Th.M. El-Sherbini

Multiple ionization of He, Ne and Ar by 2–16 keV electrons

Relative abundances of multiply charged ions formed by electron impact (2–16 keV) on some noble gases were measured using a charge analyzer with 100% transmission Large discrepancies exist between our values and those obtained in low-transmission mass spectrometers. Cross sections were determined by normalization on the absolute gross ionization cross sections of Schram et al. The measured proportionality constants of the E−1el ln Eel-term in the cross section are compared with those obtained from an analysis of photoionization data. The agreement with our values is good if besides one-electron ejection from an inner shell also the multiple electron ejection from the outer shell is taken into account.

K shell excitation of nitrogen and carbon monoxide by electron impact

Abstract The K shell energy loss spectra for N 2 and CO have been measured using 10 keV electrons. In addition to the K shell ionization continua discrete transitions at 400.8 eV for N 2 and at 287.7 eV and 534.4 eV for CO are observed corresponding to excitation of the atomic K shells. Electron-ion coincidence studies provide information on the decay processes following K shell excitation and ionization.

Oscillator strengths for multiple ionization in the outer and first inner shells of Kr and Xe

Abstract Oscillator-strength spectra are reported for the formulation of the charge states one to four following N and M ionization in Kr and O and N ionization in Xe. The spectra were derived from coincidence measurements of small-angle inelastically scattered 10-keV electrons in the two gases and the ions formed. The spectra of Kr1+ and Xe1+ gave evidence of the presence of a minimum and subsequent maximum in the contribution of 4p-ed transitions in Kr and 5p-ed transitions in Xe. Our study further shows that simultaneous ionization in inner and outer shell of the type (3d,N) in Kr and (4d,O) in Xe behaves like a direct electron-interaction process. Its contribution is quite appreciable and cannot be neglected when one compares calculated 3d (or 4d) photoabsorption with experiment. For that purpose we present a number of separate subshell oscillator-strength distributions. The present results are compared with photoionization and absorption data. Their relation with our previous work on total single and multiple ionization of Kr and Xe by electron impact is discussed.

Ionization of N2 and CO by 10 keV electrons as a function of the energy loss: II. Inner-shell electrons

Abstract The small-angle inelastic scattering of 10 keV electrons in N2 and CO has been measured in coincidence with the ions and fragments formed. Continuous oscillator-strength spectra were obtained for the production of N+2, N+(N++2), CO+, C+, O+ and CO++, via outer-shell excitations. Besides the onset of excitation to the X, A and B electronic states of N+2 and CO+ (which is observed as an increase in the oscillator strength), the N+2 spectrum shows that a considerable fraction of the oscillator strength goes to double electron transitions, i.e. to auto-ionizing Rydberg states converging to the C state. The formation of N+ ions at the lowest threshold for dissociation is discussed with regard to earlier suggestions in literature. In the case of N2, the mass-14 spectrum (i.e. N+ and N++2) clearly shows the contributions of N++2 formation at energy losses above 42 eV, while the structure in this range correponds to onset of ionization to N++2 levels known from Auger-electron spectroscopy. The measurements of CO show that most of the doubly charged ions dissociate to C+ and O+ with lifetimes shorter than about 10−8s. Comparison with optical measurements and electron-impact data shows the contribution to the total oscillator strength of fragment ions with excess energy of formation.

Multiple ionization of Kr and Xe by 2–14 keV electrons

Abstract We have measured the relative abundances of multiply charged ions formed by 2–14 keV electrons incident on krypton and xenon. Ion selection is performed in a charge analyzer of 100% transmission. The relative abundances of the multiply charged ions obtained are larger than those given by previous investigators. Partial ionization cross sections are determined by normalization on the absolute ionization cross sections of Schram et al . The energy dependence of the partial ionization cross sections is generally in agreement with the Bethe-Born approximation. A good agreement is also obtained between our experimental values for the summation of the proportionality constants of the E -1 el ln E el terms in the cross sections for all the charges and the values calculated by Kingston from photo-absorption data.

Projectile excitation in the collision of Arq+ (q=1, 2 and 3) with He and Ne

Cross sections for radiative emission from Ar II (3p44s 2P), Ar II (3p43d 2D), Ar II (3s3p6 2S), Ar III (3s3p5 1P) and Ar III (3s3p5 3P) projectile states have been measured for Arq+ (q=1, 2 and 3) ions colliding with He and Ne at impact energies between 15 and 400 keV. The experimental results are explained qualitatively by considering the MO correlation diagrams and potential energy state diagrams. It is often found that the cross section for excitation decreases with the increase of the number of intermediate transitions required in order to reach the excited state. When there is a mechanism involving radial coupling leading from initial to final states then the measured emission cross section decreases with energy, whereas mechanisms involving rotational coupling lead to cross sections that increase with increasing energy up to 200 keV or more.

Excitation and ionisation resulting from electron capture in Ar6++H2 collisions 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.