Electron-induced Ionization Research Articles

Orientation- and alignment-effects in electron-induced ionization of single-oriented water molecule

We here report the first theoretical study about the orientation-effect on the total ionization cross sections for single oriented water molecule. The theoretical description of the ionization process is performed within the 1st Born framework with a collisional system including an initial state composed of a projectile and a water target molecule described by a plane wave and an accurate one-center molecular wave-function, respectively, and a final state constituted by a slow ejected electron represented by a Coulomb wave and a scattered (fast) electron projectile described by a plane wave. Secondary electron energetic distributions and total cross sections are then compared for particular target configurations.

Double ionization of the water molecule: Influence of the target orientation on the secondary-electron angular distributions

Fivefold differential cross sections for electron-induced double ionization of isolated oriented water molecules are reported. The theoretical investigation is performed within the first Born approximation by describing the initial molecular state by means of single-center wave functions. The contributions of each final state to the double-ionization process, i.e., with target electrons ejected from similar and/or different molecular subshells, are studied and compared in terms of shape and magnitude. Furthermore, for the particular target orientations investigated, we identify clearly the signature of the main scenarios involved in (e,3e) reactions, namely, the shake-off and the two-step 1 mechanisms.

Orientation and alignment effects in electron-induced ionization of a single oriented water molecule

We here report a theoretical study about the orientation effect on the total ionization cross sections for a single oriented water molecule. The theoretical description of the ionization process is performed within the first Born framework with a collisional system including an initial state composed of a projectile and a water target molecule described by a plane wave and an accurate one-center molecular wave function, respectively, and a final state constituted by a slow ejected electron represented by a Coulomb wave and a scattered (fast) electron projectile described by a plane wave. Secondary electron energetic distributions as well as total cross sections are then compared for particular target configurations pointing out strong alignment and orientation effects on the description of the ionization process.

Electron scattering with metastable H*2(c3Πu) molecules: ionization and other total cross sections

In this paper, we report total cross sections for electron-induced ionization and excitations from both the excited metastable state H*2(c3Пu) and the electronic ground state of H2. Total cross sections are reported for incident energies from the ionization threshold to 2000 eV using the ‘complex scattering potential-–ionization contribution’ method and the relative importance of various scattering channels, including super-elastic scattering and ionization, are identified. There appear to be no previous data on the ionization cross sections for the metastable target H*2(c3Пu). This work has potential applications in the fields of plasma physics and planetary atmospheres.

Determining high precision, in situ, oxygen isotope ratios with a SHRIMP II: Analyses of MPI-DING silicate-glass reference materials and zircon from contrasting granites

The development of new techniques and instrumentation on the ANU SHRIMP II ion microprobe has made it possible to measure the oxygen isotope ratios of insulating and conducting phases (e.g. silicates, carbonates, phosphates and oxides) on a 25 µm scale with better than 0.4‰ precision and accuracy at 95% confidence. Instrumentation changes include the installation of a multiple collector, charge neutralization using an oblique-incidence low-energy electron gun, and the addition of Helmholtz coils to counter mass dispersion by the Earth's magnetic field. A redesign of sample mounts and mount holders has effectively eliminated differences in variable isotope fractionation across the mount surface during analysis. Techniques have been developed to minimize the effect of electron-induced secondary ionization of oxygen. During a 6-minute analysis involving 100–140 s of data collection, δ 18O values can be measured on one 25 µm spot with an internal precision of better than 0.2‰ (2 standard errors). Analyses of MPI-DING silicate-glass reference material demonstrate that the external reproducibility of single spots can be better than 0.4‰ at 95% confidence, and that for matrix-matched samples and reference material, accuracy is commensurate with precision. MPI-DING glasses are acceptable ion microprobe reference materials for oxygen isotope measurements of glasses, although KL2-G is possibly heterogeneous. Zircon reference materials TEMORA 2 and FC1 appear to be acceptable as preliminary oxygen isotope reference materials. SHRIMP II analyses of FC1 indicate that it has a δ 18O value of 5.4‰ (VSMOW). Analyses of zircon oxygen isotopic compositions from a gabbro, a tonalite and a granodiorite from southeastern Australia are presented. Zircon from the gabbro has a δ 18O value of 5.6‰, the tonalite has an I-type affinity and slightly heterogeneous δ 18O values around 6.6‰, and the granodiorite has an S-type affinity and a range of igneous, melt precipitated zircon δ 18O values between 8.2 and 10.2‰. These results suggest that the gabbro is mantle-derived and slightly contaminated with crustal material, and that the I-type granodiorite has evolved in a similar manner from a mantle-derived source. The δ 18O values of the zircon from the S-type granodiorite are not only higher than from the I-type, but also more heterogeneous, consistent with partial melting of a poorly-mixed, metasedimentary source.

