Infrared resonant vibrationally induced restructuring of amorphous solid
  water
  Amorphous solid water (ASW) is abundantly present in the interstellar medium,
where it forms a mantle on interstellar dust particles and it is the precursor
for cometary ices. In space, ASW acts as substrate for interstellar surface
chemistry leading to complex molecules and it is postulated to play a critical
role in proton-transfer reactions. Although ASW is widely studied and is
generally well characterized by different techniques, energetically-induced
structural changes, such as ion, electron and photon irradiation, in these
materials are less well understood. Selective pumping of specific infrared (IR)
vibrational modes can aid in understanding the role of vibrations in
restructuring of hydrogen bonding networks. Here we present the first
experimental results on hydrogen bonding changes in ASW induced by the intense,
nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2
beamline at the FELIX Laboratory at the Radboud University in Nijmegen, the
Netherlands. The changes are monitored by reflection-absorption infrared
spectroscopy. Upon resonant irradiation, a modification in IR absorption band
profile of ASW is observed in agreement with a growing crystalline-like
contribution and a decreasing amorphous contribution. This phenomenon saturates
within a few minutes of FEL irradiation, modifying upwards of 94~\% of the
irradiated ice. The effect is further analysed in terms of hydrogen bonding
donors and acceptors and the experiments are complimented with Molecular
Dynamics simulations to constrain the effect at the molecular level.
