Broadband UV Absorption Spectroscopy Research Articles

Kinetics of the Gas-Phase Reactions of syn- and anti-CH3CHOO Criegee Intermediate Conformers with SO2 as a Function of Temperature and Pressure.

Kinetics of reactions between SO2 and CH3CHOO Criegee intermediate conformers have been measured at temperatures between 242 and 353 K and pressures between 10 and 600 Torr using laser flash photolysis of CH3CHI2/O2/N2/SO2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. The kinetics of syn-CH3CHOO + SO2 are pressure-dependent and exhibit a negative temperature dependence, with the observed pressure dependence reconciling apparent discrepancies between previous measurements performed at ∼298 K. Results indicate a rate coefficient of (4.80 ± 0.46) × 10-11 cm3 s-1 for the reaction of syn-CH3CHOO with SO2 at 298 K and 760 Torr. In contrast to the behavior of the syn-conformer, the kinetics of anti-CH3CHOO + SO2 display no significant dependence on temperature or pressure over the ranges investigated, with a mean rate coefficient of (1.18 ± 0.21) × 10-10 cm3 s-1 over all conditions studied in this work. Results indicate that the reaction of syn-CH3CHOO with SO2 competes with unimolecular decomposition and reaction with water vapor in areas with high SO2 concentration and low humidity, particularly at lower temperatures.

Optical in-situ measurements and modeling of post-flame sulfation of NaOH(g) and NaCl(g)

Post-flame sulfation of gaseous sodium hydroxide (NaOH) and sodium chloride (NaCl) was investigated with optical in situ measurements at 850 to 1475 °C. A multi-jet burner was used to generate well-controlled combustion environments. The multi-jet burner also enabled the separate feeding of the sodium species and SO2 to the combustion environment where the sulfation reactions occurred. Concentrations of NaOH(g) and NaCl(g) were measured in the product gas using broadband UV absorption spectroscopy to follow the degree of sulfation. At 1475 and 1275 °C almost no sulfation occurred with an initial NaOH(g) concentration of 20 ppm and SO2 concentrations between 0 and 150 ppm. At 985 °C, the NaOH(g) concentration decreased to less than 5 ppm with SO2 concentrations above 50 ppm and at 850 °C almost all NaOH(g) was sulfated under these conditions. The experimental results for the gas-phase sulfation of NaOH were compared to previous results for the sulfation of KOH under the same conditions and the results were shown to be similar for NaOH and KOH under these conditions. Sulfation of NaOH(g) generally occurred to a more significant extent than the sulfation of NaCl(g). At 1115 to 1475 °C, no sulfation of NaCl(g) was observed. At the lowest investigated temperature, 850 °C, the NaCl(g) concentration decreased from 20 ppm to 12 ppm after the addition of 150 ppm SO2. Chemical equilibrium calculations and kinetic modeling using an updated kinetic model for the detailed Na-Cl-S chemistry were compared to the experimental results. Above 1100 °C, the system can be described by chemical equilibrium, implying that equilibrium is reached in less than 100 ms. At temperatures below 1100 °C, the measured concentration indicated kinetic control. Under these conditions, the kinetic model was in good agreement with the experimental results for NaOH(g) but over-predicted the sulfation of NaCl(g). The combined experimental data, chemical equilibrium calculations and kinetic modeling of the present study support that sulfation of alkali species can occur in the gas phase through homogeneous reactions.

Unimolecular Kinetics of Stabilized CH3CHOO Criegee Intermediates: syn-CH3CHOO Decomposition and anti-CH3CHOO Isomerization.

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate syn-CH3CHOO has been investigated at temperatures between 297 and 331 K and pressures between 12 and 300 Torr using laser flash photolysis of CH3CHI2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. Fits to experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicate that the barrier height to decomposition is 67.2 ± 1.3 kJ mol–1 and that there is a strong tunneling component to the decomposition reaction under atmospheric conditions. At 298 K and 760 Torr, MESMER simulations indicate a rate coefficient of 150–81+176 s–1 when tunneling effects are included but only 5–2+3 s–1 when tunneling is not considered in the model. MESMER simulations were also performed for the unimolecular isomerization of the stabilized Criegee intermediate anti-CH3CHOO to methyldioxirane, indicating a rate coefficient of 54–21+34 s–1 at 298 K and 760 Torr, which is not impacted by tunneling effects. Expressions to describe the unimolecular kinetics of syn- and anti-CH3CHOO are provided for use in atmospheric models, and atmospheric implications are discussed.

