A highly tetrahedral form of hydrogenated amorphous carbon (ta-C:H) wi
th maximum density of 2.9 g cm(-3), sp(3) fraction of 0.75 and hardnes
s of 61 GPa has been deposited from acetylene using a low pressure pla
sma beam source. The ion energy dependence of its properties suggests
that the subplantation deposition model of a-C also describes the depo
sition of a-C:H.