Studying Pure CH and the Interaction of CH and HO in Interstellar Ice Analogues with On-Resonance Infrared Irradiation.

Methane in the solid state is often studied in interaction with water, either as clathrate hydrates or as interstellar ice in a water-rich ice environment. Using free-electron laser infrared irradiation, we studied the interaction between CH and HO through energy dissipation after on-resonance vibrational excitation. The observed changes in the ice structure are largely independent of the vibrational mode excited and suggest an energy-dissipation-induced local heating of the ice. Local heating results in desorption for the two pure crystalline phases of CH, a phase transition to phase II for metastable CH and a 1:10 HO:CH mixture, and segregation for the 1:1 and 1:5 HO:CH ice mixtures. Local heating only occurs when the infrared irradiation is on resonance with a vibrational mode of the system in the case of the pure CH ices, suggesting that a sufficient absorption cross section is required to convert the on-resonance vibrational energy into general heating.