Laser-induced fluorescence diagnostic for ions or atoms

The laser-induced fluorescence (LIF) diagnostic is a powerful non-intrusive tool to obtain velocity distribution function of ions or atoms, with a good time (≈ μs) and space (≈ 0.1 mm) resolution. LIF uses the optical pumping of an ion or atom transition by a laser frequency shifted by Doppler effect: if ω_L is the laser frequency, ω₀ the ion or atom transition frequency at rest, and V the ion or atom velocity along the laser beam v = 2πc ω_L-ω_0/ω_L. The level of LIF emitted by ions or atoms inside the probed volume is recorded, and by scanning the laser frequency we obtain a fluorescence signal proportional to the number of pumped ions or atoms, i.e. at the resonant velocity and consequently the velocity distribution function along the laser beam for the probed species.