Two-photon spectroscopy of atomic hydrogen in high-temperature environments

Two-photon polarisation spectroscopy and two-photon laser-induced fluoresce nce have been used as sensitive techniques for the measurement of high and low atomic hydrogen number densities, respectively, in high-temperature env ironments far away from local thermodynamic equilibrium. In addition, Stark -broadened spectral line profiles of the 1S-2S transition of atomic hydroge n were measured with Doppler-free two-photon polarisation spectroscopy in o rder to test Stark broadening theory at low electron densities. The paper e xpounds briefly some of the basic properties of these techniques and presen ts experimental results.