Indirect Ionization Research Articles

Using Volatile Solvents for Ion Formation in Liquid Molecular Beam Expansion Mass Spectrometry

Cocrystallization between analyte and matrix is required by matrix-assisted laser desorption/ionization and can represent a significant limitation of the technique. A molecular beam expansion, mass spectrometric method has been developed to explore the possibility of using pure solvents as matrix to avoid cocrystallization. Two kinds of solvent, liquid CS2 and liquid or supercritical CO2, have been studied with 266-nm UV laser irradiation. We successfully ionized a number of compounds, including caffeine, guanine, cholesterol, and mixed fullerenes. Under some conditions, the mass spectra reflect parent radical cations formed by photoionization. Under other conditions, protonated, sodiated (and with CS2 even sulfated) ions are seen reflecting a nonunimolecular ionization process. When UV-transparent CO2 is used as a solvent, only analyte molecules with a UV chromophore are detected. However, with UV-absorbing CS2, we demonstrate ionization of molecules lacking a UV chromophore. This work provides strong evidence that one can form solvent clusters containing analyte, that laser photoionization of the solvent precedes ionization of the analyte, and that solvent evaporation along with the indirect ionization leads to reduced parent ion fragmentation. The exploration of this now demonstrated concept with other solvents would appear fruitful for future work.

Electron impact double ionization of Fe+ and Fe3+

Electron impact double ionization cross-sections for Fe+ and Fe3+ have been calculated in the modified binary encounter model incorporating the effects of the Coulombic field of the target on the incident electron. Accurate expression of cross-section for energy transfer ΔE given by Vriens and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. The present results show satisfactory agreement with experimental observations. It is concluded that the discrepancy in the high energy region observed in the present calculations may be attributed to non-inclusion of indirect ionization processes arising from the L-shell.

Cross sections for ionization of Mo and Mo + by electron impact

Theoretical cross sections for electron-impact ionization of the neutral Mo atom and Mo + ion are reported. Both Mo and Mo + have many metastable levels near the ground level. We calculated ionization cross sections from the ground and two lowest metastable levels for both Mo and Mo +. The total ionization cross sections for each initial state consist of direct and indirect ionization cross sections. The direct ionization cross sections were calculated by using the binary-encounter Bethe (BEB) model. The indirect ionization cross sections resulting from numerous excitation–autoionization processes were calculated by using scaled Born cross sections for the excitation of 5s electrons to 5p, 4d electrons to 5p, and 4p electrons to 4d or 5s, whose excitation energies exceeded the lowest ionization energy of the initial states of Mo and Mo +. For the ground-state Mo, indirect processes contribute only about 5%, while for metastable Mo, indirect processes contribute almost 30% to the total ionization cross section. Unlike our experience with light atoms, contributions to indirect ionization from spin-forbidden and Δ n = 1 excitations, where n is the principal quantum number, are noticeable. For Mo + indirect processes contribute less than 15%. There are no experimental data to compare for Mo. The distorted-wave Born cross sections for Mo available in the literature accounted for direct ionization only, and hence are lower than in the present work at higher incident electron energies where excitation–autoionization contributes significantly. Our total cross section for the single ionization of Mo + is in good agreement with two sets of experimental data available in the literature, while the distorted-wave Born cross section reported with one of the experiments is almost a factor of two higher than the experiments at the cross section peak.

Complete valence double photoionization of SF6

Single photon double ionization of SF(6) has been investigated at the photon energies 38.71, 40.814, and 48.372 eV by using a recently developed time-of-flight photoelectron-photoelectron coincidence spectroscopy technique which gives complete two-dimensional e(-)-e(-) spectra. The first complete single photon double ionization electron spectrum of SF(6) up to a binding energy of approximately 48 eV is presented and accurately interpreted with the aid of Green's function ADC(2) calculations. Spectra which reflect either mainly direct or mainly indirect (via interatomic coulombic decay of F 2s holes) double ionization of SF(6) are extracted from the coincidence map and discussed. A previous, very low value for the onset of double ionization of SF(6) is found to energetically coincide with a peak structure related to secondary inelastic scattering events.

