Controlling the emission profile of an H₂discharge lamp to simulate interstellar radiation fields

Abstract

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 D 2 -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.

Grain Surface Models and Data for Astrochemistry

“…Muñoz Caro et al (2010) determined a desorption yield for pure CO ice of (3.5 ± 0.5 × 10 −2 ) photon −1 which is one order of magnitude higher than that from Öberg et al (2009a). Recent investigations showed that the FUV spectrum, in particular the ratio of Lyman-α (121.6 nm) to H 2 molecular emission bands (110-180 nm), is sensitive to the gas pressure, the type of lamp, the gas composition (pure H 2 versus a mixture of H 2 and He) and the window properties (Chen et al 2014;Ligterink et al 2015). Chen et al (2014) determined CO desorption yields as high as (2.12 ± 0.03 × 10 −1 ) photon −1 for a lamp setup with pure H 2 and a window giving a ratio of 20 % flux of Lyman-α relative to the molecular emission bands.…”

A novel approach to measure photodesorption rates of interstellar ice analogues

“…Photodesorption of the ice is induced using VUV photons produced by a microwave H 2 discharge lamp. The lamp (borosilicate) has a F-type design (Ligterink et al 2015), and is evacuated using a scroll pump. The plasma is powered by a Sairem microwave generator using an Evenson cavity.…”

“…Therefore, the SED of the microwave powered H 2 -discharge lamp is used as characterised recently, using an absolute calibrated VUV spectrometer (Ligterink et al 2015). The resulting spectrum has been presented in Paardekooper et al (2016).…”

“…Recently, photodesorption experiments have been performed with fully characterised SEDs of the VUV lamp (Chen et al 2014), but they are still not able to explain the difference in reported photodesorption rates (Ligterink et al 2015).…”

“…Similar experiments by other groups (Muñoz Caro et al 2010;Chen et al 2014;Muñoz Caro et al 2016) found deviating and even higher values in the 10 −2 to 10 −1 molecules per photon range. A possible explanation for these deviations was searched for in the spectral emission pattern of the used broad-band light sources that turned out to be more sensitive on the parameter settings than assumed (Chen et al 2014;Es-sebbar et al 2015;Ligterink et al 2015). This assumption was in line with wavelength dependent CO photodesorption rates recorded at the DESIRS beamline of the SOLEIL synchrotron facility (Fayolle et al 2011) that showed that the photodesorption of CO follows a wavelength dependent desorption induced by electronic transition (DIET) mechanism that is found to be more effective around 160 nm than at Ly-α.…”