Laser induced fluorescence measurement of ion velocities in plasmas

Investigation of the sheath and pre-sheath regions adjacent to an electrode is extremely difficult due to the small size of these regions, and as any mechanical probe inserted into the plasma will significantly perturb the sheath and pre-sheath. To overcome these difficulties we are using a non-perturbing technique, Laser Induced Fluorescence (LIF) to investigate the acceleration of ions as they approach an unbiased electrode, and one to which both dc and rf voltages have been applied. In the LIF measurements reported here argon ions in the 3d' 2G9/2 metastable state were excited to the 4p' 2F07/2 state by a dye laser tuned to this transition (611.5 nm). Fluorescence at 461.0 nm resulting from the decay of this state to the 4s' 2D5/2 state was observed as the dye laser was scanned through the 611.5 nm transition: the shape of the resulting Doppler-broadened transition gives the distribution of the component of the velocity of the metastable ions in the direction of the laser beam; the area under the broadened fluorescence line is proportional to the metastable density