Ionization Cross Section Ratios Research Articles

Hypersatellite x-ray decay of 3d hollow- K -shell atoms produced by heavy-ion impact

We report on the radiative decay of double $K$-shell vacancy states produced in solid Ca, V, Fe, and Cu targets by impact with about 10 MeV/amu C and Ne ions. The resulting $K$ hypersatellite x-ray emission spectra were measured by means of high-energy-resolution spectroscopy using a von Hamos bent crystal spectrometer. The experiment was carried out at the Philips variable energy cyclotron of the Paul Scherrer Institute. From the fits of the x-ray spectra the energies, line widths, and relative intensities of the hypersatellite x-ray lines could be determined. The fitted intensities were corrected to account for the energy-dependent solid angle of the spectrometer, effective source size, target self-absorption, crystal reflectivity, and detector efficiency. The single-to-double $K$-shell ionization cross-section ratios were deduced from the corrected relative intensities of the hypersatellites and compared to theoretical predictions from the semiclassical approximation model using hydrogenlike and Dirac-Hartree-Fock wave functions and from classical trajectory Monte Carlo calculations.

Multiple Ionization of Argon Induced by Carbon Ions at 20–500 keV/u Energies

We measure the multiple ionization cross-section ratios Rk1 of Ar impacted by Cq+ (q = 1−3) ions in the energy range of 20–500 keV/u. The measured ratios Rk1 increase with the projectile energy at lower energies, and reach the maximum at energies of 50–150 keV/u, then decrease for higher energies. We also use the classical over barrier ionization model to calculate the ratios Rk1, and the calculation results are in good agreement with the data, which suggest that the multiple ionization process is described by the sequential over-barrier ionization process.

What Remains to Be Done to Allow Quantitative X-Ray Microanalysis Performed with EDS to Become a True Characterization Technique?

This article reviews different methods used to perform quantitative X-ray microanalysis in the electron microscope and also demonstrates the urgency of measuring the fundamental parameters of X-ray generation for the development of accurate standardless quantitative methods. Using ratios of characteristic lines acquired on the same X-ray spectrum, it is shown that the Cliff and Lorimer K A-B factor can be used in a general correction method that is appropriate for all types of specimens and electron microscopes, providing that appropriate corrections are made for X-ray absorption, fluorescence, and indirect generation. Since the fundamental parameters appear in the K A-B factor, only the ratio of the ionization cross sections needs to be known, not their absolute values. In this regard, the measurement of ratios of the K A-B factor (or intensities at different beam energies of the same material with no change of beam spreading in the material) permits the validation for the best models to compute the ratio of ionization cross sections. It is shown, using this method, that the nonrelativistic Bethe equation, to compute ionization cross section, is very close to the equation of E. Casnati et al. (J Phys B 15, 155-167, 1982) and also to the equations proposed by D. Bote and F. Salvat (Phys Rev A 77, 042701, 2008) for the computation of the ratio of ionization cross sections. The method is extended to show that it could be used to determine the values of the Coster-Kronig transitions factors, an important fundamental parameter for the generation of L and M lines that is mostly known with poor accuracy. The detector efficiency can be measured with specimens where their intensities were measured with an energy dispersive spectrometer detector, the efficiency of which has been measured in an X-ray synchrotron (M. Alvisi et al., Microsc Microanal 12, 406-415, 2006). The spatial resolution should always be computed when performing quantitative X-ray microanalysis and the equations of R. Gauvin (Microsc Microanal 13(5), 354-357, 2007) for bulk materials and the one presented in this article for thin films should be used. The effects of X-rays generated by fast secondary electrons and by Auger electrons are reviewed, and their effect can be detrimental for the spatial resolution of materials involving low-energy X-ray lines, in certain specific conditions. Finally, quantitative X-ray microanalysis of heterogeneous materials is briefly reviewed.

