8 pages, 4 figures. The atomic data from this work, including energy
  levels, A-values, and effective collision strengths, is available in
  electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
  (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/