Energy Transfer and Restructuring in Amorphous Solid\nWater upon Consecutive Irradiation

Abstract

Interstellar\nand cometary ices play an important role in the formation\nof planetary systems around young stars. Their main constituent is\namorphous solid water (ASW). Although ASW is widely studied, vibrational\nenergy dissipation and structural changes due to vibrational excitation\nare less well understood. The hydrogen-bonding network is likely a\ncrucial component in this. Here, we present experimental results on\nhydrogen-bonding changes in ASW induced by the intense, nearly monochromatic\nmid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline\nat the HFML-FELIX facility at the Radboud University in Nijmegen,\nThe Netherlands. Structural changes in ASW are monitored by reflection–absorption\ninfrared spectroscopy and depend on the irradiation history of the\nice. The experiments show that FEL irradiation can induce changes\nin the local neighborhood of the excited molecules due to energy transfer.\nMolecular dynamics simulations confirm this picture: vibrationally\nexcited molecules can reorient for a more optimal tetrahedral surrounding\nwithout breaking existing hydrogen bonds. The vibrational energy can\ntransfer through the hydrogen-bonding network to water molecules that\nhave the same vibrational frequency. We hence expect a reduced energy\ndissipation in amorphous material with respect to crystalline material\ndue to the inhomogeneity in vibrational frequencies as well as the\npresence of specific hydrogen-bonding defect sites, which can also\nhamper the energy transfer.