Excitation Of Positive Ions Research Articles

Excitation of positive ions by low‐energy electrons: Relevance to the Io torus

The importance of measuring electron‐ion excitation cross sections in singly and multiply charged positive ions is outlined, and recent results for Mg II and O II ions are given using the Jet Propulsion Laboratory's electron energy‐loss merged‐beams apparatus. For Mg II, collision strengths are presented for the 3s 2S → 3p 2P(h, k) resonance transition at electron energies from threshold to approximately 9 × threshold (40 eV). Theoretical comparisons are also given with two five‐state close‐coupling calculations. The energy variation of the collision strength is fitted with a semiempirical analytic function which includes approximations to polarization, resonance, and exchange contributions. In O II, first spectra anywhere of electron excitation of the optically allowed 2s22p3 4S0 → 2s2p4 4P (14.88 eV) and → 2p23s 4P (22.98 eV) transitions are presented. In addition, excitations of the two low lying, optically forbidden transitions 4S° → 2p3 2D0 (3.33 eV) and → 2p3 2P0 (5.02 eV) are detected for the first time.

Excitation of positive ions by heavy particles.

The Coulomb-Born approximation has been used to study the excitation of positive ions by heavy-particle impact. Slater-type wave functions are used for the ionic target states. This choice enables us to calculate the excitation rate coefficients between arbitrary initial and final states. We calculate the differential and total cross sections and alignment parameters for excitation of ${\mathrm{He}}^{+}$ ions from the ground state to the 2p state by proton impact. The alignment parameters for the projectile velocity range considered here are found to be positive in contrast to negative values of the corresponding parameters for electron-impact excitation of the same target states.

A New AID for Interpolating and Assessing Collision Strengths and Rate Coefficients

AbstractA new way of critically assessing and compacting data for electron impact excitation of positive ions is proposed. This method allows one (i) to detect possible printing and computational errors in the published tables, (ii) to interpolate and extrapolate the existing data as a function of energy or temperature, and (iii) to simplify considerably the storage and transfer of data without significant loss of information. Theoretical or experimental collision strengths Ω(E) are scaled and then plotted as functions of the colliding electron energy, the entire range of which is conveniently mapped onto the interval (0,1). For a given transition the scaled Ω can be accurately represented - usually to within a fraction of a percent - by a 5 point least squares spline. Further details are given in (2). Similar techniques enable thermally averaged collision strengths upsilon (T) to be obtained at arbitrary temperatures in the interval 0 < T < ∞. Application of the method is possible by means of an interactive program with graphical display (2). To illustrate this practical procedure we use the program to treat Ω for the optically allowed transition 2s → 2p in ArXVI.

Convergence of the Born and Coulomb-Born approximations for the electron-impact excitation of positive ions.

The convergence of the Born and Coulomb-Born approximations for the electron-impact excitation cross sections of positive ions was examined both formally and by sample calculations on several lithiumlike ions. It was found that the convergence of the two approximations with increasing energy is not necessarily monotonic and may not occur until many tens of threshold units. An analytic expansion of the Coulomb partial waves, in terms of spherical Bessel functions, indicates that partial-wave collision strengths in the two approximations do not necessarily converge in the limit of high angular momentum, although agreement at high incident energies occurs for all partial cross sections. Criteria for the use of the Born approximation for highly ionized atoms are discussed.

Erratum: Plasma screening effects on proton-impact excitation of positive ions

Erratum: Plasma screening effects on proton-impact excitation of positive ions

Convergence of the Coulomb-Bethe approximation for electron impact excitation of positive ions

Collision strengths of the transition Mg+ 3s-3p produced by electron impact have been calculated in the close-coupling (CC) and Coulomb-Bethe (CBe) approximations. The total collision strengths are sums of partial-wave contributions, Omega (3sl,3pl') with l'=l+or-1. It is shown that, for large l, the main source of error in the CBe partial collision strengths arises from neglect of quadrupole coupling between 3p(l-1) and 3p(l+1).

Excitation of positive ions in Coulomb-Born approximation

We have presented a method for the evaluation of the Coulomb-Born matrix element between arbitrary initial and final Slater orbitals. Since for the two- or more-electron ionic target the wave functions may be constructed by the combination of the Slater orbitals of various angular momenta, one can easily apply this method to calculate the cross sections for the electron-impact excitation of a complex ion from an arbitrary initial state to an arbitrary final state. As an example we have calculated the excitation cross sections of the ${\mathrm{He}}^{+}$ ion from the ground state to the $4f$ state at several energies.

