Quantitative measurements of singlet molecular oxygen in a low pressure ICP

Abstract

We present measurements of the densities and temperatures (rotational and translational) of the metastable a1Δg (v = 0) state of O2 in a cylindrically symmetric RF driven plasma operating in inductive mode at 100 mTorr total pressure and 300 W applied power. Line-of-sight absorption across the plasma region was determined by diode laser cavity ringdown spectroscopy on the (0, 0) vibrational band of the O2(b1Σg +) ← O2(a1Δg) transition near 1.9 μm. Four rotational quantum states were studied, with a population distribution corresponding to a rotational temperature of 346 ± 38 K. The translational temperature was determined to be 359 ± 16 K from the width of the strongest absorption line, Q(12), and in equilibrium with the rotational distribution. The absolute concentration of O2(a1Δg, v = 0) was measured as (9.5 ± 1.3) × 1013 cm−3, and corresponds to an apparent (3.5 ± 0.45)% contribution to the total number density. Time-resolved CRDS measurements following plasma extinction were used to deduce a wall loss coefficient, γ, of (2.8 ± 0.3) × 10−3 on predominantly Al surfaces. Surmising reasonable concentrations for O2(b1Σg +) and an upper limit for the vibrational temperature places the total contribution of O2(a1Δg) at between 3.6% and 5.85%. The variation of the O2(a1Δg, v = 0) state concentration with RF power shows a clear transition from the E to H mode excitation near an applied power of 150 W. Allan variance analysis yields a minimum measurable concentration of O2(a1Δg, v = 0) of 1.1 × 1012 cm−3 over 100 ringdown events, an order of magnitude more sensitive than previously reported.