HR: 0830h
AN: SP21B-04    [Abstracts]
TI: Absolute empirical rate coefficient for the excitation of the 117.6 nm line in C III
AU: * Gardner, L D
EM: lgardner@cfa.harvard.edu
AF: Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS-50, Cambridge, MA 02138 United States
AU: Daw, A N
EM: dawan@appstate.edu
AF: Appalachian State University, Department of Physics and Astronomy, Boone, NC 28608 United States
AU: Janzen, P H
EM: pjanzen@nis.lanl.gov
AF: Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545 United States
AU: Atkins, N
EM: natkins@cfa.harvard.edu
AF: Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS-50, Cambridge, MA 02138 United States
AU: Kohl, J L
EM: jkohl@cfa.harvard.edu
AF: Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS-50, Cambridge, MA 02138 United States
AB: We have measured the absolute cross sections for electron impact excitation (EIE) of C²⁺ (2s2p ³P° - 2p^{2 3}P) for energies from below threshold to 17 eV above and derived EIE rate coefficients required for astrophysical applications. The uncertainty in the rate coefficient at a typical solar temperature of formation of C²⁺ is less than ± 6 %. Ions are produced in a 5 GHz Electron Cyclotron Resonance (ECR) ion source, extracted, formed into a beam, and transported to a collision chamber where they collide with electrons from an electron beam inclined at 45 degrees. The beams are modulated and the radiation from the decay of the excited ions at λ 117.6 nm is detected synchronously using an absolutely calibrated optical system that subtends slightly over π steradians. The fractional population of the C²⁺ metastable state in the incident ion beam has been determined experimentally to be 0.42 ± 0.03 (1.65 σ). At the reported ± 15 % total experimental uncertainty level (1.65 σ), the measured structure and absolute scale of the cross section are in fairly good agreement with 6-term close-coupling R-matrix calculations and 90-term R-matrix with pseudo-states calculations, although some minor differences are seen just above threshold. As density-sensitive line intensity ratios vary by only about a factor of 5 as the density changes by nearly a factor of 100, even a 30 % uncertainty in the excitation rate can lead to a factor of 3 error in density.
This work is supported by NASA Supporting Research and Technology grants NAG5- 9516 and NAG5-12863 in Solar and Heliospheric Physics and by the Smithsonian Astrophysical Observatory.
DE: 7831 Laboratory studies
SC: Solar Physics Division - AAS [SP]
MN: 2005 Joint Assembly