IRFEL Selective Irradiation of Amorphous Solid Water: from Dangling to Bulk Modes

TL;DR

This study uses free-electron laser irradiation to selectively excite surface and bulk vibrational modes in amorphous solid water, revealing structural changes and energy transfer mechanisms relevant to astrochemical and atmospheric processes.

Contribution

It demonstrates selective vibrational excitation effects in porous amorphous solid water using free-electron laser radiation, highlighting structural modifications and energy redistribution pathways.

Findings

Selective effects observed at surface and bulk modes.

Spectral modifications indicate structural changes.

Energy injection leads to vibrational relaxation and ice restructuring.

Abstract

Amorphous solid water (ASW) is one of the most widely studied solid phase systems. A better understanding of the nature of inter- and intramolecular forces in ASW is, however, still required to correctly interpret the catalytic role of ASW in the formation and preservation of molecular species in environments such as the icy surfaces of Solar System objects, on interstellar icy dust grains and potentially even in the upper layers of the Earth's atmosphere. In this work, we have systematically exposed porous ASW (pASW) to mid-infrared radiation generated by a free-electron laser at the HFML-FELIX facility in the Netherlands to study the effect of vibrational energy injection into the surface and bulk modes of pASW. During multiple sequential irradiations on the same ice spot, we observed selective effects both at the surface and in the bulk of the ice. Although the density of states in pASW should allow for a fast vibrational relaxation through the H-bonded network, part of the injected energy is converted into structural ice changes as illustrated by the observation of spectral modifications when performing Fourier transform infrared spectroscopy in reflection-absorption mode. Future studies will include the quantification of such effects by systematically investigating ice thickness, ice morphology, and ice composition.