  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.
