Effect of Disalignment due to Electron Collision on Laser-Induced Fluorescence Polarization Spectroscopy for Measuring Electric Field in Plasmas

Abstract

The effect of disalignment due to electron collisions on the determination of electric fields in laser-induced fluorescence (LIF) polarization spectroscopy was studied. The aligned He I n1D (n=3,4) atoms were produced by laser excitation of 21S state through electric quadrupole transition in Penning discharge plasma. The polarization components of the subsequent LIF (n1D→21P) were observed as a function of electron density in the range from 2×1010 to 2×1012 cm-3. From the dependences of disalignment rates for 31D and 41D states on electron density, their rate coefficients were estimated to be (1.5±0.3)×10-5 and (4.7±1.2)×10-5 cm3/s, respectively. Critical disalignment rates, where the polarization of LIF disappeared, were also estimated using a rate-equation model taking into account the disalignment effect. On the basis of experimental rate coefficients and critical disalignment rates, we deduced the limitation of measurable electric field for electron density in LIF polarization spectroscopy.