Phase-resolved emission spectroscopy of a hydrogen rf discharge for the determination of quenching coefficients

Abstract

Collisional effects can have strong influences on the population densities of excited states in gas discharges at elevated pressure. The knowledge of the pertinent collisional coefficient describing the depopulation of a specific level (quenching coefficient) is, therefore, important for plasma diagnostics and simulations. Phase-resolved optical emission spectroscopy (PROES) applied to a capacitively coupled rf discharge excited with a frequency of 13.56 MHz in hydrogen allows the measurement of quenching coefficients for emitting states of various species, particularly of noble gases, with molecular hydrogen as a collision partner. Quenching coefficients can be determined subsequent to electron-impact excitation during the short field reversal phase within the sheath region from the time behavior of the fluorescence. The PROES technique based on electron-impact excitation is not limited—in contrast to laser techniques—by optical selection rules and the energy gap between the ground state and the upper level of the observed transition. Measurements of quenching coefficients and natural fluorescence lifetimes are presented for several helium $(3\mathrm{}{}^{1}S,4\mathrm{}{}^{1}S,3\mathrm{}{}^{3}S,3\mathrm{}{}^{3}P,4\mathrm{}{}^{3}S),$ neon ${(2p}_1{,2p}_2{,2p}_4{,2p}_6),$ argon ${(3d}_2{,3d}_4{,3d}_1^{\prime }$ and ${3d}_3),$ and krypton ${(2p}_1{,2p}_5)$ states as well as for some states of the triplet system of molecular hydrogen.