IR Irradiation on Interstellar H2O Icy Silicate Grains: from Dust Vibrational Excitation to Ice Structural Change

IR irradiation has been suggested to steer interstellar ice evolution, including molecular orientation, restructuring, and even desorption, complicating the spectral interpretation of James Webb Space Telescope (JWST) ice observations [1, 2, 3]. However, the role of the substrate beneath the ice layer has not been fully investigated. In particular, silicate dust grains, which also exhibit strong vibrational transitions at ~22 and 9.7 um, are expected to actively absorb photons in the IR range and participate in energy dissipation, vibrational coupling, and surface reactions at the ice–dust interface.

In this work, we irradiate H₂O-coated amorphous olivine-like (MgFeSiO₄) dust analogs on a ruthenium (Ru) substrate using the free-electron IR laser at the HFML-FELIX facility in Nijmegen to study the vibrational excitation and subsequent energy transfer. The selected IR laser wavelengths are 9.7 and 21.9 µm to resonantly excite silicates with and without H₂O ice coverage. The silicate-ice samples, cryogenically cooled to 10.5 K, are prepared by background deposition and monitored by a Bruker FTIR Spectrometer (VERTEX 80v) during stepwise IR irradiation up to 240 sec. The IR-induced spectral changes are further quantified as a function of photon fluence. Complementary, the potential desorption signals are recorded by a Hiden QMS (HAL/3F 501 PIC).

By targeting the resonant vibrational mode of both silicates and water ice at 9.7 µm, the experimental results show significant processing of the silicate dust grain, as evidenced by a decrease in the peak intensity of the Si-O stretching mode at 1060 cm^-1. In addition, irradiation at 9.7 µm is also shown to induce clear spectral changes in the water OH stretching mode at 3330 cm^-1, indicative of water restructuring and desorption from the silicate surface. For irradiation at 21.9 µm, which exclusively resonates with silicate dust analogs, the results show a characteristic water restructuring feature in the water OH stretching mode, suggesting vibrational energy transfer from the silicate grain to the ice mantle.