  Extensive information, such as ion temperature and wave-particle
interactions, can be obtained by direct measurement of ion distribution
functions using laser-induced fluorescence (LIF). This nonintrusive plasma
diagnostic provides an important window into the ion motions in phase-space.
Previous simulation results suggest that LIF measurements, which are performed
on metastable ions produced directly from neutral gas particles and also from
ions in other electronic states, place restrictions on the metastable lifetime.
In the case where metastable population is produced from direct ionization of
neutral atoms, the velocity distribution measured using LIF will only
faithfully represent processes which act on the ion dynamics in a time shorter
than the metastable lifetime. In this paper, the metastable lifetime effects
are explored experimentally for the first time using wave-particle interaction
in an Argon multidipole plasma. We demonstrate that this systematic effect can
be corrected using the theory addressed in the paper based on the metastable
lifetime and relative fraction of metastables produced from pre-existing ions.
