Infrared Resonant Vibrationally Induced Restructuring\nof Amorphous Solid Water

Abstract

Amorphous\nsolid water (ASW) is abundantly present in the interstellar\nmedium where it forms a mantle on interstellar dust particles and\nit is the precursor for cometary ices. In space, ASW acts as a substrate\nfor interstellar surface chemistry leading to complex molecules and\nit is postulated to play a critical role in proton-transfer reactions.\nAlthough ASW is widely studied and is generally well characterized\nby different techniques, energetically induced structural changes,\nsuch as ion, electron, and photon irradiation, in these materials\nare less well understood. Selective pumping of specific infrared (IR)\nvibrational modes can aid in understanding the role of vibrations\nin restructuring of hydrogen-bonding networks. Here, we present the\nfirst experimental results on hydrogen-bonding changes in ASW induced\nby the intense, nearly monochromatic mid-IR free-electron laser (FEL)\nradiation of the FELIX-2 beamline at the FELIX Laboratory at the Radboud\nUniversity in Nijmegen, The Netherlands. The changes are monitored\nby reflection–absorption infrared spectroscopy. Upon resonant\nirradiation, a modification in the IR absorption band profile of ASW\nis observed in agreement with a growing crystalline-like contribution\nand a decreasing amorphous contribution. This phenomenon saturates\nwithin a few minutes of FEL irradiation, modifying upward of 94% of\nthe irradiated ice. The effect is further analyzed in terms of hydrogen-bonding\ndonors and acceptors, and the experiments are complemented with molecular\ndynamics simulations to constrain the effect at the molecular level.