  Laser-Induced Fluorescence (LIF) spectroscopy is an essential tool for
probing ion and atom velocity distribution functions (VDFs) in complex plasmas.
VDFs carry information about kinetic properties of the species critical for
plasma characterization, yet their accurate interpretation is challenging due
to multicomponent distributions, broadening effects, and background emissions.
Our research introduces wavelength modulation (WM) LIF as a solution to enhance
VDF sensitivity measurements. WM-LIF, unlike standard amplitude modulation (AM)
methods, measures the derivative of the LIF signal, providing greater
sensitivity to changes in VDF shape. A numerical model was developed to compare
the efficacy of WM and AM signals in lock-in amplifier-based measurements.
Experiments were conducted in a weakly collisional argon plasma with magnetized
electrons and non-magnetized ions. The argon ion VDFs were measured using a
narrow-band tunable diode laser, with its wavelength scanned across the Ar ion
transition 4p^4 D_(7/2)-3d^4 F_(9/2) centered at 664.553 nm (in vacuum). A
lock-in amplifier detected the second harmonic WM signal, generated by
modulating the laser wavelength with an externally controlled piezo-driven
mirror. Our findings indicate that WM-LIF can be used to determine VDF
parameters, such as distribution components and their temperatures and
velocities, serving as an independent method of obtaining such parameters,
which can be used for improving analysis of conventional AM approach. This
method is especially beneficial in environments with substantial light noise or
background emissions e.g., thermionic cathodes and reflective surfaces.
