共查询到20条相似文献,搜索用时 31 毫秒
1.
N. Endstrasser F. Zappa A. Mauracher A. Bacher S. Feil D.K. Bohme P. Scheier M. Probst T.D. Mrk 《International journal of mass spectrometry》2009,280(1-3):65
We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels. 相似文献
2.
Time-of-flight mass spectrometry and two-dimensional coincidence techniques have been used to determine, for the first time, the relative precursor-specific partial ionization cross sections following electron-methane collisions. Precursor-specific partial ionization cross sections quantify the contribution of single, double, and higher levels of ionization to the partial ionization cross section for forming a specific ion (e.g. CH(+)) following electron ionization of methane. Cross sections are presented for the formation of H(+), H(2)(+), C(+), CH(+), CH(2)(+), and CH(3)(+), relative to CH(4)(+), at ionizing electron energies from 30 to 200 eV. We can also reduce our dataset to derive the relative partial ionization cross sections for the electron ionization of methane, for comparison with earlier measurements. These relative partial ionization cross sections are in good agreement with recent determinations. However, we find that there is significant disagreement between our partial ionization cross sections and those derived from earlier studies. Inspection of the values of our precursor-specific partial ionization cross sections shows that this disagreement is due to the inefficient collection of energetic fragment ions in the earlier work. Our coincidence experiments also show that the lower energy electronic states of CH(4)(2+) populated by electron double ionization of CH(4) at 55 eV are the same (ground (3)T(1), first excited (1)E(1)) as those populated by 40.8 eV photoionization. The (3)T(1) state dissociating to form CH(3)(+) + H(+) and CH(2)(+) + H(2)(+) and the (1)E(1) to form CH(2)(+) + H(+) and CH(+) + H(+). At this electron energy, we also observe population of the first excited triplet state of CH(4)(2+) ((3)T(2)) which dissociates to both CH(2)(+) + H(+) + H and CH(+) + H(+) + H(2). 相似文献
3.
The absolute total ionization cross sections from threshold to 250 eV and dissociative attachment cross sections from zero to 10 eV have been measured for the CCl2F2 (dichloro-difluoro-methane) molecule by using a parallel plate condenser type ionization chamber. The maximum of the ionization cross-section curve was found to be at an energy of about 90 eV with a cross section of 1.44 × 10?19 m2. The attachment cross-section curve shows three peaks, the most intense being at zero electron energy with a cross-section value of 1.80 × 10?20 m2, and the other two at energies of 0.6 eV and 3.5 eV, respectively. The maximal relative error in cross-section values is 0.08, for electron energies larger than 0.4 eV. 相似文献
4.
We report electron impact total cross sections, Q(T), for e-N(2)O scattering over an extensive range of impact energies approximately from 0.1 eV to 2000 eV. We employ an ab initio calculation using R-matrix formalism below the ionization threshold of the target and above it we use the well established spherical complex optical potential to compute the cross sections. Total cross section is obtained as a sum of total elastic and total electronic excitation cross sections below the ionization threshold and above the ionization threshold as a sum of total elastic and total inelastic cross sections. Ample cross section data for e-N(2)O scattering are available at low impact energies and hence meaningful comparisons are made. Good agreement is observed with the available theoretical as well as experimental results over the entire energy range studied here. 相似文献
5.
D. Koch M. Wahl A. Wucher 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,32(1):137-144
Neutral silver atoms and small clusters Ag n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar+ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ e (Ag n )/σ e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag2, Ag3 and Ag4 as a function of the electron energy between threshold and 125 eV. 相似文献
6.
《International journal of quantum chemistry》2018,118(18)
The present work provides a comprehensive set of positron impact scattering cross sections for group 14 tetrahydrides, namely, SiH4, GeH4, SnH4, and PbH4. The well‐established spherical complex optical potential and complex scattering potential‐ionization contribution methods are modified to incorporate positron scattering in the present work to calculate various cross sections. The positronium formation channel is adequately included through an improved inelastic threshold. The energy range chosen for the direct ionization cross section is from the respective ionization potential (I) of the molecule to 5 keV. Likewise, positronium formation and total ionization cross sections are reported from the positronium formation threshold to 300 eV and 5 keV, respectively, and the total cross section is computed for an extensive energy range of 1 eV to 5 keV. The positron impact total cross section for stannane molecule is computed for the first time. A characteristic valley is observed in the total cross sections with minima close to the positronium formation threshold. Further increase of cross section signifies the opening of inelastic channels especially positronium formation. In general, a reasonable agreement is found between the present results and other comparisons, wherever available. Furthermore, this is the first report of the inelastic cross sections (direct ionization, positronium formation, and total ionization) for the present set of targets. 相似文献
7.
Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binary-encounter-Bethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the -COOH pi* orbital. The authors discuss evidence that direct attachment to the lowest sigma* orbital may instead be responsible. A close correlation between the energies of the core-excited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H-. 相似文献
8.