Multiple ionization of argon accompanied by electron loss and capture of 0.22–6.35 MeV Cq+ions

In this paper a projectile ions-recoil ions coincidence technique is employed to investigate the target ionization and projectile charge state changing processes in the collision of 0.22–6.35 MeV Cq+ (q = 1 – 4) ions with argon atoms. The partial cross section ratios of the double, triple, quadruplicate ionization to the single ionization (or the single capture) of argon associated with single electron loss (or single electron capture) by the projectile are measured and compared with the previous experimental results. In the present experiment, it is observed that the ratios of ionization cross sections R associated with single loss and single capture depend strongly on the projectile charge state and vary significantly with different reaction channels as impact energy increases. In addition, this paper gets empirical scaling laws for the ionization cross section ratios R corresponding to the projectile single loss and finds that the ratios of the double ionization to the single ionization associated with single electron capture remain constant in the present energy range.

Effect of competition between relaxation and decay on the Auger-photoelectron coincidence spectroscopy (APECS) spectra of Pd metal

The high-resolution coincidence M45 photoelectron spectra and coincidence M45-VV Auger-electron spectroscopy (AES) spectra of Pd metal reported by Butterfield et al. [M.T. Butterfield, R.A. Bartynski, S.L. Hulbert, Phys. Rev. B66 (2002) 115115] are analyzed by a many-body theory. MX is the atomic shell Mx. The Auger-photoelectron coincidence spectroscopy (APECS) spectra provide the evidence of the rapid delocalization of the valence hole created in Pd metal either by the M45 shakeup or by the M4-M5V Coster-Kronig transition before the M45-hole or the M5-hole decays. The M45 shakeup state relaxes to the M45 main-line state before the M45-hole decays. As a result, the singles (noncoincidence) M45-VV AES spectral line shape becomes the sum of the intrinsic coincidence M5-VV AES one and the intrinsic coincidence M4-VV one, weighted by a M5-VV to M4-VV ratio of 1.0:0.7, which agrees well with the theoretical M45-level ionization cross section ratio of 1.0:0.69. A high-resolution APECS can reveal the core-hole dynamics in much more detailed manner than conventional noncoincidence techniques.

M-shell ionization of heavy elements by0.1–1.0MeV∕amuH1,2andHe3,4ions

The $M$-shell ionization in high-$Z$ atoms by low-energy light $_{1}^{1}\mathrm{H}$, $_{1}^{2}\mathrm{H}$, $_{2}^{3}\mathrm{He}$, and $_{2}^{4}\mathrm{He}$ ions have been studied systematically in the energy range $0.1--1.0\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\mathrm{amu}$ in order to verify the available theoretical approaches describing the $M$-shell ionization by charged particles in asymmetric collisions. The present low-energy data, combined with our earlier results reported for $M$-shell ionization by hydrogen and helium ions for higher energies, form a systematic experimental basis to test the theoretical predictions of $M$-shell ionization based on the plane-wave Born approximation (PWBA), the semiclassical approximation (SCA), and the binary-encounter approximation (BEA). In the PWBA based approaches the energy loss (E), Coulomb deflection (C), perturbed stationary state (PSS), and relativistic (R) effects were considered within the ECPSSR theory and its recent modification, called the ECUSAR theory, in which a description of the PSS effect was corrected to account for the united- and separated-atom (USA) electron binding energy limits. In the SCA calculations with relativistic wave functions the binding effect was included only in the limiting cases of separated-atom and united-atom limits. Possible contribution of the electron capture, multiple ionization, and recoil ionization to the $M$-shell vacancy production, which is dominated for light ions impact by direct single ionization process, are also discussed. The universal scaling of measured $M$-shell x-ray production and ionization cross sections was investigated in detail. Using the present data the isotopic effect has been studied by comparing the measured $M$-shell ionization cross-section ratios for equal-velocity hydrogen $_{1}^{1}\mathrm{H}$ and $_{1}^{2}\mathrm{H}$ as well as helium $_{2}^{3}\mathrm{He}$ and $_{2}^{4}\mathrm{He}$ isotopes. In addition, the ratios of measured ionization cross sections for $_{1}^{2}\mathrm{H}$ and $_{2}^{4}\mathrm{He}$ were used to investigate the role of the binding effect. The present results are of practical importance for the application of particle-induced x-ray emission technique in trace element studies.

