  Solid-phase photo-processes involving icy dust grains greatly affect the
chemical evolution of the interstellar medium by leading to the formation of
complex organic molecules and by inducing photodesorption. So far, the focus of
laboratory studies has been mainly on the impact of energetic ultraviolet (UV)
photons on ices, but direct vibrational excitation by infrared (IR) photons is
expected to influence the morphology and content of interstellar ices as well.
However, little is still known about the mechanisms through which this excess
vibrational energy is dissipated, and its implications on the structure and ice
photochemistry. In this work, we present a systematic investigation of the
behavior of interstellar relevant CO and CH3OH ice analogues upon resonant
excitation of vibrational modes using tunable infrared radiation, leading to
both the quantification of infrared-induced photodesorption and insights in the
impact of vibrational energy dissipation on ice morphology. We utilize an
ultrahigh vacuum setup at cryogenic temperatures to grow pure CO and CH3OH
ices, as well as mixtures of the two. We expose the ices to intense,
near-monochromatic mid-infrared free-electron-laser radiation to selectively
excite the species. The dissipation of vibrational energy is observed to be
highly dependent on the excited mode and the chemical environment of the ice.
All amorphous ices undergo some degree of restructuring towards a more
organized configuration upon on-resonance irradiation. Moreover, IR-induced
photodesorption is observed to occur for both pure CO and CH3OH ices, with
interstellar photodesorption efficiencies of the order of 10 molecules cm-2 s-1
(i.e., comparable to or higher than UV-induced counterparts). Indirect
photodesorption of CO upon vibrational excitation of CH3OH in ice mixtures is
also observed to occur, particularly in environments rich in methanol.
