B A Huber

Collisions of doubly charged nitrogen molecules with rare gas atoms

The authors report on single electron capture in slow collisions of doubly charged nitrogen molecules with the rare gases He, Ne and Ar. The vertical double ionization energy of N2 as well as vertical excitation energies of N22+ have been calculated by means of the multiconfiguration coupled electron pair approximation (MC-CEPA) method. Furthermore, experiments have been carried out applying the technique of translational energy spectroscopy. The energy gain spectra of the resulting N2+ molecules have been analysed, showing the single electron capture to be dominated by N22+ (X 1 Sigma +g) projectile ions, as well as by N22+ ions in the low-lying metastable c 3 Sigma +u state. The position of the experimentally observed reaction window is discussed in terms of Franck-Condon transitions and is compared with predictions of classical descriptions.

Partial and state-selective cross sections for multiple ionisation of rare-gas atoms by electron impact

Partial ionisation cross sections for single electron impact on rare-gas atoms (Ne, Kr and Xe) have been measured for charge states up to q=8. In addition, by using the translational energy spectroscopy technique (TES) the authors have determined state-selective cross sections for the production of doubly and triply charged neon ions in specific electronic states. The contributions of direct and inner-shell processes are discussed.

State-selective electron capture by Ar2+(3P, 1D, 1S) ions in He, Ne and Kr

The composition of a primary Ar2+ ion beam is analysed with respect to 3P, 1D and 1S states by aid of the translational attenuation method (TAM). The corresponding fractions are found to be 0.48, 0.37 and 0.15, respectively. Measurements with the analysed beam show that electron capture by slow Ar2+ ions depend sensitively on the initial projectile state population. Whereas in the Ne target Ar2+(3P) ions give the main contribution to the electron capture reaction, in the He target the metastable 1D2 state of Ar2+ is more effective by a factor of about 19. Finally, in krypton the translational spectrum is dominated by reactions of Ar2+(1S0).

Charge transfer of Ar2+ in He, Ne and Ar

Cross sections for charge transfer of Ar2+ in He, Ne and Ar were measured at collision energies below 2 keV. Different electronic states of the secondary particles could be separated by the translational spectroscopy technique. The measurements indicate that only a small number of reaction channels are important, leading to a strong energy dependence of the total cross sections for charge transfer. The role of the energy defect and the validity of scaling laws is discussed.

Study of electron capture reactions by means of double translational spectroscopy

The technique of double translational spectroscopy is applied in order to study electron capture by state-selected multiply charged ions. The projectile ions are prepared in specific atomic states by electron capture in an initial gas target with succeeding energy analysis. The studied capture reaction takes place in a second collision chamber. The final-state population being measured by the energy gain of the projectile ions. The advantages of this method as well as new experimental possibilities are discussed by means of the collision systems Ar3+(*)-Ar and Ar2+(*)-He, Ne.

On the importance of metastable Ne2+(1D2) ions in charge-changing Ne2+-Xe collisions

By the aid of translational spectroscopy the authors have studied the production of Ne+ ions in Ne2+-Xe collisions with an improved energy resolution (E/ delta E approximately 3000) obtaining a better identification of the reception channels. The interpretation of the spectrum has to be changed as compared with the earlier publication. The spectrum emphasises the importance of Ne2+(1D2) ions for both electron capture and transfer ionisation at low collision energies (<1 keV). Within the energy range studied the projectile core is not conserved during the capture reaction. However at higher collision energies the core conservation rule seems to hold better leading to a change of the final-state population of the projectile.

Multiple ionisation of Ar atoms by single-electron impact at energies near threshold

Multiple ionisation of Ar atoms by single-electron impact is investigated in the energy range from threshold up to about 600 eV. Partial cross sections for the production of Arq+ with 3<or=q<or=6 are measured. In the near-threshold region the ionisation curves can be fitted according to power laws sigma varies as (Eel-Ethr)K, where the exponent K is found to be smaller than q for high charge states. At higher electron energies the ionisation cross sections show a significant influence of inner-shell ionisation, which becomes the dominant mechanism for the production of high charge states.

Electron capture by slow multiply charged Mg, Al, Cs, Pb and Bi ions from He, Ne, Kr and H2

Total charge transfer cross sections involving single and double electron capture processes by multiply charged Mg, Al, Pb and Bi ions in He and H2 and Cs ions in He, Ne, Kr and H2 have been measured in the energy range of 0.5-8 keV. Charge numbers q between 2 and 8 were available for the different projectile ions. Scaling of the cross sections with q and with the binding energy of the target particles has been studied. Non-monotonic structures in the q dependence of the cross sections are supposed to be due to the oscillatory structure theoretically described for low collision energies by Ryufuku et al. (1980). The scaling with the binding energy of the target particles does not prove a definite dependence at charge states of 5 or less. For charge states q>or approximately=7 the experimental cross sections approach the theoretical values.

Electron capture and dissociation of the molecule in slow collisions

Single and multiple electron capture processes are studied in slow (8 - 16 keV) collisions between and molecular nitrogen. The contributions of dissociative and non-dissociative channels are determined as well as the kinetic energies of the fragments of the (q = 1 - 6) ions. From the kinetic energies of the fragments dissociation energies of the molecular ion are deduced. Under the assumption of isotropic dissociation the following results are obtained: at most 7% of single-electron capture leads to dissociation of into and at least 90% of the double-electron capture results in fragmentation into . If more than two electrons are captured, all -ions dissociate into . For even values of q we observe r = s, otherwise ; i.e. the maximum difference between r and s is 1. A charge distribution closest to the most symmetric one is preferred. The measured dissociation energies are smaller than those expected from the model of Coulomb explosion. Therefore, potential curves for the and molecular ions have been calculated, and dissociation energies deduced from these potential curves have been compared to the measured ones. In both cases dissociation from the lowest molecular state of the multicharged molecule seems most probable.

A spectroscopic study of double electron transfer from Cu to Ar III in an ECR-microwave discharge

We present spectroscopic measurements of rate coefficients for the population of the ArI 6d[3/2]o and 8s[3/2]o states by two electron transfer from sputtered Cu to Ar2+ in an ECR microwave discharge. Rate coefficients are obtained from absolutely measured line intensities using a modified corona model. Though the binding energies of these states differ by 8 meV only the probability of populating the 6d level exceeds by a factor of ten that of populating the 8s level. This is attributed to an energy resonance between the two electrons when promoting to their final levels. The cross section for this two electron transfer comes out to be of the order of 10−15 to 10−14 cm2.