Delayed emission of hydrogen from ion bombardment of solid methane

The decomposition of methane under ion bombardment can serve as a model for decomposition of more complex organic molecules and polymers. A large quantity of hydrogen is released from initially pure solid methane films (− 0.1 to − 3 μm thick) at low temperatures in the progression from methane toward a carbon-rich polymer which is stable at room temperature. To study this process we have observed the fluence dependence of the emission of D 2 under MeV He + and H + ion bombardment of solid deuterated methane. The process of methane decomposition through hydrogen bond breaking must begin as the first bombarding ions penetrate the low temperature solid film and indeed a small external D 2 yield is observed immediately. However, quite unexpectedly we find that the vast majority of D 2 release is delayed until a fluence threshold has been accumulated. Above this threshold the external D 2 yield rises rapidly by more than an order-of-magnitude and subsequently decays as the film is depleted in deuterium. For 1.5 MeV He + the threshold of the principal release occurs at ∼ 3 × 10 14 ions/cm 2 ; for 1.5 MeV H + , at ∼ 9 × 10 15 ion/cm 2 . We interpret the delay in D 2 emission as due to the low diffusivity of deuterium in solid methane at 12–28 K and the fluence threshold as a percolation threshold at which a sufficient fraction of the methane has been modified to a material with high deuterium diffusivity to allow deuterium to escape.