Absolute determination of local ground-state densities of atomic hydrogen in nonlocal-thermodynamic-equilibrium environments by two-photon polarization spectroscopy

Two-photon polarization spectroscopy was applied in order to establish an absolute method for the determination of atomic ground-state densities in gases and plasmas. The method has high spatial and temporal resolution, can be used in a wide range of densities, and does not require any knowledge of the thermodynamic state of the system, because the measured two-photon absorption is strictly proportional to the atomic ground-state density. For atomic hydrogen, which is of basic physical importance as well as of great interest in technology and basic plasma research, the method has carefully been worked out. The measurement range is ${\mathit{N}}_{\mathit{H}}$≥${10}^{19}$ ${\mathrm{m}}^{\mathrm{-}3}$ with an uncertainty of less than 10%. To achieve this accuracy, an arc plasma was shown to provide a standard of atomic hydrogen density with 5% uncertainty, and nonresonant two-photon absorption in xenon gas was established to serve as a transfer standard for easy application. In addition to the density measurements, the developed method allows for precise measurements of two-photon transition probabilities of other atomic species.