Time-resolved cavity ring-down spectroscopy (CRDS) has been applied to determine gas phase and surface loss rates of Si and SiH3 radicals during plasma deposition of hydrogenated amorphous silicon. This has been done by monitoring the temporal decay of the radicals densities as initiated by a minor periodic modulation applied to a remote SiH4 plasma. From pressure dependence, it is shown that Si is reactive with SiH4 [(1.4 +/- 0.6) x 10(-16) m(-3) s(-1) reaction rate constant], while SiH3 is unreactive in the gas phase. A surface reaction probability beta of 0.9 < β &LE; 1 and β = 0.30 +/- 0.05 has been obtained for Si and SiH3, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.