Energy transfer and restructuring in amorphous solid water upon consecutive irradiation

TL;DR

This study investigates how vibrational energy transfer and structural changes occur in amorphous solid water (ASW) under IR irradiation, revealing energy redistribution mechanisms and the role of hydrogen bonds in amorphous ice.

Contribution

It provides experimental and simulation evidence of vibrational energy transfer and structural reorganization in ASW induced by IR laser irradiation, highlighting the influence of hydrogen-bonding networks.

Findings

FEL irradiation induces local structural changes in ASW.

Vibrational energy transfers without breaking hydrogen bonds.

Reduced energy dissipation in amorphous compared to crystalline water.

Abstract

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.