Infrared free-electron laser irradiation of carbon dioxide ice

Interstellar ice grains are believed to play a key role in the formation of many of the simple and complex organic species detected in space. However, many fundamental questions on the physicochemical processes linked to the formation and survival of species in ice grains remain unanswered. Field work at large-scale facilities such as free-electron lasers (FELs) can aid the investigation of the composition and morphology of ice grains by providing novel tools to the laboratory astrophysics community. We combined the high tunability, wide infrared spectral range and intensity of the FEL beam line FELIX-2 at the HFML-FELIX Laboratory in The Netherlands with the characteristics of the ultrahigh vacuum LISA end station to perform wavelength-dependent mid-IR irradiation experiments of space-relevant pure carbon dioxide (CO2) ice at 20 K. We used the intense monochromatic radiation of FELIX to inject vibrational energy at selected frequencies into the CO2 ice to study ice restructuring effects in situ by Fourier Transform Reflection–Absorption Infrared (FT-RAIR) spectroscopy. This work improves our understanding of how vibrational energy introduced by external triggers such as photons, electrons, cosmic rays, and thermal heating coming from a nascent protostar or field stars is dissipated in an interstellar icy dust grain in space. Moreover, it adds to the current literature debate concerning the amorphous and polycrystalline structure of CO2 ice observed upon deposition at low temperatures, showing that, under our experimental conditions, CO2 ice presents amorphous characteristics when deposited at 20 K and is unambiguously crystalline if deposited at 75 K.