Advertisement

Dynamics of Low Energy Electron Collisions with Molecules and Clusters

  • H. Hotop

Abstract

The dynamical behaviour of slow electrons traversing ases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances)1. For quite a few atoms and molecules, special behaviour of the s - wave (L = 0) phase shift leads to a deep Ramsauer-Townsend minimum in the scattering cross section between 0 and 1 eV which strongly affects the electron mobility in these gases. Even more importantly, resonances (compound states of the electron-molecule system with lifetimes ranging typically from 10-15 to 10-11 s) are often found to dominate the dynamics of electron-molecule collisions over the energy range 0 to 10 eV. The extended time interval (compared with the direct transit time which is below 1 fs), spent by the incoming electron close to the target while in the resonance state (lifetime τ = ħ/Γ, Γ= resonance width), has profound effects especially on collision channels which involve a reaction of the nuclear framework, i.e. on vibrational excitation (VE) and on dissociative attachment (DA). Apart from well-known shape resonances such as H2 -(2u), N2 -(2g), CO-(2∏), O2 -(2g, v ≥ 4) which are located below the lowest limit for DA and owe their lifetime to the centrifugal barrier of the electron, repulsive anion states above the DA limit are important for VE as well as DA. The importance of resonances for vibrational excitation (VE) as well as negative ion formation via dissociative attachment (DA) is illustrated in Fig. 1.

