Intensity and polarization of laser-induced fluorescence due to forbidden excitation of He atoms immersed in an electric field in plasmas

Laser-induced fluorescence (LIF) processes through the Stark and electric-quadrupole moment (QDP) transitions of He I (2 S-1 -> n D-1 -> 2 P-1,n=3,4) have been investigated for reliable electric-field measurements in plasmas. A linear-polarization model is formulated for various configurations of electric fields, magnetic fields, and laser polarization. To extend the model to higher-particle-density plasmas we develop a rate-equation model involving a collisional disalignment term. Disalignment rates of n D-1 states, R-da, due to a collision with He gas were measured. Spatial distributions in intensity and polarization of LIF were observed in a discharge plasma. For n=3 with small R-da, the same electric-field distribution in the sheath was obtained from either of the intensity ratios of the Stark to QDP component and the polarization, and the sheath potential agreed well with that by an electric probe. For n=4 with large R-da, the distribution was also correctly obtained from partially depolarized LIF wave forms by using the extended model. These results show that our extended model provides an accurate measurement of the electric field. The minimum detectable field strength was 80 V/cm for n=4. Application and limitations of the methods are discussed.