Double 1s shell ionization of Si induced in collisions with protons and heavy ions

The double 1s ionization of Si induced in collisions with protons and heavier ions (C, Ne) was studied by measuring the K X-ray emission of a solid Si target. In order to resolve the hypersatellite contributions in the spectra, high-resolution crystal diffractometry was employed yielding subelectronvolt energy resolution. Experimentally obtained hypersatellite yields were used to determine the double to single K shell ionization cross-section ratios σ KK/ σ K corresponding to the investigated collisions. The experimental ratios for collisions with heavy ions, where direct Coulomb ionization and electron capture need both to be considered, were compared to the theoretical values calculated within the independent electron approximation employing single electron ionization probabilities calculated by the three body classical trajectory Monte-Carlo (CTMC) method. For proton collisions where direct ionization solely contributes to 1s ionization the semiclassical approximation (SCA) was employed.

Energy dependence of the Penning ionization electron spectrum of Ne*(3s 3P2,0)+Ar

Crossed supersonic beam measurements of the Penning ionization electron spectrum for the title system are reported for a collision energy range E=1.6 to 7.4 kcal/mol. The spectra are deconvoluted to obtain separate line shapes for the four possible combinations of J(Ne) and J′(Ar+). The reagent fine structure ionization cross-section ratio QJ=0/QJ=2 is found to increase from 1.1 to 1.5 over this energy range, in good agreement with other studies. The anomalous product branching ratios QJ,J′=3/2/QJ,J′=1/2 found by Hotop et al. [J. Electron Spectrosc. Relat. Phenom. 23, 347 (1981)] are reproduced; these also depend weakly on E. The width and blueshift of all lines increase with E, while line shape asymmetry increases rapidly at low E and levels off at high. Comparison is made with quantum-mechanical calculations based on previously proposed potentials; the calculations predict oscillatory E dependence of the linewidth and asymmetry.

Transferability of the Valence Ionization Intensities of Chemical Functional Groups between Molecules

The transferability of the valence ionization intensities of chemical functional groups is investigated with three common laboratory ionization sources (Ne Iα, 16.7 eV; He Iα, 21.2 eV; and He IIα, 40.8 eV) for a series of molecules. Each molecule contains two different functional groups that are well separated electronically and spatially by an alkane chain linker. The optimum length of the alkane chain between the functional groups is investigated. In this initial study, the functional groups are RCl, RBr, RSH, and RFc, where R is an alkane chain and Fc is ferrocene with an alkane chain bonded to one cyclopentadienyl ring. An additional feature of this study is that each functional group contains a molecular orbital with nearly pure atomic character, allowing comparison to theoretical atomic ionization cross-section ratios. In general, the observed photoelectron intensity changes are more gradual than those predicted by either theoretical atomic photoionization cross sections or by a complete Gelius analysis based on molecular orbital compositions obtained from electronic structure calculations. Experimentally, there is a high degree of transferability of the relative ionization intensities of the chemical functional groups between the molecules of this set. These results indicate that an empirical library of the relative ionization cross sections of chemical functional groups as a function of photon energy will be a useful aid to the study of more complex molecules and will provide an experimental foundation for further theoretical studies of molecular photoelectron cross sections.

Ionization cross section ratios of rare-gas atoms(Ne, Ar, Kr and Xe) by electron impact from thresholdto 1 keV

Electron impact multiple ionization relative cross sections ofrare gas atoms (Ne, Ar, Kr and Xe) have been measured using apulsed electron beam and a pulsed ion extraction combined witha time-of-flight analysis of the charge. The measurements covera larger range of energies and charges than in earlierexperiments, from threshold to 1 keV and from charge n = 1 to 4 (Ne),5 (Ar), 7 (Kr) and 6 (Xe). The average number(γ) of electrons ejected from the atom per ionizationevent is calculated and used to evaluate the contributions ofthe inner-shell processes. The results are also compared withthose in the case of photon impact. As regards the majorions produced, relative uncertainty as low as 1.3% for theionization cross section ratios is achieved for all the samplegases, except near threshold. As for the γ-values,relative uncertainty is estimated to be as low as 1.2%.