Keywords

Vibrational Excitation Electron Collision Electron Attachment Feshbach Resonance Dissociative Electron Attachment 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. 1.
    Electron-molecule collisions, I. Shimamura and K. Takayanagi (Eds.), Plenum Press, New York (1984)Google Scholar
  2. 2.
    F. H. Read, J. M. Channing, Production and optical properties of an unscreened but localized magnetic field, Rev. Sci. Instrum. 67, 2372-2377 (1996)CrossRefGoogle Scholar
  3. 3.
    M. Allan, Excitation of the 23S state of helium by electron impact from threshold to 24 eV: measurements with the ‘magnetic angle changer’, J. Phys. B 33, L215-L220 (2000)CrossRefGoogle Scholar
  4. 4.
    D. Klar, M.-W. Ruf, H. Hotop, Attachment of electrons to molecules at meV resolution, Aust. J. Phys. 45, 263-291 (1992)Google Scholar
  5. 5.
    D. Klar, M.-W. Ruf, H. Hotop, A tunable laser-atomic beam photoelectron source with sub-milli-electron-volt resolution: design, operation and application to threshold electron attachment studies, Meas. Sci. Technol. 5, 1248-1261 (1994)CrossRefGoogle Scholar
  6. 6.
    A. Schramm, J. M. Weber, J. Kreil, D. Klar, M.-W. Ruf, H. Hotop, Laser photoelectron attachment to molecules in a skimmed supersonic beam: diagnostics of weak electric fields and attachment cross sections down to 20 µeV, Ph s. Rev. Lett. 81, 778-781 (1998)CrossRefGoogle Scholar
  7. 7.
    L. G. Christophorou, J. K. Olthoff, M. V. V. S. Rao, Electron interactions with CF4, J. Phys. Chem. Ref. Data 25, 1341-1388 (1996)CrossRefGoogle Scholar
  8. 8.
    L. G. Christophorou, J. K. Olthoff, Electron interactions with plasma processing gases: an update for CF4, CHF3, C2F6, and C3F8, J. Phys. Chem. Ref. Data 28, 967-982 (1999)CrossRefGoogle Scholar
  9. 9.
    L. G. Christophorou, J. K. Olthoff, Electron interactions with SF6, J. Phys. Chem. Ref. Data 29, 267-330 (2000)CrossRefGoogle Scholar
  10. 10.
    J. Lotter, A. Kühn, E. Illenberger, Formation of CF4-from CF4 clusters, Chem. Phys. Lett. 157,171-174(1989)CrossRefGoogle Scholar
  11. 11.
    S. Motlagh, J. H. Moore, Cross sections for radicals from electron impact on methane and fluoroalkanes, J. Chem. Phys. 109, 432-438 (1998)CrossRefGoogle Scholar
  12. 12.
    L. Mi, R. A. Bonham, Electron-ion coincidence measurements: The neutral dissociation cross section for CF4, J. Chem. Phys. 108, 1910-1914 (1998)CrossRefGoogle Scholar
  13. 13.
    H. Sugai, H. Toyoda, T. Nakano, M. Goto, Absolute cross sections for the electron-impact dissociation of CF4 and CHF3 into the CFX (x = 1 - 3) neutral radicals, Contrib. Plasma Phys. 35,415-420 (1995)CrossRefGoogle Scholar
  14. 14.
    M.-C. Bordage, P. Ségur, L. G. Christophorou, J. K. Olthoff, Boltzmann analysis of electron swarm parameters in CF4 using independently assessed electron-collision cross sections, J. Appl. Phys. 86, 3558-3566 (1999)CrossRefGoogle Scholar
  15. 15.
    E. Vogt, G. H. Wannier, Scattering of ions by polarization forces, Phys. Rev. 95, 1190-1198(1954)zbMATHCrossRefGoogle Scholar
  16. 16.
    I. I. Fabrikant, H. Hotop, Low-energy behavior of exothermic dissociative electron attachment, Phys. Rev. A 63, 022706-1-10 (2001)CrossRefGoogle Scholar
  17. 17.
    H. A. Bethe, Theory of disintegration of nuclei by neutrons, Phys. Rev. 47, 747-759 (1935)zbMATHCrossRefGoogle Scholar
  18. 18.
    C. Desfrancois, H. Abdoul-Carime, J. P. Schermann, Ground-state dipole-bound anions, Int. J. Mod. Phys. B 10, 1339-1395 (1996)CrossRefGoogle Scholar
  19. 19.
    W. Domcke, L. S. Cederbaum, On the interpretation of low-energy electron - HC1 scattering phenomena, J. Phys. B 14, 149-173 (1981)CrossRefGoogle Scholar
  20. 20.
    J. P. Gauyacq, A. Herzenberg, Nuclear-excited Feshbach resonances in e + HC1 scattering, Phys. Rev. A 25, 2959-2967 (1982)CrossRefGoogle Scholar
  21. 21.
    G. Knoth, M. Gote, M. Rädle, K. Jung, H. Ehrhardt, Nuclear-excited Feshbach resonances in the electron scattering from hydrogen halides, Phys. Rev. Lett. 62, 1735-1737 (1989)CrossRefGoogle Scholar
  22. 22.
    A.-Ch. Sergenton, L. Jungo, M. Allan, Excitation of vibrational levels of HF up to v = 4 by electron impact, Phys. Rev. A 61, 062702-1-6 (2000)CrossRefGoogle Scholar
  23. 23.
    A. Schramm, I. I. Fabrikant, J. M. Weber, E. Leber, M.-W. Ruf, H. Hotop, Vibrational resonance and threshold effects in inelastic electron collisions with methyl iodide molecules, J. Phys. B 32, 2153-2171 (1999)CrossRefGoogle Scholar
  24. 24.
    J. M. Weber, E. Leber, M.-W. Ruf, H. Hotop, Nuclear-excited Feshbach resonances in electron attachment to molecular clusters, Phys. Rev. Lett. 82, 516-519 (1999)CrossRefGoogle Scholar
  25. 25.
    E. Leber, S. Barsotti, I. I. Fabrikant, J. M. Weber, M.-W. Ruf, H. Hotop, Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters, Eur. Phys. J. D 12, 125-131(2000)CrossRefGoogle Scholar
  26. 26.
    E. Leber, S. Barsotti, J. Bömmels, J. M. Weber, I. I. Fabrikant, M.-W. Ruf, H. Hotop, Vibrational Feshbach resonances in electron attachment to nitrous oxide clusters: decay into heterogeneous and homogeneous cluster anions, Chem. Phys. Lett. 325, 345-353 (2000)CrossRefGoogle Scholar
  27. 27.
    T. D. Mark, Free electron attachment to van der Waals clusters, Int. J. Mass. Spectrom. IonProc. 107, 143-163 (1991)CrossRefGoogle Scholar
  28. 28.
    Y. Hatano, Electron attachment and recombination in dense molecular media, Aust. J. Phys. 50, 615-625 (1997)Google Scholar
  29. 29.
    J. Kreil, M.-W. Ruf, H. Hotop, I. Ettischer, U. Buck, Threshold electron attachment and electron impact ionozation involving oxygen dimers, Chem. Phys. 239,459-473 (1998)CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media New York 2001

Authors and Affiliations

  • H. Hotop
    • 1
  1. 1.Fachbereich PhysikUniversität KaiserslauternKaiserslauternGermany

Personalised recommendations