Laser-Induced Fluorescence and Doppler-Free Polarization Spectra in a Low Density Hydrogen Plasma

The H_α spectral line profile in a low-density hydrogen plasma of a microwave discharge has been measured by the laser-induced fluorescence and the Doppler-free polarization spectroscopy methods with a pulsed high-power tunable laser. The saturation characteristics of the output signals in each method were examined in detail with observations of typical saturation broadening. The original homogeneous width of the line was estimated in the limit of no laser pumping, and various factors governing this width are discussed while demonstrating the importance of the clarification of molecular processes in the discharge. In the Doppler-free spectra a large distortion of the spectrum was also found under different operating conditions; it was attributed to the plasma opacity effect.