Electron impact excitation of positive ions in the Coulomb-Born-Bely approximation: calculation for helium-like ions

The Coulomb-Born-Bely approximation, which is an extension of the Ochkur-Rudge approximation to the excitation of the positive ion by Bely (1966) has been used to obtain the cross sections for electron impact excitation of helium-like ions (3<or=Z<or=10, Z=14, 20 and 26). The present cross sections for 11S to 21S and 11S to 21P transitions have been compared with those of the Coulomb-Born, Coulomb-Born-Oppenheimer and distorted-wave approximation.

Absolute emission cross section for electron-impact excitation ofGa+to the4P1level

Crossed beams of electrons and ${\mathrm{Ga}}^{+}$ ions have been used to measure the absolute emission cross section for the process ${e}^{\ensuremath{-}}+{\mathrm{Ga}}^{+} (4^{1}S)\ensuremath{\rightarrow}{e}^{\ensuremath{-}}+{\mathrm{Ga}}^{+} (4^{1}P)$ from below thresh-old (8.77 eV) to 400 eV. Total uncertainties at 68% confidence level are typically 16%. The cross section exhibits the sharp onset at a threshold characteristic of positive-ion excitation processes. Evidence of structure is observed from just above threshold to 15 eV. The semiempirical Gaunt-factor formula is in reasonable agreement with the measurements over the energy range measured. A lower limit for the lifetime of the $4^{3}P_{0,2} {\mathrm{Ga}}^{+}$ metastable state has been established at $\ensuremath{\tau}\ensuremath{\ge}0.4$ sec.

Absolute emission cross sections for electron-impact excitation ofZn+(4p P2)and (5s S2) terms. I

Absolute emission cross sections for electron-impact excitation of the $3{d}^{10}4p ^{2}P$ and $3{d}^{10}5s ^{2}S$ terms of ${\mathrm{Zn}}^{+}$ have been measured from below threshold to about 790 eV ($^{2}P$) and 388 eV ($^{2}S$) using the crossed-charged-beams technique. Both transitions have the abrupt onset at threshold characteristic of positive-ion excitation. The $^{2}P$ cross section shows considerable structure in the interval from threshold to near 20 eV, above which it falls off smoothly. Agreement with five-state close-coupling theory is excellent below 100 eV when cascading is included in the theory. Above 100 eV, the data lie above the theory. The peak value of the $^{2}P$ cross section is 9.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ essentially at threshold, while the peak value of the $^{2}S$ cross section is about 0.47\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$. The net linear polarization of the $3{d}^{10}4p ^{2}P$ emission was measured (unresolved from the $3{d}^{10}4d ^{2}D\ensuremath{\rightarrow}3{d}^{10}4p ^{2}P$ cascading transition), and these data were used to correct the cross-section data for anisotropy of the emitted light. The effective lifetime of the $3{d}^{9}4{s}^{2}^{2}D_{\frac{3}{2}}$ level was measured by observing exponential decay of the 589.6-nm photons resulting from its decay.

Absolute emission cross section for electron-impact excitation ofLi+to the (2P3) level

Crossed beams of electrons and ${\mathrm{Li}}^{+}$ ions have been used to measure the absolute emission cross section for the process, $e+{\mathrm{Li}}^{+}(1^{1}S)\ensuremath{\rightarrow}e+{\mathrm{Li}}^{+}(n^{3}l)\ensuremath{\rightarrow}h\ensuremath{\nu}$ (548.5 nm), from below the threshold at 61.26 to 162 eV. The cross section exhibits the sharp onset at threshold characteristic of positive-ion excitation. The cross section at threshold is deduced to be 2.0 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$, decreasing to 0.2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$ at 162 eV. Total uncertainties at a 68% confidence level are typically about \ifmmode\pm\else\textpm\fi{}15%. Pronounced structure is observed in the near-threshold region. From about 1.5 times the threshold energy to the highest energies measured, the cross section follows the generally predicted ${E}^{\ensuremath{-}3}$ behavior for $^{1}S$ to $^{3}P$ transitions. Theoretical predictions of the cross sections differ from one another by up to a factor of 2, and none appears to fit the data really well.

