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 a 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 the 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 upward of 94% of the irradiated ice. The effect is further analyzed in terms of hydrogen-bonding donors and acceptors, and the experiments are complemented with molecular dynamics simulations to constrain the effect at the molecular level.