Infrared free electron laser induced photodesorption of CO and N₂ from solid amorphous water at cryogenic temperatures

Photon induced processing of molecular ices is known to be important in a number of astrophysical environments. However, to date, astrophysical models tend to only incorporate ultra-violet induced photoprocesses. This is despite the fact that the flux of infrared (IR) photons is comparable to, and sometimes exceeds that of, ultra-violet photons in a number of environments. In order to determine whether IR photoprocesses are important under astrophysical conditions, we have undertaken experiments to investigate the IR-induced desorption of CO and N₂ from CO:H₂O and N₂:H₂O mixed ices grown at 9 K. These studies were performed at FELIX, the free electron laser facility at Radboud University, Nijmegen, The Netherlands. Irradiation of the ices was performed in the mid-IR range from 2.9 – 12 μm and shows that photon induced desorption (PID) of CO and N₂ only occurs when the ices are irradiated at wavelengths that excite the vibrational modes of the H₂O ice. No PID is observed when the internal stretching modes of the dopant molecules are excited. The observed PID traces, recorded during the irradiation with a mass spectrometer, can be fitted with a biexponential decay function that shows the presence of a fast and a slow photodesorption process. These can be assigned to direct and indirect substrate-mediated resonant photodesorption processes. Power dependence studies show saturation and suggest that a complex process is leading to the observed PID. The data reported here clearly show that IRdriven photodesorption should be considered in astrophysical models.