Gas dependence of paired luminous rings in capacitive radio-frequency discharges

Paired luminous rings separated by a narrow dark gap have been studied in p ulsed capacitive radio-frequency (rf) discharges using seven gases. These p aired rings are clearly observed when H-2 or D-2 is used. The isotope effec t between H-2 and D-2 is observed in the pressure dependence of the emissio n profile and different generation time of rings. When He, N-2, O-2, Ar, an d Xe are used, only a few pairs of rings are observed. The atomic-mass depe ndence of the gas pressure, at which a pair of ring emission is excited (P- 1) at the electrode separation d=6 cm, is measured. The axial electric fiel d E is calculated at P-1 for each gas using two methods: One is the energy- balance method and the other is the ionization-coefficient method. In the f ormer method, the energy-balance equation is solved to obtain E so that an electron is accelerated and reaches ionization energy at 3 cm from the init ial position. In the latter method, the equation for the ionization coeffic ient alpha /P=A x exp(-BP/E) is solved to find E by substituting P=P-1 and 1/alpha =3 cm. The calculated E using two methods agree relatively well wit h each other in H-2, D-2, He, N-2, and O-2. The empirical scaling-law, P-1= (8.61 +/-0.40)/A and E=(0.302 +/-0.0057)BP1, has been obtained from the ion ization-coefficient method. (C) 2001 American Institute of Physics.