The Collaborative International Dictionary of English v.0.48:

ionize \i"on*ize\, v. t. [imp. & p. p. Ionized; p. pr. & vb.
   n. Ionizing.] (Physics, Chemistry)
   To convert (an atom or molecule) into ions, e. g. by
   dissolving in water, by reaction with acid or alkali, or by
   interaction with ionizing radiation. See ionization.
   [Webster 1913 Suppl. +PJC]