Charge Transfer Collisions in Ionospheres, Exospheres, Magnetospheres, and Interstellar Clouds

Fine-Structure Effects in O⁺ - O Collisions

O⁺(fast) + O(slow) -> O(fast) + O⁺(slow)

The long-range nature of such resonant charge exchange collisions means that the kinetic energy of each nucleus tends to be conserved. Thus, exchange of an electron has the same effect as a very efficient transfer of momentum and leads to an apparent slowing down of the ion.

Laboratory experimental measurement of resonant oxygen atom charge exchange cross sections at atmospherically relevant energies has proved to be extremely difficult. Lehigh University and SRI International collaborated on a new theoretical approach that included, for the first time, the fine-structure splitting of atomic oxygen as well as spin-orbit and rotational coupling between the states of the oxygen molecular ion, O₂⁺, during the collision [1,2].

Accidental Resonance in H⁺ - O and O⁺ - H Collisions

O + H⁺ <--> O⁺ + H

have very large cross sections in both directions, and thus play central roles in interactions with the solar wind and atomic escape in the exospheres of Venus, Earth, and Mars, as well as in interstellar clouds. We are performing ab initio electronic structure calculations [3] and quantum mechanical coupled channels calculations, including spin-orbit coupling, of differential and momentum transfer cross sections for charge exchange and fine structure excitation in this system. The work builds on previous theoretical [4-7] and photodissociation spectroscopy studies [5,8] of the OH⁺ molecule and parallels our previous calculations on collisions of O⁺ with O [1,2] (described above). For several transitions, collision mechanisms based on curve crossings have been identified. The results indicate that long range coupling terms in the OH⁺ potentials (internuclear distances 10-15 a₀) contribute to the cross sections.

References

1. A. P. Hickman, M. Medikeri-Naphade, C. D. Chapin, and D. L. Huestis, "Fine Structure Effects in the O⁺-O Collision Frequency," Geophys. Res. Lett. 24, 119-122 (1997)
2. A. P. Hickman, M. Medikeri-Naphade, C. D. Chapin, and D. L. Huestis, "Calculation of Fine Structure Effects in the O⁺-O Collisions," Phys. Rev. A 56, 4633-4643 (1997).
3. J. A. Spirko, J. T. Mallis, and A. P. Hickman, "Calculation of Adiabatic and Diabatic ³Sigma^- States of OH⁺," J. Phys. B 33, 2395-2407 (2000).
4. G. Chambaud, J. M. Launay, B. Levy, P. Millie, E. Roueff, and F. T. Minh, "Charge Exchange and Fine-Structure Excitation in O-H⁺ Collisions," J. Phys. B 13, 4205-4216 (1980).
5. H. Helm, P. C. Cosby, and D. L. Huestis, "Photofragment Spectroscopy of the Shape Resonances in OH⁺," Phys. Rev. A 30, 851-857 (1984).
6. R. P. Saxon and B. Liu, "Theoretical Study of OH⁺: Potential Curves, Transition Moments, and Photodissociation Cross Sections," J. Chem. Phys. 85, 2099-2104 (1986).
7. P. C. Stancil, D. R. Schultz, M. Kimura, J.-P. Gu, G. Hirsch, and R. J. Buenker, "Charge Transfer in Collisions of O⁺ with H and H⁺ with O," Astron. Astrophys. Supp. Ser. 140, 225-236 (1999).
8. J. Levin, U. Hechtfischer, L. Knoll, M. Lange, G. Saathoff, R. Wester, A. Wolf, D. Schwalm, and D. Zajfman, "Photodissociation Spectroscopy of OH⁺ Molecular Ions at the TSR Storage Ring," Hyperfine Interactions 127, 267-270 (2000).

Acknowlegements

Principal Investigators

• A. Peet Hickman, Lehigh <aph2@Lehigh.edu>
• David L. Huestis, SRI (E-mail) (CV) (Publications)

Related Web Pages

• Lehigh University Home Page

• SRI International Home Page
• SRI's Molecular Physics Laboratory
• Atmospheric Chemistry and Space Physics at SRI
• Electronic and Atomic Collisions at SRI

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