dehydrogenate rapidly at reaction temperature to HCN so that they can be considered as true gas phase intermediates.

Cross sections of slow electron scattering by cadmiumatoms

The ionization efficiency curve up to 16 eV, the total electronscattering cross section up to 15 eV, the electron excitationcross sections for metastable 5 3P0,1,2 andresonance 5 1P1 levels up to 11 eV have beenmeasured for cadmium atoms. The energy dependence of the totalcross section has revealed two distinctly well resolvedresonance features at energies of 0.33 and 3.74 eV, respectively. The total excitation cross sections measured forthe 5 3P0,1,2 and 5 1P1 levels refine the earlier measured optical excitation functions for these levels. These results show the possibility of measuring, inthe same experiment, the total cross section of electronscattering by cadmium atoms, the ionization efficiency and the low-lying level excitation function, as well as providing their absolutecalibration.

Study of Electron Impact Ionization Cross-sections of Sodium-containing Molecules.

Electron impact ionization is a common way to provide positive ions for mass spectrometry. For most of molecular vapor species, however, experimental values of their electron impact ionization cross-sections are so limited that theoretic estimations become necessary. It is found that classic model such as additive rule or its modified version is no longer valid near the ionization threshold. Knowledge in this area is important for alkaline metals like sodium because their molecules may reach the maximum cross-section just a few eV above the ionization potential and easily crack in higher incident energies. A Binary-Encounter-Bethe (BEB) model developed by NIST was employed to calculate the cross-sections of NaO, NaOH, Na2, and Na2O molecules. The total ionization cross-section is computed from partial cross-sections of molecular orbitals. Gaussian and GAMESS code were employed to calculate the necessary molecular orbital parameters in the ground state. Calculated cross-sections were plotted as a function of electron impact energy. Reliability of the data predicted by BEB model was investigated by measuring corresponding ionization efficiency curves for some of the molecules. It was found that I(E)/Imax is generally equal to σ(E)/σmax within 15% in the testing range from 5 to 30 eV for sodium-containing molecules. It indicates that the BEB predictions seem correct at low electron incident energies.

Structure and energetics of Lin(OH)n−1 (n=2–5) clusters deduced from photoionization efficiency curves

Ionization efficiency curves (IECs) were measured for Lin(OH)n−1 (n=2–5) clusters generated by reactions of laser-ablated lithium metal with water vapor, using photoionization time-of-flight mass spectrometry. The measured IECs are well-reproduced with a simulation involving Franck–Condon factors, enabling to identify the global-minimum structure of the clusters predicted in our theoretical study [Tanaka et al., J. Chem. Phys. 113, 1821 (2000)]. Hyperlithiated structures (HLS), in which an excess electron is delocalized, are identified for the cluster with n=2 and 3, while electron-localized or segregated structure (ELS) are for those with n=3–5. Adiabatic ionization energies were determined as 4.053±0.003 eV for HLS of Li2OH, 3.687±0.003 eV for HLS of Li3(OH)2, 4.133±0.003 eV for ELS of Li3(OH)2, and 3.418±0.009 eV for ELS of Li4(OH)3. The ionization energy of Li5(OH)4 was determined as 3.60±0.11 eV by a linear extrapolation of IEC. For Li3(OH)2, the energetics of two lowest-energy isomers is discussed based on the observed abundance ratio.

Mass spectrometric determination of appearance energies for ions formed from CoF(4) and CoF(3) molecules.

Knudsen cell mass spectrometry was applied to the evaluation of the ionization efficiency curves for the ions originating from CoF(4) molecules. Cobalt tetrafluoride was obtained in the gas phase over the CoF(3)(s)-TbF(4)(s) system in the temperature range from 640 to 690 K. From the ionization efficiency curves the appearance energies of the ions formed from the CoF(4) molecules were determined by means of Vogt's deconvolution method. Clausius-Clapeyron plots for the ions from CoF(4) molecules were measured. Evaporation of pure CoF(3)(s) was carried out, and the appearance energies of the ions formed from CoF(3) molecules were determined. The ionization energies for CoF(4) and CoF(3) molecules were found to be (14.3 +/- 0.2) and (13.3 +/- 0.1) eV, respectively.

