Total Excitation Cross Sections Research Articles

The Coulomb Ochkur-Rudge approximation applied to excitation of triplet levels in helium-like ions

Bely's modification of the Ochkur-Rudge exchange approximation (1966, 1967) which allows for Coulomb distortion has been used to obtain total cross sections for electron impact excitation of some 11S to n3L transitions in helium-like positive ions. A comparison with the recent Li+ (11S to 23P) measurement of Rogers, Olsen and Dunn (1978) as well as with other theoretical results, enables an assessment to be made of the merits of this simple approximation. The authors conclude that the Coulomb Ochkur-Rudge approximation probably provides a reasonable estimate (i.e. to within about a factor of 2) for excitation of triplet P levels but considerably overestimates (underestimates) excitation to n3S(n3D). Furthermore, since it differs from the exact non-relativistic result when Z= infinity one can argue that the validity of this approximation does not necessarily improve with increasing Z.

A Poor Man’s Approach to Semi-Quantitative Analysis with Electron Energy Loss Spectroscopy

In an earlier paper1 it was found that to a good approximation, the efficiency of collection of electrons that had lost energy due to an inner shell excitation could be written as where σE was the total excitation cross-section and σE(θ, Δ) was the integral cross-section for scattering within an angle θ and with an energy loss up to an energy Δ from the excitation edge, EE. We then obtained: where , with P being the momentum of the incident electron of velocity v. The parameter r was due to the assumption that d2σ/dEdΩ∞E−r for energy loss E. In reference 1 it was assumed that r was a constant.

Cross sections for excitation of the (2s22p23s)4P and (2s2p4)4P states of atomic nitrogen by near-threshold electron impact

Using a modified trapped-electron technique in a crossed-beam geometry, the authors have measured the total relative cross sections for electron impact excitation of the (2s22p23s)4P and (2s2p4)4P states of atomic nitrogen in the region near threshold. The (2s22P23s)4P cross section exhibits a sharp resonance at threshold, within their resolution of about 100 meV, and decreases to half the peak value 0.45 eV above threshold. The cross section for the (2s2p4)4P state increases monotonically from threshold in agreement with theory. The significance of their results to recent in situ measurements of 1200 AA and 1134 AA emission in the airglow is discussed.

Excitation and ionisation resulting from electron capture in Ar6++H2collisions at ion projectile energies of 200-1200 keV

Photon emission between 20 and 250 nm, and slow electron and ion production cross sections have been measured for collisions between Ar6+ and H2 at impact energies from 200 to 1200 keV. These studies indicate that single electron charge transfer into excited states of the product ion is the most important inelastic process. The capture occurs mainly into n=4 levels with the excitation of the higher angular momentum states dominating over most of the projectile energy range. Slow electrons are apparently produced primarily by the capture ionisation process in which the projectile captures an electron and an additional electron is emitted from the target. The capture ionisation cross section is appreciable, amounting to 30 to 40% of the total excitation cross section.

Effect of the tensor force on gamma-ray de-excitation angular distributions from dipole states populated in theC12(p,p′) reaction

Angular distributions have been measured for the de-excitation gamma rays from the isoscalar and isovector ${1}^{+}$ states populated in the $^{12}\mathrm{C}$($p$,${p}^{\ensuremath{'}}$) reaction at ${E}_{p}(\mathrm{lab})=22\ensuremath{-}27$ MeV. Fluctuations in the ${A}_{0}$ coefficients, which measure the total excitation cross sections, are observed to decrease with energy toward the high end of the region investigated. The ${a}_{2}$ coefficients are still fluctuating at 27 MeV, but exhibit reasonably well-defined average values of about -0.40 and 0.15 for the $T=0$ and $T=1$ levels, respectively. Microscopic distorted-wave calculations based on a realistic $G$ matrix interaction, while not sufficient to completely describe the present data, suggest that the isospin dependence of the ${a}_{2}$ coefficients is associated with the tensor force. The theoretical calculations have been extended to 50 MeV and this effect of the tensor force persists.NUCLEAR REACTIONS $^{12}\mathrm{C}$($p$,${p}^{\ensuremath{'}}\ensuremath{\gamma}$) ${E}_{p}=22\ensuremath{-}27$ MeV; measured gamma-ray angular distribution; $M1$ states miscroscopic DWBA analysis.