Penning and associative ionizationof argon atoms by excited metastable neon

The associative-to-Penning ionization cross section ratio issimulated with a classical model for theNe*(3P2,0)-Ar collision. We used the semi-classicaloptical potential curve of Gregor and Siska together with calculated ab initio Ne-Ar+ potential curves todescribe the entry and exit channels, respectively. Thismodel which had already been adjusted to fit the total crosssection failed to reproduce both the experimental associativeionization cross section and the associative-to-Penning ionization cross section ratio. The contrasting results obtained between theory and experimental data are understood to bethe result of an unsuitable real part of the potential for theentrance channel. We did not attempt to modify the averagesemi-classical potential to fit the experimental data,and it is concluded that an accurate ab initio complexpotential is needed for a quantitative analysis of all theobservable quantities.

Study of cross-sections for the K-shell ionization of atoms by electron and positron impact

In this paper, the K-shell ionization cross-sections of some atoms by electron impact in the energy region less than several ten keV recently measured by our group and by other groups have been compared with the theoretical results of plane-wave Born approximation (PWBA) with Coulomb, relativistic, and exchange corrections as well as the Luo and Joy's theory. These experimental data have also been compared with Casnati et al.'s empirical formula and Deutsch-Margreiter-Märk (DMM) semi-empirical formula, which is modified by us based upon the most recently precisely measured K-shell ionization cross-sections of Cr, Ni, Cu elements of Llovet et al. Moreover, the experimental K-shell ionization cross-section ratios of electron and positron impact have been re-analyzed using the PWBA with Coulomb, relativistic, and exchange corrections. It is found that for most of the elements analyzed in this paper the experimental data are in good or acceptable agreement with the PWBA theoretical results and Luo and Joy's theory and that our modified DMM semi-empirical formula can also provide a good or better description to the experimental data of K-shell ionization cross-section by electron impact.

Detection of sputtered and evaporated carbon aggregates: relative and absolute electron ionization fragmentation yields

The present study is a first attempt to determine electron impact ionization efficiencies for C 2 and C 3. A novel method has been applied to obtain the partial cross-section values for the reactions C 2+e→C +,C 2+e→C 2 + and C 3+e→C +,C 3+e→C 2 + and C 3+e→C 3 +. The neutral target consisting of C, C 2 and C 3 is produced by thermal evaporation from a heated graphite sample and the neutral precursors in the subsequent ionization process can be distinguished by their different flight-time distributions acquired in the evaporation process. The partial ionization cross-section ratios obtained in this experiment have been calibrated with calculated absolute total ionization cross section curves of C 2 and C 3 using the Deutsch-Märk (DM) formalism.

Anisotropy Effects in Methyl Chloride Ionization by Metastable Neon Atoms at Thermal Energies

Quasiclassical trajectory calculations have been carried out for the Penning ionization reaction Ne*(3P2,0) + CH3Cl → CH3Cl+(X, A, B) + Ne + e-. The calculations have been performed, within the rigid rotor approximation, for the translational energy range of 0.04−0.2 eV. The interaction potential of the colliding Ne*−CH3Cl system has been semiempirically estimated. The ionization probability has been determined, within the assumption that ionization occurs at the turning point, using a model which takes into account both geometry of the intermediate complex and ab initio electron density distribution of the orbitals involved on ionization. Calculations carried out for different rotational states (j = 0, 7, 14, 21, and 28, being j = 14 the most populated rotational level at 300 K) manifest the influence of CH3Cl rotation on partial (that is for specific X, A, B electronic states of CH3Cl+) and total ionization. Results averaged for the populated rotational levels at 300 K show that total ionization cross section decreases with translational energy. The branching ratio to X and A states also decreases, while that to the B state increases. These results fairly reproduce previous experimental results on the energy dependence total ionization cross section and branching ratios for the production of CH3Cl+, CH2Cl+, and CH3+ ions.

On the high-energy limit for the ratio of helium double- to single-ionization in collisions with highly charged ions

The established high-velocity limit of the double-to-single ionization cross-section ratio for helium is ≈2.5×10 −3. It was claimed that the highest projectile charge state for which this limit can be reached is around Z p=13. In this Letter we give some arguments that in ultrarelativistic collisions this limit can be reached for noticeably higher projectile charge states.

