Controlling the emission profile of an H2 discharge lamp to simulate interstellar radiation fields

Context. Microwave discharge hydrogen-flow lamps have been used for more than half a century to simulate interstellar ultraviolet radiation fields in the laboratory. Recent discrepancies between identical measurements in different laboratories, as well as clear wavelength dependent results obtained in monochromatic (synchrotron) experiments, hint at a more elaborate dependence on the exact discharge settings than assumed so far. Aims. We have investigated systematically two lamp geometries in full dependence of a large number of different running conditions and the spectral emission patterns are characterized for the first time with fully calibrated absolute flux numbers. Methods. A sophisticated plasma lamp calibration set-up has been used to record the vacuum-ultraviolet emission spectra with a spectral resolution of 0.5 nm and bandwidth of 1.6 nm in the 116-220 nm region. Spectra are compared with the output of a calibrated D2-lamp which allows a derivation of absolute radiance values. Results. The general findings of over 200 individual measurements are presented, illustrating how the lamp emission pattern depends on i) microwave power; ii) gas and gas mixing ratios; iii) discharge lamp geometry; iv) cavity positioning; and v) gas pressure.