Study of the 2p 1/2 ? excitation probability in collision systems withZ 1+Z 2>150 using line width analysis

The 90°-line width ofK x-ray lines induced in heavy ion collisions is mainly determined by the kinematical broadening. Its conversion into mean impact parameters together with total excitation cross section allows to obtain mean excitation probabilities. Within the energy range (1.4–5.9) MeV/u mean excitation probabilities for 2p1/2σ-excitation are deduced. Collision systems in theZ-range (150–180) are investigated. A very poor agreement between experiment and theoretical predictions has been found. The excitation probabilities are larger than predicted and are nearly independent of the atomic numberZ of the united atom. The collision energy dependence is much weaker than predicted. Even a decrease of the mean excitation probabilities at high collision energies has been found.

Excitation in gas-phase collisions at intermediate and high velocities

A summary of experimental and theoretical total excitation cross sections of light ion atom systems for intermediate and high velocities is given. The velocity dependence of cross sections is compared and similarities for different collision systems are pointed out. The experimental data are compared with theoretical calculations.

Electron – hydrogen atom collision in the presence of a circularly polarized laser field

Electron–hydrogen collision in the presence of a circularly polarized laser field is studied within a formalism based in an appropriate space-translations transformation and the Green's function formalism. The Born–Oppenheimer approximation for the scattering amplitude is obtained and the dependence of the differential and total excitation cross section on the electromagnetic field polarization is studied.

Dielectric properties for SF6 and SF6 mixtures predicted from basic data

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric-field–to–gas-density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron-beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self-consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6-He and SF6-N2 mixtures. Published experimental values of α for the SF6-He mixtures lie between the values of α calculated with and without ionization of SF6 by excited He atoms. Published experimental values of (E/N) * agree with our calculations to within 5% in both the SF6-He and the SF6-N2 mixtures.

Target dependence of excitation resulting from electron capture in collisions of 200 keV Ar6+ions with noble gases

An experimental investigation has been made of the target dependence of projectile-ion excitation resulting from electron capture in collisions of 200 keV Ar6+ ions with He, Ne, Ar, Kr and Xe target atoms. The studies show strongly rising total capture excitation cross sections and shifts in the post-collision projectile excited-state distributions to higher n levels, with increase in target Z. In collisions of multi-charged ions with atoms at low and intermediate velocities (U 6 1 au), the dominant inelastic processes usually involve moderately exothermic electron capture into excited states of the resulting projectile. Recent investigations have been concerned with studies of both the radiative and non-radiative modes of decay of these states (Winter eta1 1977a, b, c, 1978, Bloemen et a1 1978, El-Sherbini el a1 1978). Most of the attention has been directed towards energy and charge-state dependences of capture excitation. However, additional studies are also being made in our laboratory, on the influence of the target-electron binding energy of these processes. In this letter, we report some results of a recent study of capture excitation processes of the form: Ar6'+X+ (Ar5+)*+X++Ar5++ X++hv

CONTINUOUS EMISSION, LOWERING OF THE IONIZATION POTENTIAL AND TOTAL EXCITATION CROSS-SECTIONS OF AN ATMOSPHERIC THERMAL PLASMA

The authors present a Partial LTE (PLTE) model in which the ground state is overpopulated with respect to the other excited states. The relative overpopulation of the ground state as a function of the electron temperature, T/sub e/, is fairly sensitive to small variations in both the transition probability, A, and the value for the lowering of the ionisation potential, Delta chi /sub 0/. The total cross-section for excitation from the neutral ground state was derived from the measured degree of overpopulation. The authors calculated the emissivity of the free-bound UV recombination radiation, which proved to be quite large in the 70-80 nm spectral range. Some results for the continuum radiation and for the Stark parameters at lambda approximately=700 nm were also given

Measurement of excitation cross section of the 6 ^3P_1 state of Hg using self-calibrating apparatus

The total cross section for excitation of ground state mercury atoms to the 63P1 state by 50 eV electrons was experimentally determined to be 0.434 ± 0.04 × 10−16 cm2. The apparatus used a pair of single-photon detectors to observe correlated optical photons in the 7 1S0 → 6 3P1 → 6 1S0 and 7 3S1 → 6 3P1 → 6 1S0 cascades. The product of the number of singles counts in the two detectors divided by the number of time-correlated cascade events was used along with angular distribution and correlation factors to determine N, the total number of atoms excited to the 6 3P1 state in the interaction region. The correct number N was arrived at in this way without making a separate determination of the overall photon detection efficiency. No standard lamp was needed for calibration nor was reference made to another standard cross section. The apparatus was self-calibrating. The absolute excitation cross section was determined from N and other parameters of the bombardment apparatus.

