A practical fit formula for ionization rate coefficients of atoms and ions
by electron impact: Z = 1-28.
G. S. Voronov (1997, Atomic Data and Nuclear Data Tables, 65, 1). 

Fit parameters for direct ionization rates.
Byte-per-byte description of file: ci.dat (406 records)
-------------------------------------------------------------------------------
   Bytes Format  Units   Label        Explanations
-------------------------------------------------------------------------------
   1-  5  A5     ---     Ion          Element symbol and stage of ionization
   7-  8  I2     ---     Z            Atomic number
  10- 11  I2     ---     N            Number of electrons
  13- 19  F7.1   eV      dE           Ionization energy             
  21- 21  I1     ---     P            Fit parameter
  23- 31  E9.3   cm^3/s  A            Fit parameter
  33- 38  F6.4   ---     X            Fit parameter
  40- 43  F4.2   ---     K            Fit parameter
  45- 48  I4     eV      Tmin         Lower bound of electron temperature
  50- 53  I4     keV     Tmax         Upper bound of electron temperature
-------------------------------------------------------------------------------