Energy transfer and restructuring in amorphous solid water upon consecutive irradiation by Herma M. Cuppen et al. on Wednesday 30 November

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.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.16217v1