Ionization Cross Section Functions Research Articles

The energies of the triply excited n = 2 intrashell He − resonances 2s 22p and 2s2p 2 revisited

Using a recently constructed high-resolution crossed-beam apparatus consisting of a hemispherical electron monochromator and a quadrupole mass spectrometer, we have measured in detail the He + ionization cross-section function in the electron energy range from ∼56–59 eV. By fitting the two structures corresponding to the presence of the two triply excited n = 2 intrashell He − resonances (2s 22p) 2P and (2s2p 2) 2D with the theoretical natural line shape, we have deduced that their energy positions lie at 57.06 ± 0.05 and 58.15 ± 0.05 eV, respectively. Between these two states, an additional previously unseen structure has been observed and tentatively linked to the doubly excited autoionizing (2s 2) 1S state of He. When comparing the presently deduced resonance energies with previous data, there is good agreement within the error bars with those experiments which used both a similar experimental technique to detect these states and a data analysis based on the Fano line shape. Experimental results using other methods to detect and characterize these states (electron scattering, electron excitation) appear to lie at slightly higher energies; nevertheless, the energy separation between the two states is the same for almost all experiments.

Electron impact ionization of C60and C70: production and properties of parent and fragment ions studied with a two-sector field mass spectrometer

Abstract Electron impact ionization and dissociation studies of C60 and C70 carried out in our laboratory over the past three years have revealed many exciting and novel features of this new class of molecules (clusters). The most salient results are summarized in this paper, including first a description of the crossed beams apparatus and the various mass spectrometric (two-sector field) techniques used. This is followed by a discussion of the production of parent and fragment ions (with up to eight charges) via electron impact ionization of C60 and C70 including results on mass spectral fragmentation patterns and measured and calculated absolute ionization cross-section functions. The experimental results concerning the energetics of produced parent and fragment ions are presented in the next section and measured appearance energies and breakdown curves are analysed in the frame of various theoretical concepts (Rrkm, Fhbt) thereby allowing us to derive the corresponding binding energies. The last section is devoted to (i) the different aspects of the stability of singly- and multiply-charged fullerene ions (i.e. the quantitative study of various unimolecular decay channels such as monomer evaporation, sequential decay series, charge separation reactions, etc.) and, based on the measured properties of these decay reactions, to (ii) a discussion of the possible decay mechanisms.

Absolute electron impact ionization cross sections for highly charged ions: Scaling law for hydrogen-like ions

Scaling laws are of great importance in the description of elementary processes in plasma physics and plasma chemistry. This is particularly true for electron impact induced ionization of atoms, molecules and their respective ions. The recently proposed semi-classical DM (Deutsch-Märk) approach for the calculation of the single ionization cross section of atoms is applied here to the single ionization of hydrogen-like ions and the formula derived can be used to rationalize the empirically-based scaling law for the ionization cross section of hydrogen-like ions, i.e. the Z 4 power dependence, where Z is the nuclear charge. The results obtained are compared with previous experimental and theoretical ionization cross section functions.

A semiclassical approach to the calculation of electron impact ionization cross-sections of atoms: From hydrogen to uranium

Absolute ionization cross-sections for single ionization by electron impact (from threshold up to 200 eV) of free gaseous atoms have been calculated by utilizing a recently proposed semiclassical approach (DM formulation). The formula used here is derived within the framework of classical and quantum mechanical approximations, and the necessary input parameters are determined and discussed. Where possible the calculated ionization cross-section functions are compared with experimental data sets and with the most widely used previous formulae such as the Gryzinski and Lotz formulae. It is concluded that the present approach leads on average to a better agreement with experimental results than previous classical, semiclassical and empirical treatments.

A semiclassical approach to the calculation of electron impact ionization cross-sections of atoms: from hydrogen to uranium

Absolute ionization cross-sections for single ionization by electron impact (from threshold up to 200 eV) of free gaseous atoms have been calculated by utilizing a recently proposed semiclassical approach (DM formulation). The formula used here is derived within the framework of classical and quantum mechanical approximations, and the necessary input parameters are determined and discussed. Where possible the calculated ionization cross-section functions are compared with experimental data sets and with the most widely used previous formulae such as the Gryzinski and Lotz formulae. It is concluded that the present approach leads on average to a better agreement with experimental results than previous classical, semiclassical and empirical treatments.

Calculation of Absolute Outer‐Shell Electron Impact Ionization Cross Sections

AbstractThe recently developed semiempirical DM‐approach for the calculation of electron impact single ionization cross sections is applied here to the ionization of the rare gases (Ne, Ar, Kr and Xe) via ejection of an outer s‐shell electron. The results obtained are compared with previous experimental and theoretical s‐shell ionization cross section functions.

