A comparison of ion irradiation and UV photolysis of CH4 and CH3OH

We have studied by infrared absorption spectroscopy the effects induced by fast ions (30 keV) and Lyman-alpha photons (10.2 eV) on some molecular ices at low temperature (10-20 K). It is well known that in both cases the physical and chemical properties of the ices are modified. However while the energy released by ions depends mainly on their energy and on the target species, the effects induced by photons also depend on the optical properties of the sample. Here we show that the effects of ion irradiation and UV photolysis are comparable on fresh ices (i.e. at low doses) but are increasingly different as processing is continued (i.e. at high doses).