G. C. Shields P. A. Steiner P. R. Nelson M. C. Trauner T. F. Moran 《Journal of mass spectrometry : JMS》1987,22(2):64-69
Cross sections for charge transfer reactions of organic ions containing oxygen have been obtained using time-of-flight techniques. Charge transfer cross sections have been determined for reactions of 2.0 to 3.4 keV ions produced by electron impact ionization of oxygen containing molecules such as methanol, ethanal and ethanol. Experimental cross section magnitudes have been correlated with reaction energy defects computed from ion recombination energies and target ionization energies. Large cross sections are observed for reacting systems with small energy defects. 相似文献
9.
Single and double differential ionization cross sections for the production of ions resulting from dissociative, single and double ionization of SF(6) by electron impact have been calculated using a semiempirical formulation based on the Jain-Khare approach. In addition, triple differential cross sections have been obtained for some of the doubly charged fragment ions at an incident electron energy of 100, 150, and 200 eV, respectively, and a fixed scattering angle of 30 degrees. As no previous data seem to exist for differential cross sections we have derived from these differential cross sections corresponding partial and total ionization cross sections from threshold up to 900 eV and compared those with the available theoretical and experimental data. 相似文献
10.
Relative partial ionization cross sections and precursor specific relative partial ionization cross sections for fragment ions formed by electron ionization of C2H2 have been measured using time-of-flight mass spectrometry coupled with a 2D ion-ion coincidence technique. We report data for the formation of H+, H+2, C2+, C+/C2+ 2, CH+/C2H+2, CH+2, C+2, and C2H+ relative to the formation of C2H+2, as a function of ionizing electron energy from 30-200 eV. While excellent agreement is found between our data and one set of previously published absolute partial ionization cross sections, some discrepancies exist between the results presented here and two other recent determinations of these absolute partial ionization cross sections. We attribute these differences to the loss of some translationally energetic fragment ions in these earlier studies. Our relative precursor-specific partial ionization cross sections enable us, for the first time, to quantify the contribution to the yield of each fragment ion from single, double, and triple ionization. Analysis shows that at 50 eV double ionization contributes 2% to the total ion yield, increasing to over 10% at an ionizing energy of 100 eV. From our ion-ion coincidence data, we have derived branching ratios for charge separating dissociations of the acetylene dication. Comparison of our data to recent ab initio/RRKM calculations suggest that close to the double ionization potential C2H2+2 dissociates predominantly on the ground triplet potential energy surface (3Sigma*g) with a much smaller contribution from dissociation via the lowest singlet potential energy surface (1Delta g). Measurements of the kinetic energy released in the fragmentation reactions of C2H2+2 have been used to obtain precursor state energies for the formation of product ion pairs, and are shown to be in good agreement with available experimental data and with theory. 相似文献
11.
12.
M. Azizi C. Wülker W. Hink W. Sandner 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,30(2):161-167
We have investigated the cross section for inner (5p) shell ionization of barium by high energy (6 keV) electron impact. The target atoms were in the ground state and in the laser excited metastable 6s 5d (1,3 D) state, respectively. Comparison of the relative ionization cross sections yields information on the configuration mixed wave functions of neutral and ionic barium. 相似文献
13.
14.
《International journal of mass spectrometry》2002,213(1):5-8
We used the Deutsch-Märk (DM) formalism to calculate atomic K-shell electron impact ionization cross sections for the elements Fe, Co, Mn, Ti, Zn, Nb, and Mo. The calculated K-shell ionization cross sections are compared with recently measured K-shell ionization cross sections. Good to satisfactory agreement was found for all atoms with the exception of Ti. Moreover, when compared to other available K-shell ionization cross sections for these atoms, calculated using other theoretical methods and semiempirical formulae, the predictions of the DM formalism achieve a level of agreement with experimental data that is as good or better than the predictions from the other methods. 相似文献
15.
16.
A. Borella A. Moens P. Schillebeeckx R. Van Bijlen G. L. Molnár T. Belgya Zs. Révay L. Szentmiklósi 《Journal of Radioanalytical and Nuclear Chemistry》2005,265(2):267-271
Summary The total capture cross section of 209Bi was determined at the cold neutron beam PGAA-NIPS facilities at the Budapest Neutron Centre. The measurements were performed using a coaxial HPGe detector with Compton suppression. The total and partial gamma-ray production cross sections were deduced relative to the 14N(n,γ) partial gamma-ray production cross section. By using a bismuth nitrate stoichiometric compound as the sample, we excluded various systematic uncertainties. The total capture cross section is in very good agreement with the compilation of Mughaghab, but is slightly lower than the most recent value determined at the high flux reactor of the ILL in Grenoble, France. We also performed measurements using a 0.5 mm thin Bi metal disc. The relative intensities determined from the Bi disc and the compound samples are in good agreement. 相似文献
17.