Interaction of free electrons with C 60: ionization and attachment reactions

Electron impact ionization and attachment to C 60 has been studied as a function of electron energy using a crossed molecular beam—electron beam double focusing mass spectrometer system. It was possible to determine absolute ionization cross section functions (up to 80 eV) for the reactions C 60 + e → C + 60, C + 58, C + 56 and C + 54, respectively. The huge kinetic shifts observed for the appearance of the various fragment ions are interpreted in terms of RRKM theory yielding a corresponding dissociation energy for C 2 evaporation of (7.1±0.4) eV. The attachment cross section curve observed for the production of C − 60 exhibits a strong zero energy resonance and evidence of equally strong autoscavenging processes up to energies of 14 eV.

Comment on the shape of the ionization cross section function for C 60 + e → C [rad]+ 60 + 2e from threshold to maximum

Recently, Baba [M.S. Baba, T.S.L. Narasimhan, R. Balasubramanian and C.K. Mathews, Int. J. Mass Spectrom. Ion Processes, 114 (1992) R1] reported on the measured electron impact ionization cross section function for the production of C + 60 and they noted a small peak situated at about 30 eV. In contrast, present measurements with an apparatus dedicated to the accurate determination of cross section functions reveals only a smoothly increasing function around this electron energy, thus excluding the existence of a possible resonance process in this energy region.

Discrimination effects for ions with high initial kinetic energy in a Nier-type ion source and partial and total electron ionization cross-sections of CF 4

Fragment ions produced by dissociative electron impact ionization of molecules often have large amounts of kinetic energy (up to several electronvolts). Presently performed computer simulations of the ion trajectories in a Nier-type ion source show strong discrimination effects in the extraction characteristics of such ions, leading to huge errors in measured fragmentation patterns and absolute partial ionization cross-section functions. Using the simulation results a correction function (extraction efficiency vs. fragment kinetic energy) is claculated here for a given potential distribution of a Nier-type ion source extraction system. This correction procedure is applied to previously measured partial ionization cross-sections of CF 4. The corrected partial and total CF 4 cross-sections are in excellent agreement with other recent measurements using advanced methods.

Electron impact ionization cross sections of molecules: Part I. Experimental determination of partial ionization cross sections of SF 6: a case study

Electron impact ionization of SF 6 has been studied as a function of electron energy from threshold up to 180 eV using a Nier type ion source in combination with a double focusing sector field mass spectrometer (reversed geometry). Absolute partial ionization cross section functions for the production of SF + 5, SF + 4, SF + 3, SF + 2, SF +, S + , F +, SF 2+ 4 and SF 2+ 2 have been determined using an improved experimental method based on ion trajectory calculations. In addition, the total ionization cross section function of SF 6 has been obtained. The present data are compared with previous experimental and theoretical results in order to evaluate the reliability and accuracy of the method used.

Direct experimental evidence for the Coulomb explosion of doubly charged argon cluster ions. Ar n2+

Direct experimental evidence for the Coulomb explosion (within 1 μs after ion formation) of doubly charged cluster ions Ar n 2+ (with sizes below the appearance size n 2) is presented. This finding was made possible by a detailed study and analysis of the electron impact ionization cross section functions of Ar n + and Ar n 2+ as a function of cluster size and production conditions. The present method also allows the determination of size distributions of Ar n 2+ with even n. Complete Ar n 2+ spectra thus obtained show different abundance anomalies than Ar n + spectra. The origin of these differences is discussed in terms of structure and stability of these ions.

Determination of absolute partial and total electron impact ionization cross-sections for CF 2Cl 2 from threshold up to 180 eV: an improved experimental method

Electron impact ionization of CF 2Cl 2 has been studied as a function of electron energy from threshold up to 180 eV using a double focussing sector field mass spectrometer (reversed geometry) with an improved ion transmission characteristic. With this set-up complete and/or controlled collection of mass selected parent and fragment ions can be achieved, thus permitting the accurate measurement of quantitative electron impact ionization cross-sections. Absolute partial ionization cross-section functions for the production of CF 2Cl + 2, CFCl + 2, CF 2Cl +, CCl + 2, Cl + 2, CFCl +, FCl +, CF 2 +, CCl +, Cl +, CF +, F +, C +, CFCl 2+ 2, CF 2Cl 2+, CFCl 2+, CCl 2+ 2, CCL 2+ and Cl 2+ have been determined. In addition, the total ionization cross-section function of CF 2Cl 2 is obtained and compared with previous experimental and theoretical results.

