Infrared Irradiation of HO:CO Ice: A Combined Experimental and Computational Study of the Dissipation of CO Vibrational Excitations.

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 CO. To better understand the energy dynamics and its possible influence on the processes in the ice, we investigated a HO:CO 1:4 ice mixture exposed to infrared irradiation on-resonance with the CO vibrations. Experimentally, we find changes in the OH stretch of HO after irradiating the asymmetric stretch of CO 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 CO readily dissipates to other asymmetric stretches in the environment, but only dissipates to the CO libration and HO 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 CO bending vibration, the simulations reveal a fast distribution of energy and coupling to the intermolecular interactions that lead to thermal heating of the HO vibrational modes. This is not observed on the time scale of the experiments. Still, both simulations and experiments reveal nonthermal annealing of the HO component of the mixed ice when exposed to infrared irradiation on-resonance with the CO vibrations.