On-line capillary electrophoresis-mass spectrometry using dopant-assisted atmospheric pressure photoionization: Setup and system performance

The on-line coupling of capillary electrophoresis (CE) and mass spectrometry (MS) via atmospheric pressure photoionization (APPI) is demonstrated. To achieve CE-APPI-MS, an adapted coaxial sheath-flow interface was combined with an ion-trap mass spectrometer equipped with an APPI source originally designed for liquid chromatography-MS. Effective photoionization of test compounds was accomplished after optimization of several interface and MS parameters, and of the composition and flow rate of the sheath liquid. Further enhancement of the ionization efficiency could be achieved by adding a dopant, such as acetone or toluene, to the sheath liquid to aid indirect ionization. Acetone significantly increased the ionization of the polar test compounds by proton transfer, while toluene was more useful for the enhanced formation of molecular ions from nonpolar compounds. The effect of several common CE background electrolytes (BGEs) on the APPI-MS response of the analytes was also studied. It appeared that in contrast with electrospray ionization, nonvolatile BGEs do not cause suppression of analyte signals using APPI. Therefore, in CE-APPI-MS, a variety of buffers can be chosen, which obviously is a great advantage during method development. Remarkably, also sodium dodecyl sulfate (SDS) did not affect the photoionization of the test compounds, indicating a strong potential of APPI for the on-line coupling of micellar electrokinetic chromatography (MEKC) and MS.

Electron-impact ionization of Ti3+ ions

Absolute cross sections for electron-impact single ionization of potassium-like titanium have been measured from threshold to 1000 eV using a crossed-beams experimental set-up. A 0.5 eV resolution energy-scan technique revealed structures in the cross section in the range from 40 eV to 150 eV due to dominant contributions of indirect ionization mechanisms. A large fraction of the cross section is due to excitation–autoionization proceeding via intermediate excited states 3p5(3d2 3F) 2F, 3p5(3d2 3P) 2P, 3p5(3d2 3F) 2D which subsequently decay by super-Coster–Kronig transitions. The results of the measurements are in satisfactory agreement with previous advanced theoretical calculations.

Strong contributions of indirect processes to the electron-impact ionization cross section of Sc^{+} ions

We present experimental measurements and theoretical calculations for the electron-impact single ionization cross section of ${\mathrm{Sc}}^{+}$ ions covering an energy range from threshold to $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. An electron-ion crossed-beams setup was employed for the measurements of absolute cross sections as well as for a high-resolution energy scan to uncover fine details in the energy dependence of the cross section. Direct ionization is described by configuration-averaged distorted-wave theory and indirect ionization by $R$-matrix theory. Indirect processes contribute to the total ionization cross section by up to $\ensuremath{\sim}40%$. This finding is related to the existence of strong $3p\ensuremath{\rightarrow}3d$ excitation channels in ${\mathrm{Sc}}^{+}(3{p}^{6}3d4s)$. The shape of the related cross-section feature is reminiscent of the very strong $4d\ensuremath{\rightarrow}4f$ excitation, found in the ionization of xenon and its neighboring elements.

Electron-impact single and multiple ionization of Mg+ ions

Absolute cross sections for single, double and triple electron-impact ionization of Mg+ ions have been measured by employing a crossed-beams experimental set-up. Contributions of indirect resonant processes to the total cross section for single ionization such as resonant-excitation–double-autoionization and resonant-excitation–auto-double-ionization have been observed utilizing a high-resolution energy scan technique with statistical uncertainties as low as 0.04%. The experimental cross sections are compared with results of recent R-matrix calculations showing agreement in the overall features but pronounced differences in the details of indirect ionization contributions. Double ionization was found to be dominated by the direct removal of a 2p-electron with subsequent autoionization. There are also indications of indirect processes in the triple ionization of Mg+ but no clear evidence could be established.

Onset corona fields and electrical breakdown criteria

Predictions are made of the electric field necessary for the onset of corona in air for spheres and cylinders from the general breakdown criterion, ; α′ is the net ionization coefficient, r is the radius and Q is a constant. It is found that Q = 104 gives a fair fit to experimental results for both wires and points, and for positive and negative corona, for radii varying from 0.01 to 20 cm. By assuming a linear variation of α′ with the difference of the electric field from the critical field where ionization equals attachment, an analytic formula can be derived for the breakdown field for the onset of corona as a function of the radius of the sphere, which is similar to, but different from, Peek's formula for the onset of corona for cylinders or wires. The formulae also give the breakdown field as a function of temperature and pressure. For Townsend and Streamer breakdown mechanisms, values of Q are typically 3 × 104 and 108, respectively. Thus the value of Q derived from experimental values of the onset field for corona for both wires and points is significantly lower than would be expected from either the Townsend breakdown criterion or the Streamer breakdown criterion. It is suggested that the most likely collision process to explain the observed low value of Q is indirect ionization, for example by collisions between excited molecules.

