Investigation of micropores in amorphous hydrogenated carbon by a pulsed positron beam

The positron lifetime (PLT) in a thin film of amorphous hydrogenated carbon (a-C:H) was studied by a pulsed positron beam. For the first time the PLT was measured in a layer which is as thin as 100 nm. PLT's of 400(5) ps for the surface state and 348(3) ps in the volume of the film were obtained. The diffusion length of the positrons was measured to be only 4(1) nm. From these results we conclude that in the specimen investigated at least 3% of the total volume is occupied by micropores about 0.5 nm in diameter which may make possible the diffusion of molecular hydrogen in a-C:H.