Vacuum Ultraviolet Emission Spectrum Measurement of a Microwave-Discharge Hydrogen-Flow Lamp in Several Configurations: Application to Photodesorption of Co Ice

Abstract

We report measurements of the vacuum ultraviolet (VUV) emission spectra of a microwave-discharge hydrogenflow lamp (MDHL), a common tool in astrochemistry laboratories working on ice VUV photoprocessing. The MDHL provides hydrogen Ly-α (121.6 nm) and H 2 molecular emission in the 110-180 nm range. We show that the spectral characteristics of the VUV light emitted in this range, in particular the relative proportion of Ly-α to molecular emission bands, strongly depend on the pressure of H 2 inside the lamp, the lamp geometry (F type versus T type), the gas used (pure H 2 versus H 2 seeded in He), and the optical properties of the window used (MgF 2 versus CaF 2). These different configurations are used to study the VUV irradiation of CO ice at 14 K. In contrast to the majority of studies dedicated to the VUV irradiation of astrophysical ice analogs, which have not taken into consideration the emission spectrum of the MDHL, our results show that the processes induced by photons in CO ice from a broad energy range are different and more complex than the sum of individual processes induced by monochromatic sources spanning the same energy range, as a result of the existence of multistate electronic transitions and discrepancy in absorption cross sections between parent molecules and products in the Ly-α and H 2 molecular emission ranges.

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

“…The systematic approach presented here allows the optimization of lamp settings for specific experiments, using the MDHL in a more wavelength selective way. The study complements the work of Chen et al (2014) and more recently Es-sebbar et al (2015). The main difference between the present work and these two recent studies is that the spectral irradiance of the MDHLs is characterized absolutely, using a fully calibrated measurement procedure.…”

“…Photodesorption rates for several molecules using synchrotron radiation (Fayolle et al 2011(Fayolle et al , 2013 as well as direct cross section measurements of the main constituents of interstellar ice show strong wavelength dependences in the VUV domain. (Cruz-Diaz et al 2014a,b) Recently, Chen et al (2014) showed that the differences in CO photodesorption rates reported by various groups over the past decade may be partly explained by differences in MDHL emission spectra. Carbon monoxide is an excellent example, since it strongly absorbs 160 nm photons while Lyman-α photons are clearly off resonance, as has been shown in independent experiments (Fayolle et al 2011;Cruz-Diaz et al 2014a).…”

Chemical Evolution of a Co Ice Induced by Soft X-Rays

“…The experiments have been performed with the interstellar photoprocess system (IPS; Chen et al 2014). The soft X-ray source is the Spherical Grating Monochromator beamline at NSRRC covering the energy range from 0.25 to 1.2 keV.…”

“…Thus, the spectral shape of the illuminating source became crucial in the interpretation of the experiments. Chen et al (2014) performed an accurate analysis of the vacuum ultraviolet emission spectra of a microwave-discharge hydrogen flow lamp commonly used in studies of ice processing, owingto its intense Lyα emission at 121.6 nm and molecular hydrogen (mainly) Lyman and Werner emission bands. These authors show that the spectral characteristics of the radiation depend on a number of technical details, such as the relative proportions of atomic and molecular hydrogen, and that such characteristics determine the yield of CO photodesorption.…”

“…Laboratory studies of the evolution of interstellar ice analogs under the effect of photon irradiation and ionized particles have greatly contributed to our understanding of the chemistry of complex organic molecules synthesized in space. In particular, the chemical modifications of CO ices, both pure or in mixture, have been studied under the effect of vacuum (Gerakines et al 1996;Gerakines & Moore 2001;Cottin et al 2003;Loeffler et al 2005;Chen et al 2014) and extreme-ultraviolet radiation (Wu et al 2005), ions (Gerakines & Moore 2001;Trottier & Brooks 2004;Loeffler et al 2005;Domaracka et al 2010) and electrons (Jamieson et al 2006). Muñoz Caro et al (2010) measured the photodesorption yield of CO ice under ultraviolet irradiation, finding the release yield at 15 K to be about one order of magnitude higher than previous estimates.…”