A semi-empirical concept for the calculation of electron-impact ionization cross-sections of neutral and ionized fullerenes

We have developed a semi-empirical approach to the calculation of cross-section functions (absolute value and energy dependence) for the electron-impact ionization of several neutral and ionized fullerenes C60q+ (q=0–3), for which reliable experimental data have been reported. In particular, we propose a modification of the simplistic assumption that the ionization cross-section of a cluster/fullerene is given as the product of the monomer ionization cross-section and a factor ma, where “m” is the number of monomers in the ensemble and “a” is a constant. A comparison between our calculations and the available experimental data reveals good agreement for q=0, 1, and 3. In the case of the ionization of C602+ (q=2), our calculation lies significantly below the measured cross-section which we interpret as an indication that additional indirect ionization processes are present for this charge state.

Monte Carlo simulation study of the Fano factor, w value, and energy resolution for the absorption of soft x rays in xenon–neon gas mixtures

Xenon gas proportional-scintillation counters (GPSC) have many applications in the detection of soft x rays where their energy resolution, R, is comparable to solid-state detectors when large window areas are required. However, R is known to deteriorate for energies Exr below 2–3 keV due to electron loss to the entrance window. Since the addition of a lighter noble gas increases the absorption depth, we have investigated the use of Xe–Ne gas mixtures at atmospheric pressure as detector fillings. The results of a Monte Carlo simulation study of the Fano factor, F, the w value, and the intrinsic energy resolution, R=2.36(Fw/Exr)1/2, are presented for Xe–Ne mixtures and pure Xe and Ne. The results show that the addition of Ne to Xe reduces the intrinsic energy resolution ℛ but this never compensates for the reduction in scintillation yield in GPSC applications, implying that the instrumental energy resolution R will only improve with the addition of Ne when electron loss to the window in pure Xe is significant. The simulation reproduces the photoionization process of the Xe and Ne atoms, the vacancy cascade decay of the residual ions, and the elastic and inelastic scattering of electrons by the gas atoms. The contribution of energy and charge transfer mechanisms such as Penning, associative, and transfer ionization is discussed in detail. It is shown that Penning and associative ionization are the crucial indirect ionization processes which determine the behavior of F and w at low concentrations of Xe. The importance of the nonmetastable Ne states is also assessed.

Autoionizing resonances in electron-impact ionization ofO5+ions

We report on a detailed experimental and theoretical study of electron-impact ionization of ${\mathrm{O}}^{5+}$ ions. A high-resolution scan measurement of the K-shell excitation threshold region has been performed with statistical uncertainties as low as 0.03%. At this level of precision a wealth of features in the cross section arising from indirect ionization processes becomes visible, and even interference of direct ionization with resonant-excitation/auto-double-ionization (READI) is clearly observed. The experimental results are compared with R-matrix calculations that include both direct and indirect processes in a unified way. Radiative damping of autoionizing Li-like states is found to be about 10--15 %. The calculations almost perfectly reproduce most of the experimental resonance features found in the present measurement including READI. They also agree with the direct-ionization converged close-coupling results of I. Bray [J. Phys. B 28, L247 (1995)] and the absolute total ionization cross section measurement of K. Rinn et al. [Phys. Rev. A 36, 595 (1987)].

Slow photoelectron imaging.

Experiments are reported on the detection of slow photoelectrons resulting from the photoionization of Xe atoms in a dc electric field by electron imaging. In the far-field photoelectron velocity distributions we can distinguish between direct and indirect ionization processes (involving long range Coulomb interactions with the Xe+ ion). Also, a new modulation of the velocity distribution is observed which cannot be explained by previously discussed mechanisms. Classical and quantum mechanical calculations are presented to support the interpretations.

A collisional radiative model for mercury in high-current discharges

A collisional radiative model is presented for mercurydischarges with electron temperatures between 0.75-2 eV and electrondensities between 1018-1020 m-3. Such plasma parametersare encountered in a number of modern light sources, such asmercury-operated induction lamps and the compact fluorescent lamp. Theanalytical top model has been used, which allows the majority of thenon-equilibrium levels to be taken into account implicitly. As a result,indirect ionization processes involving highly excited atomic mercurystates are taken into account in spite of the relatively low number oflevels (19) which has been considered.The influence of higher atomic mercury levels on the ionization ratecoefficient has been carefully analysed, and has been found to contributesignificantly. Furthermore, a consistent means of quantifying theproduction of radiation by the plasma will be presented by introducing thespecific effective emissivities. These enable one to express thetotal radiated power in terms of the densities of the transport-dominatedstates and the electron density and temperature. These coefficients, aswell as the net coefficients of ionization and recombination, will bepresented and discussed, enabling their usage in plasma transport models.