Ionization of aligned Rydberg atoms by ion impact

The classical trajectory Monte Carlo method has been used to calculate capture and ionization cross sections and final-state electron momentum distributions in collisions to show how the collision mechanisms evolve over the range of intermediate projectile speeds . A circular Rydberg target was aligned in two orientations defined by the orbital angular momentum vector of the electron being either antiparallel or perpendicular to the incident momentum vector of the projectile. Aligning the target atom in this manner helps us to single out specific collision processes otherwise obscured in a randomly oriented atom. A plot of the capture and ionization cross sections shows a crossover in the capture and ionization curves as soft collisions get replaced by harder, more head-on collisions, which leads to more ionization events than captures. The ratio of ionization cross sections, , indicates a further change in collision dynamics around . It is believed that the collision dynamics for become dominated by the geometric size of the target. Plots of the ionized electron final momentum spectra for both alignments, projected onto the projectile-projectile collision plane, show specific types of two- and three-body interactions and show their evolution as a function of . The presence of these interactions can be identified and monitored by observing electrons captured to the continuum, backward scattered and saddle-point electrons, and binary collision rings and how each interaction evolves as a function of .

Measurements of electron-impact ionization cross sections of neon by comparison with photoionization

A method and apparatus have been developed for precise measurements of the ratios of total cross sections for electron-impact ionization and photoionization in rare gases based on the comparison of the total ion yields of the two ionization processes. Low uncertainties for the cross-section ratios are achieved above all by using a cryogenic electrical substitution radiometer as a primary detector standard in the soft-x-ray and vacuum ultraviolet spectral ranges in order to determine the impinging photon flux. On the basis of our measured cross-section ratios and well-known total photoionization cross sections we deduce absolute total electron-impact ionization cross sections of rare gases. As a result we present ratios of total electron-impact ionization cross sections to total photoionization cross sections and deduce total electron-impact ionization cross sections of neon in the energy range of electrons from 140 to 4000 eV and of photons from 100 to 1500 eV. Relative uncertainties as low as 1.3% for the cross-section ratios and 2.8% for the total electron-impact ionization cross sections have been achieved. A comparison of our cross-section data with published experimental and theoretical data is presented.

The investigation of the multiple ionization of argon by energetic oxygen ions

The cross section ratios of the double, triple and quadruplicate ionization to the single ionization of argon were measured by 1.5–7.5 MeV partially stripped oxygen ion ( q = 1–4) bombardment. The Auger and the charge exchange processes are discussed and subtracted from the total multiple ionization ratios in order to study the mechanism of the direct multiple ionization of argon by partially stripped ions in the present energy range. It is found that the obtained ratios of the multiple ionization cross sections to those of the direct single ionization of argon agree well with the (ν q eff ) −2n+2 law for 1.5–7.5 MeV O q+ ( q = 1–4) impinging ions.

On the accuracy of the L-subshell ionization cross sections for proton impact I. Spectrum fitting

The L X-ray spectrum of thorium following 1 MeV proton impact has been measured using a Si(Li) detector. The spectrum was analysed by various commonly used procedures. It is shown that the separation of the L γ group into X-ray transitions of L 1 and L 2 subshells is affected by the natural lineshape model. The common practice of neglecting the natural lineshape (Lorentzian or other shapes where there are many-body effects) introduces systematic errors which are larger than the frequently-cited error bars. In the example used here, the effects of neglecting natural lineshape are similar in size to the difference in theoretical L 1/L 3 and L 2/L 3 subshell ionization cross section ratios predicted for protons by the ECPSSR and coupled channel calculations. Similar systematic errors will arise in other contexts. The optimum accuracy inherent in Si(Li) spectroscopy will only be realised if full account is taken of natural lineshapes in the treatment of the X-ray spectra.

The ionization of helium by 1.5–7.5 MeV partially stripped C q+ ions

The double to single ionization cross section ratios of helium by partially stripped carbon ions have been measured for projectile charge states ranging from +1 to +4 and impact energies from 1.5 to 7.5 MeV. The effective charge effect in partially stripped ion-helium collision is presented. It is found that the effective charge q eff increases as the impinging energy increases and q eff shows a modest dependence upon the projectile charge state in the present energy range. The projectile charge state, projectile and target electronic state dependences of the effective charge effect is discussed using orbital interpenetrating. The nCTMC calculations were carried out to investigate the energy dependence of the effective charge. By introducing effective charge the cross section ratio of helium obtained in the present work is in good agreement with the ( V q eff ) −2 law.