Doppler effect in fluorine K-Auger line produced in electron-induced core ionization of SF6

An experimental evidence is reported on the observation of the Doppler effect in fluorine K-Auger line emitted from a core-ionized SF6 molecule under an impact of 16 keV electrons. The emitting source of the Auger line is found to acquire a kinetic energy of 4.7+/-0.3 keV. We propose that such large energy is released from the Coulomb repulsion taking place between F+ and SF5+ fragment ions under influence of an intense focusing field of the incident electrons. In the presence of the Coulomb field of these ions, the Auger line obtains a polarization P = 76%+/-7%.

A high-frequency electrospray driven by gas volume charges

High-frequency (>10kHz) ac electrospray is shown to eject volatile dielectric liquid drops by an entirely different mechanism from dc sprays. The steady dc Taylor conic tip is absent and continuous spraying of submicron drops is replaced by individual dynamic pinchoff events involving the entire drop. We attribute this spraying mechanism to a normal Maxwell force produced by an undispersed plasma cloud in front of the meniscus that produces a visible glow at the spherical tip. The volume charge within the cloud is formed by electron-induced gas ionization of the evaporated liquid and produces a large normal field that is much higher than the nominal applied field such that drop ejection occurs at a voltage (at high frequencies) that is as much as ten times lower than that for dc sprays. The ejection force is sensitive to the liquid properties (but not its electrolyte composition), the ac frequency and trace amounts of inert gases, which are believed to catalyze the ionization reactions. As electroneutral drops are ejected, due to the large (>100) ratio between individual drop ejection time and the ac frequency, this mechanism can produce large (microns) electroneutral drops at relatively low voltages.

Measurement of Zinc K-Shell Ionization Cross Sections by Electron Impact

Electron-induced Zn ionization cross sections, which are scarce, have been obtained from measurement of Kα x-ray emission at energies from near threshold to 25 keV. The influence of substrate of thin target on ionization cross sections has been corrected by using the bipartition model of electron transport. The experimental results are satisfactory as compared with empirical formula of Green and Cosslett.

Impossibility of distinguishing between identical particles in quantum collision processes

The very successful convergent close-coupling (CCC) method [I. Bray and D. V. Fursa, Phys. Rev. A $54,$ 2991 (1996)] for calculating cross sections for the ionization of atoms by electron impact is examined for the case of hydrogen ionization. The theory is recast in an operator theoretic form, allowing explicit formulas to be given for the solutions of the truncated CCC equations. The limit to which these solutions tend as ever more terms are included in the truncation is shown to be the exact transition operator for the system. This analytical result is not consistent with the numerical CCC results, which yield amplitudes that violate the symmetrization principle of quantum mechanics and which depend for success on treating the electrons as classically, rather than quantally, indistinguishable particles. This new result is also inconsistent with the ``step function hypothesis'' [I. Bray, Phys. Rev. Lett. $78,$ 4721 (1997)] that was introduced to restore the consistency of the CCC calculations with the symmetrization postulate. In addition, the CCC method does not include long-range effects of the Coulomb interaction that are expected to be important in certain kinematic configurations at low energies. Consequently, it is not justified yet to interpret the numerical successes of the CCC method as confirmation of a fundamental dynamical understanding of electron-induced atomic ionization.

Absolute density determination of CH radicals in a methane plasma

Laser-induced fluorescence was applied at the B–X transition of the CH radical to measure absolute densities of the CH radicals in an electron cyclotron resonance methane plasma. The absolute experimental uncertainty is only about 30% due to a new calibration procedure. The CH density correlates well with optical emission from the CH A–X and B–X transitions over a wide pressure range. Experimental results are in satisfactory agreement with predictions from a model based on rate equations for the electron-induced dissociation and ionization of the parent methane gas. This model also includes the interaction of the plasma species with the surrounding walls and the particle transport due to pumping.

Angular momentum branching ratios for electron-induced ionization: Atomic and model calculations.