Kinetics of the gas phase reaction of the Criegee intermediate CH2OO with SO2 as a function of temperature.

The kinetics of the gas phase reaction of the Criegee intermediate CH2OO with SO2 have been studied as a function of temperature in the range 223-344 K at 85 Torr using flash photolysis of CH2I2/O2/SO2/N2 mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Measurements were performed under pseudo-first-order conditions with respect to SO2, revealing a negative temperature dependence. Analysis of experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicates that the observed temperature dependence, combined with the reported lack of a pressure dependence in the range 1.5-760 Torr, can be described by a reaction mechanism consisting of the formation of a pre-reaction complex leading to a cyclic secondary ozonide which subsequently decomposes to produce HCHO + SO3. The temperature dependence can be characterised by kCH2OO+SO2 = (3.72 ± 0.13) × 10-11 (T/298)(-2.05±0.38) cm3 molecule-1 s-1. The observed negative temperature dependence for the title reaction in conjunction with the decrease in water dimer (the main competitor for the Criegee intermediate) concentration at lower temperatures means that Criegee intermediate chemistry can play an enhanced role in SO2 oxidation in the atmosphere at lower temperatures.

Quantitative K-Cl-S chemistry in thermochemical conversion processes using in situ optical diagnostics

The sulfation of gas-phase KOH and KCl was investigated in both oxidizing and reducing atmospheres at temperatures of 1120 K, 1260 K, 1390 K, and 1550 K. Well-defined gas environments were generated in a laminar flame burner fuelled with CH4/air/O2/N2. Atomized K2CO3 and KCl water solution fog and SO2 were introduced into the hot gas as sources of potassium, chlorine, and sulfur, respectively. The in situ concentrations of KOH, KCl, and OH radicals were measured using broadband UV absorption spectroscopy, and the concentration of K atom was measured using TDLAS at 769.9 nm and 404.4 nm. The nucleated and condensed K2SO4 aerosols were visualized as illuminated by a green laser sheet. With SO2 addition, reduced concentrations of KOH, KCl, and K atom were measured in the hot gas. The sulfation was more significant for the low temperature cases. KOH was sulfated more rapidly than KCl. K2SO4 aerosols, formed by homogeneous nucleation, were observed at temperatures below 1260 K. At 1390 K, no aerosols were formed, indicating that the consumed KOH was transformed into gaseous KHSO4 or K2SO4. K atoms formed in the hot flue gas with KOH addition enhanced the consumption of OH radicals except at the high-temperature case at 1550 K. At 1120 K and 1260 K, the sulfation of KOH with SO2 seeding reduced the concentration of K atom, resulting in less OH radical consumption. Studies were also conducted in a hot reducing environment at 1140 K, with the flame at an equivalence ratio of 1.31. Similar to the observation in the oxidizing atmosphere, the concentrations of KOH and K atom decreased dramatically with SO2 seeding. An unknown absorption spectrum observed was attributed to UV absorption by KOSO. The experimental results were used to evaluate a detailed K-Cl-S reaction mechanism, and a reasonable agreement was obtained.

Kinetics of the Gas Phase Reactions of the Criegee Intermediate CH2OO with O3 and IO.

The kinetics of the gas phase reactions of the Criegee intermediate CH2OO with O3 and IO have been studied at 296 K and 300 Torr through simultaneous measurements of CH2OO, the CH2OO precursor (CH2I2), O3, and IO using flash photolysis of CH2I2/O2/O3/N2 mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Experiments were performed under pseudo-first-order conditions with respect to O3, with the rate coefficients for reactions of CH2OO with O3 and IO obtained by fitting to the observed decays of CH2OO using a model constrained to the measured concentrations of IO. Fits were performed globally, with the ratio between the initial concentration of O3 and the average concentration of IO varying in the range 30-700, and gave kCH2OO+O3 = (3.6 ± 0.8) × 10-13 cm3 molecule-1 s-1 and kCH2OO+IO = (7.6 ± 1.4) × 10-11 cm3 molecule-1 s-1 (where the errors are at the 2σ level). The magnitude of kCH2OO+O3 has a significant effect on the steady state concentration of CH2OO in chamber studies. Atmospheric implications of the results are discussed.

