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 a 1 Δ g (v = 0) state of O 2 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 O 2 (b 1 Σ g + ) ← O 2 (a 1 Δ g ) 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 O 2 (a 1 Δ g , v = 0) was measured as (9.5 ± 1.3) × 10 13 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 O 2 (b 1 Σ g + ) and an upper limit for the vibrational temperature places the total contribution of O 2 (a 1 Δ g ) at between 3.6% and 5.85%. The variation of the O 2 (a 1 Δ g , 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 O 2 (a 1 Δ g , v = 0) of 1.1 × 10 12 cm −3 over 100 ringdown events, an order of magnitude more sensitive than previously reported.

Cavity ringdown studies of the E–H transition in an inductively coupled oxygen plasma: comparison of spectroscopic measurements and modelling

“…The increase in emission intensity (which is also visible to the naked eye) between 120 W and 140 W provides strong evidence of the E-H switchover. Previous observations in this chamber support this interpretation [30]. Both the broadband optical emission from the plasma and atomic emission at 777 nm also show a marked increase in intensity above 120 W.…”

“…Loss of species at plasma chamber walls is one of the biggest sources of uncertainty in plasma modelling and makes direct measurement of wall loss rates very desirable. These rates have been measured previously for O( 3 P) [12] and O 2 (a 1 Δ g ) [30] and we now present a measurement of wall loss for O 2 (b 1 Σ g + ) in our chamber obtained by time resolved observation of the b → X emission. A 760 nm optical filter (FWHM = 10 nm) was placed in front of a PMT, chosen for its high sensitivity and fast response time (∼1 ns), to restrict the emission incident on the detector to that arising from the A-band.…”

“…In order to optically probe the plasma two high reflectivity mirrors (R > 99.995% Layertec GmbH, Germany) are housed in the vacuum mirror mounts forming an optical cavity and enabling CRDS to be performed as detailed previously [12,30]. A general CRDS experimental arrangement [31] is depicted in figure 1.…”

“…Later measurements, utilizing off-axis ICOS, were the first to observe the (1, 0) band, this time in the afterglow of a 20-32 Torr ICP [26,27] and later in the afterglow of a 28 Torr microwave discharge [28]. Földes et al demonstrated detection on the (1, 0) band using cw-CRDS in a 2.25 Torr microwave plasma [29] and most recently O 2 (a 1 Δ g ) was probed using cw-CRDS on the (0, 0) band in an ICP at 100 mTorr [30].…”

“…. These results are presented in section 3 and are contextualised by combining them with previous measurements of the absolute number densities of O 2 (a 1 Δ g ) in its ground vibrational level, [O 2 (a, v = 0)] [30], in order to inform a volume averaged kinetic plasma model. This model is outlined in section 4 and although it employs only a relatively small number of reactions.…”