Photoionization of gases

For the measurements, a double ionization chamber was developed at IPM and used at the MLS with synchrotron radiation in a wavelength range from 25 nm to 90 nm. The chamber consists of a gas volume at pressures ranging up to a few 10 Pa, where photoabsorption causes the incident radiant power to be attenuated and photoelectrons to be emitted – specifically, positively charged photoions are generated. Since these photoions are detected after being electrostatically extracted at two consecutively arranged anodes, it is possible to measure the light attenuation and thus to determine the cross section for photoabsorption and photoionization. Furthermore, the two ion signals provide evidence of the incident radiant power.

During the experiments carried out at the MLS, the double ionization chamber was thoroughly characterized, with a specific focus on the influence of pressure gradients on the signals between the synchrotron radiation entering and exiting the chamber. The relative standard measurement uncertainties for the photoionization cross sections of He, Ne, Ar, Kr, and Xe around a few 10^–3 are up to one order of magnitude lower than those from previously available data and are in excellent agreement with them within the combined uncertainties.

The results have already been used to validate the measurement data obtained with a double ionization chamber of the same type; this chamber was used in the SolACES module of the International Space Station (ISS) from 2008 to 2017 to investigate solar radiation. Moreover, the new interaction cross sections represent an improved basis for the quantitative detection of free-electron laser radiation using gas monitor detectors optimized specifically for this application. These datasets are also very important for the development of a gas analysis method that is traceable to the SI.