Evaluation of the Radial Integrals in the Coulomb-Born Approximation

All the radial integrals required in the Coulomb-Born calculation of the excitation of positive ions by electron impact have been expressed in terms of the Gauss hypergeometric functions for the monopole and dipole cases when the atomic radial functions are represented analytically as a linear combination of Slater-type orbitals. The cross sections for the 1s→2s and 1s→2p transitions in hydrogen-like ions with Z =2∼10 and 50 have been obtained. The present results are in agreement with others which have been evaluated numerically.

Resonances in 1s-2s and 1s-2p collision strengths for electron impact excitation of hydrogenic ions

Accurate rate coefficients for the electron impact excitation of positive ions are required for the interpretation of the spectra of hot plasmas. In making calculations, account should be taken of resonances in collision strengths at energies which contribute to the rate coefficients. For 1s-2s and 1s-2p excitation of hydrogenic ions it is shown that (3l3l') resonances occur at energies close to the excitation thresholds. The effect of resonances of the type (3ln'l') with n'>or=3 is to increase rate coefficients by about 20% for 1s-2s and 10% for 1s-2p.

Improved microchannel plate performance with a resistive anode encoder

Performance of microchannel plates in the high-gain chevron configuration has been improved by the addition of an interplate voltage VB. For VB=100 V, and positive ion excitation, the output electron distribution FWHM decreased from 166% to 97%. Using a resistive anode encoder (RAE) readout, and 1216-Å uv excitation, we observed a corresponding spatial resolution improvement of 16%.

Experimental proof of a theoretical relation for the probability of positive ion excitation by bombardment of solid surfaces with 12 keV Ar+ ions

Experimental proof of a theoretical relation for the probability of positive ion excitation by bombardment of solid surfaces with 12 keV Ar+ ions

A comparison of the Born and Coulomb-Born approximations for electron impact excitation of positive ions

It is shown that for transitions 2s to np in lithium-like ions the Born and Coulomb-Born approximations differ by less than 10% for incident energies greater than about 8 times threshold. This contradicts the results obtained by Kastner et al (1972) for excitation of Si11+.

Electron impact excitation of positive ions: cross-sections and reaction rates

A classical impulse approximation is used to calculate the electron impact excitation cross-sections of lithium-likeions and sodium-like ions. A comparison of the results is made with the available quantum mechanical calculations. The excitation rate coefficients are calculated for these ions assuming a Maxwellian distribution for the velocity of the incident electrons.

The Relevance to Astrophysics of the Results of Recent Experiments With Colliding Charged-Particle Beams

AbstractA summary is given of some recent measurements of the ionization and excitation of positive ions by electrons, and the detachment of electrons from H− by proton and electron impacts. The results are discussed and compared with theory. Conclusions are drawn which are relevant to atomic collision processes which occur in astrophyscial plasmas.

Measurement of Thin Oxide Surface Film Thicknesses and Atomic Densities by the Analysis of Positive Ion Excited Soft X-Ray Spectral

AbstractCharacteristic soft x-ray emission lines created as a result of low energy (10-100keV) positive ion excitation of a target material has been applied to the study of thin films of oxygen that form naturally on metal surfaces. The specific metals investigated were aluminum and nickel aa well as a number of their corresponding alloys.Low energy positive ions are well suited as projectiles for the study of thin films in that they have low penetrating powers, yet produce high x-ray yields for K-, L-, and M-shell elemental x-ray lines. Perhaps the most important feature is the fact that no background bremsstrahlung continuum x-radiation is generated in the slowing processes of the ions. Exploiting the advantages of these facts, a special high sensitivity detection technique has been developed for the soft x-ray analysis of the constituents that make up a particular thin surface film.The physical basis for thickness and atomic density measurements of thin surface films by positive ion excitation are discussed. Calibration data are presented for thin films of known thickness and composition establishing the sensitivity of the method for the measurement of the elemental atomic densities of thin films down to a few monolayers or approximately 10 Å in thickness. Finally, the results of the analysis of oxide surface films of aluminum, nickel and some of their alloys are presented and discussed.The objective of this continuing program is to gain an understanding of the mechanisms and principles governing the formation of oxide surface films on metals with the basic goal of producing new alloying techniques for metal passification in a saline environment.