Calculation of electron impact ionization cross-sections. The fluorine anomaly

Electron ionization cross-sections as a function of electron energy for molecules containing fluorine have been calculated by utilizing empirically modified collision theories. Previous calculations of these cross-section functions did not result in the correct magnitude due to an anomalous behavior of fluorine in these compounds as evidenced in recent studies based on the additivity of atomic ionization cross-sections. Previous theories (empirical, classical, and semi-classical) were corrected approximately for these effects and then used to calculate ionization cross-section functions for F 2, CCl 2F 2, CF 4, SF 6, UF 6, and N 2 as a special test case. The compound cross-sections were compared with experimental values of the total ionization cross-section (if available) and constitute a considerable improvement over past theoretical values. For UF 6, no previous values exist.

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+3, CF+2, CF+, C+, F+, CF2+3, and CF2+2 in CF4 have been determined. In addition, the total (and the counting) ionization cross section function of CF4 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 (CF2+3) =41.8±0.3 eV, AE (CF2+2) =42.9±0.3 eV, and AE (CF2+)=52.1±0.5 eV. In accordance with previous results no stable CF+4 parent ion has been detected, however, a metastable dissociation process CF+*4→ CF+3+F has been observed.

Absolute partial and total electron ionization cross sections for CCl4 from threshold up to 180 eV

Electron impact ionization of carbon tetrachloride was studied as a function of electron energy from threshold up to 180 eV. A double-focusing mass spectrometer system in combination 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 sections for the occurrence of CCl3+, CCl2+, CCl+, Cl2+, Cl+, C+, CCl32+, and CCl22+ have been determined. In addition, the total ionization cross-section function of CCl4 is reported and compared with theoretical predictions based on a classical binary encounter approximation. Using nth root extrapolation the following ionization energies of the doubly ionized fragment ions have been derived: AE(CCl32+)=30.4±0.3 eV; and AE (CCl22+)=31.8±0.3 eV. In accordance with theoretical predictions and previous results, no stable CCl4+ has been detected, however, metastable dissociation processes CCl4+ → CCl3+ have been observed.

Absolute partial electron impact ionization cross sections of Xe from threshold up to 180 eV

Partial electron ionization cross section ratios and functions of Xe were determined in the low energy regime (≤180 eV) using a refined mass spectrometric technique. The experimental results are compared with previous measurements and calculations.

Single and double ionization of nitric oxide by electron impact from threshold up to 180 eV

Electron impact ionization of nitric oxide was studied as a function of electron energy up to 180 eV. A double focusing mass spectrometer in combination with an improved electron impact ion source (Stephan et al.) was used, alleviating the problems of ion extraction from the source and the transmission of the extracted ions through the mass spectrometer system. Relative partial ionization cross section functions were measured for the processes NO+e→NO++2e and NO+e→NO2++3e. An accurate determination of the cross section ratio q(NO2+/NO)/q(NO+/NO) was made. In addition, absolute partial ionization cross sections for the production of NO+ and NO2+ were determined by normalizing the relative cross sections with two different methods. Using nth root extrapolation an appearance potential of NO2+(X 2Σ+) was determined. The result is compared with previous measurements and theoretical calculations.

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.

Single and double ionization of nitrogen dioxide by electron impact from threshold up to 180 eV

Electron impact ionization of nitrogen dioxide has been studied as a function of electron energy up to 180 eV with a double focussing mass spectrometer Varian MAT CH5 and an improved Nier type electron impact ion source. We report the first observation of NO2++ produced via electron impact. Relative partial ionization cross section functions have been obtained for the processes NO2+e→NO2++2e and NO2+e→NO2+++3e. An accurate measurement of the cross section ratio q (NO2++/NO2) /q(NO2+/NO2) has been performed. In addition an estimate of the absolute partial cross sections has been made normalizing the measured relative cross section against the Ar+ cross section with the method of molecular effusive flow. Using the nth root extrapolation the following ionization potentials have been derived from the cross section functions near threshold: NO2+ = (10,4±0,3) eV and NO2++ = (35,0±0,5) eV.

Cross section for single and double ionization of carbon monoxide by electron impact from threshold up to 180 eV

Electron impact ionization of carbon monoxide has been studied as a function of electron energy up to 180 eV with a double focusing mass spectrometer Varian MAT CH5. Absolute partial ionization cross section functions have been obtained for the processes CO +e→CO++2e and CO+e→CO+++3e by normalization against the Ar+ cross section with help of the method of molecular effusive flow. In the case of CO++ the results suggest, in accordance with previous authors, that this ion exists in a metastable state with a mean half-life in the range of 6–15 μs. Using nth root extrapolation, the following minimum ionization potentials have been derived from the low energy cross section function: CO+(X 2Σ+) = (14.07±0.05) eV and CO++= (41.8±0.5) eV. All experimental results presented are compared with previous determinations where available.