We present calculations of the matrix elements for electron-induced ionization of core electrons of atoms. We use both self-consistent atomic potentials for accuracy and model potentials to gain physical insight. We pay particular attention to the angular momentum distribution of the two final-state electrons, especially when one of them lies near what would be the Fermi energy in a solid (i.e., as in an absorption fine-structure experiment). For nodeless core wave functions, in the dominant channel both final-state electrons have angular momentum one greater than that of the initial core state. For sufficiently deeply bound states, this first approximate selection rule holds until the incident electron energy exceeds the ionization threshold by at least 500 eV, i.e., over the experimentally relevant range. It is also possible to determine the angular momentum distribution of the final-state electron. The EXAFS-like electron tends to have angular momentum one greater than that of the initial core state, even in some cases where the first approximate selection rule does not hold. (EXAFS is extended x-ray-absorption fine structure.) The strongest trend is that the dipole component in a partial-wave expansion of the Coulomb interaction dominates the matrix element. In these studies, careful treatment of not just the core state but also the unbound states is crucial; we show that the conventional orthogonalized plane-wave approximation is inadequate, giving incorrect ordering of the channels. For model potentials with an adjustable screening length, low-lying bound resonances are found to play an important role.

Auger yields in a distorted wave theory for the excitation autoionization of Li and Be sequence ions

We compute the excitation-autoionization contribution to the cross-section for electron-induced single ionization of Li and Be sequence target ions, using distorted wave theory and LS coupling. Excitations of 1 s target electrons to nl, with n = 2, are considered explicitly, and Auger yields are obtained for these excited states. Results are compared with experiment and with theoretical predictions based on other methods.

Cross-section and rate formulas for electron-impact ionization, excitation, deexcitation, and total depopulation of excited atoms

For electron-induced ionization, excitation, and deexcitation, mainly from excited atomic states, a detailed analysis is presented of the dependence of the cross sections and rate coefficients on electron energy and temperature, and on atomic parameters. A wide energy range is covered, including sudden as well as adiabatic collisions. By combining the available experimental and theoretical information, a set of simple analytical formulas is constructed for the cross sections and rate coefficients of the processes mentioned, for the total depopulation, and for three-body recombination. The formulas account for large deviations from classical and semiclassical scaling, as found for excitation. They agree with experimental data and with the theories in their respective ranges of validity, but have a wider range of validity than the separate theories. The simple analytical form further facilitates the application in plasma modeling.

Angular dependence of proton kinetic-energy distributions from dissociative ionization ofH2

Results are reported on the angular dependence of the ${\mathrm{H}}^{+}$ translational kinetic-energy distributions resulting from electron-induced dissociative ionization of ${\mathrm{H}}_{2}$. These measurements confirm a fact clearly implied by earlier measurements, namely, that considerable variation of the energy distributions with angle can occur even for electron energies as much as 40 eV above threshold, and bring into question conclusions previously drawn from comparison of ion and/or high-Rydberg-excited fragment energy distributions measured at widely separated angles.

Theory of High Field Conduction in a Dielectric

A dimensionless solution is given for continuous current flow in a solid dielectric, which undergoes electron-induced collision ionization. A method is developed for finding the current-voltage characteristic of the dielectric from this solution, given the emission function of the cathode and a blocking anode. The results are used to infer the thickness dependence of dielectric breakdown strength based on the model considered.

Ionization and Displacement Damage in Silicon Transistors

An investigation of electron-induced ionization and displacement damage in N-P-N and P-N-P transistors at medium and low powers is presented. Measurements of base and collector currents as a function of base-emitter voltage with radiation as a parameter were made. The transistors were irradiated sequentially with 125 keV and 1 MeV electrons. Initial saturation of surface-damage effects with 125 keV made possible the separation of displacement and surface damage. The results show that the dominant displacement damage occurs within the emitter-base transistor region for fluence levels less than or equal to 5×1015 1 MeV e/cm2 and emitter currents less than or equal to 10 milliamperes in the 2N2102 and 2N1132 transistors. Above this fluence level, displacement damage in the bulk of the base region becomes effective. Isochronal and isothermal annealing measurements indicate that the results can be explained by attributing the damage in the transistion region to recombination current losses at K-centers for N-P-N transistors and A-centers for P-N-P transistors.

Electron probe surface mass spectrometry study of CO on molybdenum

The technique of electron probe surface mass spectrometry has been utilized to study the system CO on polycrystalline molybdenum in the temperature range 300 to 1500 °K. The adsorption of the α and β phases of CO has been studied and is shown to be adequately described by the Roberts-Miller theory. Electron induced surface ion desorption of O s + is correlated with adsorbed CO indicating a cross section for the α phase CO of σ α ( O s +)=3×10 −18 cm 2 and for the β phase CO of σ β ( O s +)=6×10 −21 cm 2. Desorbed surface ions of CO and C are not observed. The electron-induced surface ionization efficiency curves for the α and β phases are very similar, both having a threshold of 20 eV. The results obtained are interpreted to indicate that the primary interaction of electrons with adsorbed CO may not be significantly different from that occurring when electrons interact with gas phase CO.