  Extensive information can be obtained on wave-particle interactions and wave
fields by direct measurement of perturbed ion distribution functions using
laser-induced fluorescence (LIF). For practical purposes, LIF is frequently
performed on metastable states that are produced from neutral gas particles and
ions in other electronic states. If the laser intensity is increased to obtain
a better LIF signal, then optical pumping can produce systematic effects
depending on the collision rates which control metastable population and
lifetime. We numerically simulate the ion velocity distribution measurement and
wave-detection process using a Lagrangian model for the LIF signal for the case
where metastables are produced directly from neutrals. This case requires more
strict precautions and is important for discharges with energetic primary
electrons and a high density of neutrals. Some of the results also apply to
metastables produced from pre-existing ions. The simulations show that optical
pumping broadening affects the ion velocity distribution function (IVDF)
$f_0(v)$ and its first-order perturbation $f_1(v,t)$ when laser intensity is
increased above a certain level. The results also suggest that ion temperature
measurements are only accurate when the metastable ions can live longer than
the ion-ion collision mean free time. For the purposes of wave detection, the
wave period has to be significantly shorter than the lifetime of metastable
ions for a direct interpretation. It is more generally true that metastable
ions may be viewed as test-particles. As long as an appropriate model is
available, LIF can be extended to a range of environments.
