Infrared Irradiation of H2O:CO2 Ice: A Combined Experimental and Computational Study of the Dissipation of CO2 Vibrational Excitations

JGM Schrauwen and TM Dijkhuis and S Ioppolo and DR Galimberti and B Redlich and HM Cuppen, ACS EARTH AND SPACE CHEMISTRY, 9, 1580-1592 (2025).

DOI: 10.1021/acsearthspacechem.5c00030

In interstellar ices, the ice matrix can have a great influence on the chemical reactions. The hydrogen-bonding network in pure water ices facilitates fast energy dissipation that, for example, stabilizes the HOCO complex, a crucial step in the formation of CO2. To better understand the energy dynamics and its possible influence on the processes in the ice, we investigated a H2O:CO2 1:4 ice mixture exposed to infrared irradiation on-resonance with the CO2 vibrations. Experimentally, we find changes in the OH stretch of H2O after irradiating the asymmetric stretch of CO2 for several minutes with the intense monochromatic light of the FELIX free electron lasers. Using molecular dynamics simulations, we found that an excitation of the asymmetric stretch of CO2 readily dissipates to other asymmetric stretches in the environment, but only dissipates to the CO2 libration and H2O twist modes after roughly 2 ns because of its minimal anharmonicity and coupling with other modes. This is significantly longer than the off-time between laser pulses of 1 ns, suggesting ladder climbing or that the stacking of the excitation boosts the experimentally observed changes. For infrared excitation of the CO2 bending vibration, the simulations reveal a fast distribution of energy and coupling to the intermolecular interactions that lead to thermal heating of the H2O vibrational modes. This is not observed on the time scale of the experiments. Still, both simulations and experiments reveal nonthermal annealing of the H2O component of the mixed ice when exposed to infrared irradiation on-resonance with the CO2 vibrations.

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