Mass spectrometric study of the ionization and dissociation of sulphur hexafluoride by monoenergetic electron impact

The ionization and fragmentation of SF6 under electron impact has been examined by means of a trochoidal electron velocity selector and a mass spectrometer. The ionization efficiency curves of the positive ions were recorded and the appearance energies of different electronic states of these ions were determined. The experimental results were combined with auxiliary thermochemical data to evaluate the dissociation energies D°°(F5S–F) = 4.01 eV, D°°(F4S–2F) = 6.39 eV, D°°(F3S–3F) = 9.98 eV, the ionization energy of SF5, = 11.27 eV and heats of formation ΔH°f(SF5+) = 1.81 eV, ΔH°f(SF4+) = 4.12 eV, ΔH°f(SF3+) = 4.11 eV and ΔH°f(SF3) = − 5.13 eV. © 1998 John Wiley & Sons, Ltd.

Direct experimental evidence for the thermal nature of delayed electron emission from a superhot C60 molecule

Delayed electron emission from superhot neutral C60 was found to strongly depend on the initial molecular thermal energy. This dependence is clearly demonstrated by measurements of delayed ionization efficiency curves following electron impact for widely different C60 average vibrational energies in the range of 8–14 eV. These measurements provide direct experimental evidence for the thermal nature of delayed ionization of superhot clusters. A simulation based on decay kinetics with thermal rate constants was found to be in good agreement with experiment.

Ionization cross-sections and ionization efficiency curves from polarizability volumes and ionization potentials

It has been shown by simple electrostatic considerations and confirmed using literature experimental data that a strong correlation exists between the maximum electron impact ionization cross-section and the square root of the ratio of the atomic or molecular polarizability volume to the ionization potential. The observed shape of the ionization efficiency curves for a wide range of molecules may be rationalized in terms of the interaction of an incoming electron wave with the molecule, which has an effective diameter determined from the polarizability volume. The peak in the curve occurs when the electron wavelength matches this diameter. An expression for the ionization cross-section at any incident electron energy, which is a function only of the molecular polarizability and ionization potential, has been deduced. The results of this treatment are in good agreement with experiment and the predictions are generally at least as good as those of more sophisticated theoretical treatments of the electron impact ionization process.

Absolute electron impact ionization cross sections for , where X = H, F, Cl, Br, and I

Measurements of electron impact ionization cross sections have been made for methane and the series methyl fluoride to methyl iodide. The results for methane and methyl fluoride to methyl bromide have been compared with ionization efficiency curves calculated using Deutsch - Märk (DM) and binary-encounter-Bethe (BEB) methods, and also with the results of an ab initio model which gives the maximum cross section as a function of molecular orientation. In addition, the ab initio and DM methods have been used to calculate the steric ratios for the electron impact ionization of methyl chloride which have been compared with experimental measurements made previously.

Mass Spectrometric Study of the Dissociative Ionization of Sulphuryl Fluoride by Monoenergetic Electron Impact. Part I. Heteronuclear Fragment Ions

The dissociative ionization of sulphuryl fluoride (SO2F2) under electron impact has been examined by using a trochoidal electron velocity selector and a magnetic sector mass spectrometer. The ionization efficiency curves of SO2F

Influence of isotopic effect on the shift of the ionization potentials of hydrogen desorbed from the metal hydride surface

The sets of vertical ionization potentials for molecules of hydrogen (deuterium) desorbed from metal hydrides as well as for thermodynamically equilibrated molecular hydrogen (deuterium) at room temperature were obtained by means of the ionization efficiency curves method using mass-spectrometry measurements. Some sets of term differences are explained by thermoemission vibrational excitation of molecules. Comparative analysis to the data previously obtained is given. It was established that the anomalies of electrical transfer observed in the gas phase of metal-hydride-hydrogen systems are caused by vibrational excitation of H 2-molecules, generated during recombination of H-atoms on the metal hydride sorbent surface.

Mass Spectrometric Investigation of Fullerenes. II. Determination of the Electron Impact Cross-section of C60 and C70

Absolute and relative ionization cross sections of C60 and C70 were measured in the range from threshold to 100 eV, using a Knudsen cell mass spectrometric method. Pb was used as internal standard to calibrate the instrument sensitivity on an absolute scale. The vapour pressure of the fullerenes was obtained by a mass loss method as (0.230±5%) Pa at 757 K and (0.429±5%) Pa at 843 K for C60 and C70 respectively. In order to avoid artifacts in the ionization efficiency curve (IEC) shapes, special attention was devoted to the ion source construction, the performance of which was tested using a Ag–Al alloy. The maximum partial ionization cross sections, σ(C