The influence of C2H2 and dust formation on the time dependence of metastable argon density in pulsed plasmas

Diode laser absorption at 772.38 nm is used to measure the time resolved density of Ar* (3P2) metastable atoms in a capacitively coupled radio-frequency (RF) discharge running in an argon/acetylene mixture at 0.1 mbar. The RF power is pulsed at 100 Hz and the density of Ar* (3P2) atoms in the 5 ms ON time and in the afterglow is recorded. Different plasma conditions, namely (1) pure argon, (2) argon +7% acetylene before powder formation, (3) argon +7% acetylene after dust particles have been formed and (4) argon with dust particles remaining in the plasma volume but without acetylene, are studied. The measured steady-state Ar* (3P2) density in the middle of the reactor is always about 10 times larger in the dusty argon plasma than in the pure argon discharge. This is mainly a consequence of the enhancement of electron temperature after dust formation. Both steady-state densities and decay times in the afterglow indicate that the degree of dissociation of C2H2 in the plasma volume can be as high as 99%. It is shown that under our plasma conditions, the loss of Ar* (3P2) atoms on the surface of dust particles is negligible compared to their loss by diffusion to the electrodes.