Experimental identification of the precursors and processes leading to particles is essential for understanding particulate contamination in deposition plasmas. We have investigated particle formation in rf plasmas using light scattering (elastic and inelastic) and quadrupole ion mass spectrometry as complementary plasma diagnostics. Negative ions reach high masses (at least 500 amu) and are the only elementary species with a residence time on the scale of the powder formation time. Furthermore, a negative ion polymerisation scheme shows that the densities of high mass anions are strongly diminished at kHz power modulation frequencies, where reduced powder production is also observed. We conclude that negative ions are the particle precursors and that initial clusters grow by negative ion polymerisation in silane plasmas. In situ light scattering techniques are described to self-consistently determine particle size, number density and refractive index. Novel, visible photoluminescence measurements from particles suspended in the plasma are also reported. These diagnostics demonstrate that particle evolution proceeds by an agglomeration phase and that the particle properties are different from the bulk material early in the particle development. (author) 9 figs., 45 refs.