Effects of indirect ionization on the charge state distributions observed with highly charged ion sources

Presently, most charge state distributions produced with highly charged ion sources are predicted with models that approximate the ionization process with the Lotz formula. The Lotz ionization cross sections decrease approximately geometrically with increasing charge state except for ions with very few vacancies, for ions with very few electrons, and for electron impact energies which barely exceed the ionization energy. The geometrical decrease causes these models to predict a maximum abundance for most of the charge states, which is only weakly dependent on the charge state. Experimental results, however, yield much higher abundances for ions with an empty M shell than ions with a partly filled M shell. This difference is explained with indirect ionization processes that are neglected by the Lotz approximation, and normally can be neglected for the ionization of the L shell, but can dominate the ionization of the M shell.

Direct and indirect ionization processes near the Li `hollow-atom' threshold

Using a unified theoretical treatment of different ionization processes occurring in photon and electron impact on atoms and ions, we examine lithium near its `hollow-atom' threshold. By introducing a set of `core base states' for the target, and optimizing the collisional representation within an R-matrix model, we combine ab initio, outer- and inner-shell effects, excitation and ionization channels, direct and indirect ionization processes, resonant and non-resonant behaviour, for photon and electron impact processes. We use the method to interpret experimental data on electron impact ionization of ground state and photoionization of Li, and look at double photoionization of Li and electron impact ionization out of metastable . We examine and quantify hollow-atom interference effects and resonances, tabulating all 480 states for , and show the chaotic behaviour of overlapping series.

Electron impact ionization cross section of Ar +, Kr +, In + and Ge +

The single electron impact ionization cross sections of Ar +, Kr +, In + and Ge + were measured on the improved electron-ion impact apparatus at Fudan University. The Ar +, Kr + and In + experimental results are in good agreement with data from other laboratories. The cross section of Ge + was measured for the first time. The experimental results indicate that the indirect ionization process gives a larger contribution to the total ionization cross section of In +, as compared to the other three targets. The experimental data were compared with our Distorted-Wave-Born-Exchange-Approximation (DWBEA) calculation, the Lotz formula, and Configuration-Averaged-Distorted-Wave (CADW) calculations.

Competing photodissociation pathways for physisorbed O 2/graphite investigated using Ar spacer layers

We have studied photon stimulated ion desorption from O 2 physisorbed on graphite, using synchrotron radiation in the energy range 16–40 eV. Ar spacer layers were employed in order to distinguish between direct and indirect (photoelectron driven) dissociation channels, specifically, (i) direct dipolar dissociation, (ii) direct dissociative ionization, (iii) photoelectron driven dissociative attachment and (iv) photoelectron driven dipolar dissociation. We find that the O − yield is mainly due to direct dipolar dissociation, while for O + direct dipolar dissociation and both direct and indirect dissociative ionization are identified. The Ar layer reduces the image potential arising from the graphite substrate. Some dissociation pathways in the energy region 24–27 eV are enhanced for O 2/graphite compared with the gas phase.

137 - Solid-state detector dosimetry for male gonads radioprotection in radiotherapy

Basic clinical interest and, in some cases, regulatory recommendations request measurement or, at least, estimation of the dose to the male gonads in radiotherapic treaments. Subdiaphragmatic, rectal and post-surgical seminomatarget treatments in adults have been taken into account. The proximity of target volume or the particular request for fertility evaluationmake the problem more relevant. Solid-state detectors give interesting opportunities for in vivo dosimetryout of the therapeutic beams. In vivo and in phantom dosimetry have been performed, in order to set direct data and indirect estimates Ionization chamber measurements have been collected and used as reference values for semiconductors calibration Also thermoluminescent dosimetry has been used for a comparison. Feasibility, accuracy and reproducibility results show that the use of solid state dosimetry for radioprotection in radiotherapy is very stimulating A procedure for the evaluationof male gonads dose in radiotherapy is proposed Solid-state and thermoluminescent dosimetry in some clinical cases have been compared with our protocol evaluations, showingencouragingresults

Alignment and Orientation Effects for the Indirect Ionization of CH3Cl in the |111.rtbbrac. Rotational State by Fast Electron Impact

Alignment and Orientation Effects for the Indirect Ionization of CH3Cl in the |111.rtbbrac. Rotational State by Fast Electron Impact