Differential scattering experiments for e-H2S collisions in the low-energy range

Vibrationally elastic ( Delta v=0) scattering and vibrational excitation of H2S by electron impact has been studied for collision energies up to 10 eV in a closed-beam experiment. In the vibrationally elastic scattering the cross sections show pronounced forward scattering and a steep increase towards small energies, characterising the influence of the direct dipole process. In contrast, the vibrational excitation is clearly dominated by resonance processes. The excitation function exhibits a narrow peak close to threshold, which is attributed to the final-state interaction of the scattering electron in the dipole field and a strong shape resonance at about 2.3 eV of 2A1 symmetry. In the shape resonance the absolute value of the total integral cross section for vibrational excitation has been found to exceed 10-15 cm2, which is about 30% of the vibrationally elastic one.

Measurement of shape and absolute value of 1s? excitation probability in 4.7 MeV/u Xe+Au collisions

The impact parameter dependence of the 1sσ excitation probability for 4.7 MeV/u Xe + Au collisions has been measured for impact parameters between 40 and 200 fm. A determination of the total 1sσ excitation cross section from the data gives (10,8 ± 2)b in good agreement with results of an independent measurement.

Experimental and theoretical analysis of the electron energy distribution functions in Townsend discharges in xenon

The electron energy distribution functions in non-self-sustaining Townsend discharges in xenon have been measured at E/N of 304.9, 462.5 and 612.5 Td (1 Td=10-17 V cm2), using a retarding-potential-difference technique. Also the distribution functions are studied theoretically from the Boltzmann equation. The agreement between the experimental and the theoretical distribution functions is reasonably good. In addition, the total excitation cross section in xenon is predicted from the present theoretical analysis.

Total cross sections for electron impact excitation of the 11S→23S transition in He-like ions

Total cross sections for collisional excitation of the 11S to 23S transition in helium-like ions with atomic number 2<or=Z<or=8 have been computed using a modified form of the Oppenheimer approximation which is equivalent to Geltman's Coulomb-projected Born approximation when exchange is taken into account. Although this approximation cannot be derived from a variational principle it nevertheless yields a result for helium which compares surprisingly well with most of the available experimental data for incident energies ranging from near threshold up to 300 eV. This suggests that the results obtained for the ionised members of the sequence may be quite reliable.

The excitation of heavy atoms by electrons in multiple-collision diffraction theory

The total cross sections for excitation of optical lines in Ne and Ar by electrons are calculated in the single inelastic collision approximation of Glauber theory. Good agreement between theory and experiment is obtained. The results obtained are compared with those of PWBA and DWBA calculations.

Calculation of cross sections for vibrational excitation and de-excitation of CO2by electronic collisions

Total cross sections for resonance vibrational excitation and de-excitation of CO2 in its ground electronic state in the electron energy range 2.2-5.6 eV have been calculated. The calculations were made in the symmetric-stretch channels only using Herzenberg's boomerang model (1968).

Study of the simultaneous emission of two photons in collisions between neon atoms using photon coincidence techniques

The energy dependence of the total cross section for simultaneous excitation of the target and projectile atom in Ne+Ne collisions to states of the 2p53p configuration has been studied for collision energies from 50 to 700 eV. It is estimated that simultaneous excitation occurs in about 25% of the inelastic events. Cross sections for the population of two individual 2p53p states, Q(2pn mod 2pn') (Paschen notation), have been studied in detail for the case n=1. The relative magnitude of these cross sections depends on the population of the molecular states involved. Relative populations of these states have been determined from the reported measurements.

Inelastic scattering of electrons by metastable helium: First Born and Glauber cross sections for2S3−3S3excitation

We calculate the differential and the total cross sections for the direct excitation of the 3/sup 3/S state of helium from the metastable 2/sup 3/S state by electron impact using the Glauber and Born methods. For the Glauber cross-section calculation we use the analytic method of Thomas and Chan to reduce the Glauber amplitude for charged-particle-neutral-atom collisions to a one-dimensional integral representation involving modified Lommel functions. The calculations are made for incident electron energies between 3 and 1000 eV, and for two different sets of 3/sup 3/S wave functions. The results of the differential and the total excitation cross sections are compared with all other available theoretical calculations.