Summary:

In interstellar ices, the surrounding ice matrix is known to play a critical role in mediating chemical reactions. In pure water ice, the hydrogen-bonding network enables rapid energy dissipation, which can stabilize transient species such as the HOCO complex—a key intermediate in CO₂ formation. To gain insight into how energy transfer dynamics in mixed ices may affect such processes, a H₂O:CO₂ 1:4 ice mixture was subjected to infrared irradiation resonant with CO₂ vibrational modes. Experimentally, changes in the OH stretch of H₂O were observed following several minutes of irradiation targeting the asymmetric stretch of CO₂, using the intense monochromatic light produced by the FELIX free electron lasers. Molecular dynamics simulations indicated that excitation of the CO₂ asymmetric stretch is initially redistributed among neighboring CO₂ asymmetric modes. However, dissipation into CO₂ libration and H₂O twist modes occurs only after approximately 2 ns, due to weak anharmonic coupling. This delay exceeds the 1 ns off-time between laser pulses, suggesting that ladder climbing or cumulative excitation effects may be responsible for the observed spectral changes. By contrast, excitation of the CO₂ bending mode leads to rapid energy redistribution and efficient coupling to intermolecular interactions, resulting in effective heating of the H₂O vibrational modes in simulations—though such thermal effects were not detected on the experimental timescale. Nonetheless, both experimental and computational results indicate that nonthermal annealing of the H₂O component occurs in the mixed ice when irradiated with infrared light resonant with CO₂ vibrations.

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