Molecular decomposition of solid methane films and emission of molecular fragments under MeV ion bombardment.

Electronic and collisional excitation of thin films of condensed molecular gases (H sub 2 O, O sub 2, N sub 2, CO sub 2, NH sub 3, SO sub 2, CH sub 4, etc.) by MeV and KeV ions results in sputtering of the primary molecules. In addition, new molecular species, formed from fragments of the original molecules, are also emitted. Solid methane is a particularly interesting case. Hydrogen is a principal emission product of methane as it is in the bombardment of all hydrocarbons. However, in methane electronically excited by MeV light ions, the major hydrogen release occurs only after a well defined threshold ion fluence. This suggests a percolation threshold for the escape of hydrogen: material modification of the solid methane must proceed to a point at which a continuous diffusion path of high diffusion coefficient exists to the surface from within the film before the major emission can take place.