W. L. Morgan 《Plasma Chemistry and Plasma Processing》1992,12(4):449-476
The available information on low-energy electron impact cross sections for Cl2, F2, and HCAl is reviewed and critically evaluated. Of interest are the cross sections for momentum transfer; rotational, vibrational, and electronic excitation (with possible dissociation); and attachment and ionization. The bulk of the data available for these molecules are on dissociative attachment, as that is the property, that has been of greatest interest in applications and is relatively easy to measure. Much of the process of critical evaluation is performed by computing electron transport, or swarm, coefficients using the published cross sections and comparing with published measurements. What little is known about electronic excitation or dissociation processes is theoretical or even guessed at using analogies to processes in other molecules. Recommended cross sections are identified where feasible. 相似文献
18.
E. Cengiz V. Aylıkcı N. Kaya G. Apaydın E. Tıraşoğlu 《Journal of Radioanalytical and Nuclear Chemistry》2008,278(1):89-96
In this study, the chemical effects on σKi
(i = α, β), σLα cross sections, Kβ/Kα X-ray intensity ratios and vacancy transfer probabilities from K to L (η
KL) for pure Nb and Nb compounds were investigated. The samples were excited by 59.5 keV γ-rays from 241Am and 5.96 keV photon energy from a 55Fe annular radioactive sources. K and L X-rays emitted by samples were counted by an Ultra-LEGe detector with a resolution
of 150 eV at 5.9 keV. While it was observed that the chemical bonding had an effect on the σKβ, σLα cross sections and Kβ/Kα X-ray intensity ratios for compounds, it was almost negligible for σKα cross section because Kα transitions (2P3/2,1/2→1S1/2) occurred in inner shells. It is well known that interactions between central element atom and ligands come into existence
in valence state, so outer energy levels are sensitive to the chemical environment. The experimental values of σKα cross section and η
KL are in good agreement with theoretically calculated and other experimental values of pure niobium, but the experimental values
of the σKβ, σLα cross sections and Kβ/Kα X-ray intensity ratios have differences for some compounds because valence electrons have different bond distances and binding
energies in different compounds. 相似文献
19.
We report new measurements of the absolute electron-impact double ionization cross sections for Ar and Kr and of the ratios of double-to-single ionization for impact energies from threshold to 200 eV using the crossed electron-beam — fast-atom-beam technique. The work was motivated by the recently highlighted spread of about 30% in the Ar2+/Ar+ ionization cross section ratios obtained by several groups using different experimental techniques. Such a spread is inconsistent with statistical uncertainties of typically 3% or less that were quoted for the various reported ratios. A similar situation exists for Kr where the spread among the recently published Kr2+/Kr+ ionization cross section ratios is about 15%. We made an attempt to identify all potential systematic errors inherent to the fast-beam technique that could affect the measurement of cross section ratios with special emphasis on those systematic errors that could influence the detection of singly and doubly charged product ions differently. We found Ar2+/Ar+ and Kr2+/Kr+ cross section ratios of, respectively 0.066 ±0.007 and 0.087 ±0.008 at 100 eV which confirm earlier measurements using the same experimental technique. The error limits on cross sections ratios measured in our fast-beam apparatus were determined to be at least ±9% for cross section ratios of multiple-to-single ionization for the same target atom and at least ±10% for ratios of single ionization cross sections for different target species. Our error limits are dominated by systematic uncertainties of the apparatus which do not cancel when cross section ratios are measured, since the ratios are obtained under similar, but not identical experimental conditions. 相似文献
20.
Probing hemoglobin structure by means of traveling-wave ion mobility mass spectrometry 总被引:1,自引:0,他引:1
Charlotte A. Scarff Vibhuti J. Patel Konstantinos Thalassinos James H. Scrivens 《Journal of the American Society for Mass Spectrometry》2009,20(4):625-631
Hemoglobin (Hb) is a tetrameric noncovalent complex consisting of two α- and two β-globin chains each associated with a heme group. Its exact assembly pathway is a matter of debate. Disorders of hemoglobin
are the most common inherited disorders and subsequently the molecule has been extensively studied. This work attempts to
further elucidate the structural properties of the hemoglobin tetramer and its components. Gas-phase conformations of hemoglobin
tetramers and their constituents were investigated by means of traveling-wave ion mobility mass spectrometry. Sickle (HbS)
and normal (HbA) hemoglobin molecules were analyzed to determine whether conformational differences in their quaternary structure
could be observed. Rotationally averaged collision cross sections were estimated for tetramer, dimer, apo-, and holo-monomers
with reference to a protein standard with known cross sections. Estimates of cross section obtained for the tetramers were
compared to values calculated from X-ray crystallographic structures. HbS was consistently estimated to have a larger cross
section than that of HbA, comparable with values obtained from X-ray crystallographic structures. Nontetrameric species observed
included apo- and holo- forms of α- and β-monomers and heterodimers; α- and β-monomers in both apo- and holo- forms were found to have similar cross sections, suggesting they maintain a similar fold
in the gas phase in both the presence and the absence of heme. Heme-deficient dimer, observed in the spectrum when analyzing
commercially prepared Hb, was not observed when analyzing fresh blood. This implies that holo-α-apo-β is not an essential intermediate within the Hb assembly pathway, as previously proposed. 相似文献