Stark broadening measurements of the hydrogen 2S-level in the transition region to ion-impact-broadening

Doppler-free Stark broadening measurements of the two-photon excited 2S-level of hydrogen were extended to electron densities down to 1020 m−3 in order to see, how the previously observed discrepancies between measured and calculated Stark profiles continue in the range of lower densities. The results of the entire investigation in the electron density range from 31⋅1020 m−3 down to 1.1⋅1020 m−3 are reported in this paper. Measurements at a fixed electron density but different neutral densities revealed, that neutral broadening contributions are negligible compared to Stark broadening under the conditions of this investigation.