Stark broadening of the 2S-level of hydrogen at low electron densities measured by Doppler-free two-photon polarization spectroscopy

Doppler-free two-photon polarization spectroscopy was used for measuring the Stark-broadening of the 2S-level (Lyman-alpha transition) of hydrogen in an are plasma at electron densities around 10(21) m(-3) in order to examine corresponding theories. This electron density range, not accessible so far, is of special interest because Stark-broadening is comparable with the fine-structure splitting. previous investigations in are plasmas with somehow higher electron densities already demonstrated, that atomic fine-structure is causing a distinct asymmetry in the central part of the Stark-profiles. Precise profile measurements were possible using a recently developed pulsed UV-laser spectrometer with a bandwidth of 300 MHz tunable around 243 nm. In addition, the electron densities were determined with highly sensitive dispersion interferometry.