  Interstellar and cometary ices play an important role in the formation of
planetary systems around young stars. Their main constituent is amorphous solid
water (ASW). Although ASW is widely studied, vibrational energy dissipation and
structural changes due to vibrational excitation are less well understood. The
hydrogen-bonding network is likely a crucial component in this. Here we present
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 HFML-FELIX facility at the Radboud University in Nijmegen, the
Netherlands. Structural changes in ASW are monitored by reflection-absorption
infrared spectroscopy and depend on the irradiation history of the ice. The
experiments show that FEL irradiation can induce changes in the local
neighborhood of the excited molecules due to energy transfer. Molecular
Dynamics simulations confirm this picture: vibrationally excited molecules can
reorient for a more optimal tetrahedral surrounding without breaking existing
hydrogen bonds. The vibrational energy can transfer through the
hydrogen-bonding network to water molecules that have the same vibrational
frequency. We hence expect a reduced energy dissipation in amorphous material
with respect to crystalline material due to the inhomogeneity in vibrational
frequencies as well as the presence of specific hydrogen-bonding defect sites
which can also hamper the energy transfer.
