Atomic Physic Division

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Atomic Spectroscopy Group

Atomic Structure and Collision Theory

Yong-ki Kim, Phillip M. Stone, M. Asgar Ali, and Karl K. Irikura


  The main thrust of this activity is to develop ab initio theories which are accurate yet simple enough to be applicable to a wide range of atoms and molecules to calculate their properties such as binding and excitation energies, transition probabilities, and electron-impact cross sections for excitations and ionization. Relativistic wave functions are used to generate atomic data for both light and heavy atoms [1], while primarily nonrelativistic wave functions are used for molecules.

Ionization cross sections for H, He, and over 70 molecules, fragments, and ions of interest to scientific and industrial applications such as astrophysics, magnetic fusion (e.g., CH4) and plasma processing of semiconductors (e.g., CF4) are compared to available experimental and theoretical data on a public website, http://physics.nist.gov/ionxsec. A brief outline of our theory and relevant references can also be found there.

Fig. 1
Fig. 1. BEf-scaled cross section for the 6s2 - 6s6p 1P excitation of Hg by electron impact. Squares, relativistic distorted-wave Born theory by Srivastava et al., [J. Phys. B 26, 1025 (1993)]; upright triangles, experiment by Peitzmann and Kessler [J. Phys. B 23, 2629 (1990)]; inverted triangles, experiment by Panajatovic et al. [J. Phys. B 26, 1005 (1993)]; short dashes, unscaled plane-wave Born (PWB) theory, long dashes, BE-scaled PWB theory; solid curve, BEf-scaled PWB theory [2].

A simple theory (called BEf scaling and Ef scaling) has been developed for excitation of atoms [2,3]. The BEf scaling is for neutral atoms and the Ef scaling for singly-charged ions. The BEf/Ef scaling uses binding energy (B), excitation energy (E), and the dipole f value. The BEf/Ef scaling is combined with the binary-encounter-Bethe (BEB) model for direct ionization [4] to generate reliable total ionization cross sections for open-shell atoms, where excitation of a core electron to a valence orbital followed by autoionization (=excitation-autoionization) could substantially increase the total ionization cross section [5]. A BEf scaled excitation cross section for mercury is shown in Fig. 1. The total ionization cross section of gallium, which has a significant contribution from excitation-autoionization, is presented in Fig. 2.

Fig 2

Fig 2. Ionization cross section of Ga for the production of singly-charged ions by electron impact. Short dashes, BEB theory for direct ionization [5]; solid curve, BEB theory plus excitation-autoionization from BE scaling [4]; circles, experiment by Shul et al. [Phys. Rev. A 39, 5588 (1989)]; triangles, experiment by Patton et al. [J. Phys. B 29, 1409 (1996)]; dot dashes, semiempirical theory by Margreiter et al. [Int. J. Mass Spectrom. Ion Processes 139, 127 (1994)]; long dashes, semiempirical theory by Lotz [Z. Phys. 232, 101 (1970)].

 
References:
  1. Y.-K. Kim, Relativistic atomic structure theory for highly-charged ions, in Trapping Highly-Charged Ions: Fundamentals and Applications, edited by J.D. Gillaspy, (NOVA Science Publications, Huntington, New York, 2001), Chap. 16, p. 397.
     
  2. Y.-K. Kim, Scaling of Coulomb-Born cross sections for electron-impact excitation of singly charted ions, Phys. Rev. A 65, (February 2002 issue).
     
  3. Y.-K. Kim, Scaling of plane-wave Born cross sections for excitation of neutral atoms, Phys. Rev. A 64, 032713 (2001).
     
  4. Y.-K. Kim and P.M. Stone, Ionization of boron, aluminum, gallium, and indium by electron impact, Phys. Rev. A 64, 052707 (2001).
     
  5. Y.-K. Kim and M.E. Rudd, Binary-encounter-dipole model for electron impact ionization, Phys. Rev. A 50, 3954 (1994).

For technical questions: philip.stone@nist.gov

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Online: July 2002