Shedding light on the governing mechanisms for insufficient CO and H2 burnout in the presence of potassium, chlorine and sulfur

Based on the experiences of insufficient burnout in industrial fluidized bed furnaces despite adequate mixing and availability of oxidizer, the influence of potassium on CO and H2 oxidation in combustion environments was investigated. The combustion environments were provided by a laminar flame burner in a range relevant to industrial furnaces, i.e. 845 °C to 1275 °C and excess air ratios ranging from 1.05 to 1.65. Potassium, in the form of KOH, was homogeneously introduced into the hot gas environments to investigate its effect on the radical pool. To quantitatively determine key species that are involved in the oxidation mechanism (CO, H2, KOH, OH radicals, K atoms), a combination of measurement systems was applied: micro-gas chromatography, broadband UV absorption spectroscopy and tunable diode laser absorption spectroscopy. The inhibition effect of potassium on CO and H2 oxidation in excess air was experimentally confirmed and attributed to the chain-terminating reaction between KOH, K atoms and OH radicals, which enhanced the OH radical consumption. The addition of chlorine or sulfur could reduce the concentrations of KOH and K atoms and consequently eliminated the inhibition on CO and H2 oxidation. Existing kinetic mechanisms underestimate the inhibiting effect of potassium and they fail to predict the effect of temperature on CO and H2 concentration when potassium and sulfur co-exist. This work advances the need to revise existing kinetic mechanisms to fully capture the interplay of K and S in the oxidation of CO and H2 in industrial fluidized bed furnaces.

CH2OO Criegee intermediate UV absorption cross-sections and kinetics of CH2OO + CH2OO and CH2OO + I as a function of pressure.

The UV absorption cross-sections of the Criegee intermediate CH2OO, and kinetics of the CH2OO self-reaction and the reaction of CH2OO with I are reported as a function of pressure at 298 K. Measurements were made using pulsed laser flash photolysis of CH2I2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy at pressures between 6 and 300 Torr. Results give a peak absorption cross-section of (1.37 ± 0.29) × 10-17 cm2 at ∼340 nm and a rate coefficient for the CH2OO self-reaction of (8.0 ± 1.1) × 10-11 cm3 s-1, with no significant pressure dependence of the absorption cross-sections or the self-reaction kinetics over the range investigated. The rate coefficient for the reaction between CH2OO and I demonstrates pressure dependence over the range investigated, with a Lindemann fit giving k0 = (4.4 ± 1.0) × 10-29 cm6 s-1 and k∞ = (6.7 ± 0.6) × 10-11 cm3 s-1. The origins of IO in the system have been investigated, the implications of which are discussed.

UV spectroscopic determination of the chlorine monoxide (ClO) / chlorine peroxide (ClOOCl) thermal equilibrium constant.

The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228 and 301K in a discharge flow apparatus using broadband UV absorption spectroscopy. A third-law fit of the equilibrium values determined from the experimental data provides the expression K eq = 2.16 × 10-27 e (8527±35 K/T) cm3 molecule-1 (1σ uncertainty). A second-law analysis of the data is in good agreement. From the slope of the van't Hoff plot in the third-law analysis, the enthalpy of formation for ClOOCl is calculated, (298 K) = 130.0 ± 0.6 kJ mol-1. The equilibrium constant results from this study suggest that the uncertainties in K eq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.

Unimolecular decomposition kinetics of the stabilised Criegee intermediates CH2OO and CD2OO.

Decomposition kinetics of stabilised CH2OO and CD2OO Criegee intermediates have been investigated as a function of temperature (450-650 K) and pressure (2-350 Torr) using flash photolysis coupled with time-resolved cavity-enhanced broadband UV absorption spectroscopy. Decomposition of CD2OO was observed to be faster than CH2OO under equivalent conditions. Production of OH radicals following CH2OO decomposition was also monitored using flash photolysis with laser-induced fluorescence (LIF), with results indicating direct production of OH in the v = 0 and v = 1 states in low yields. Master equation calculations performed using the Master Equation Solver for Multi-Energy well Reactions (MESMER) enabled fitting of the barriers for the decomposition of CH2OO and CD2OO to the experimental data. Parameterisations of the decomposition rate coefficients, calculated by MESMER, are provided for use in atmospheric models and implications of the results are discussed. For CH2OO, the MESMER fits require an increase in the calculated barrier height from 78.2 kJ mol-1 to 81.8 kJ mol-1 using a temperature-dependent exponential down model for collisional energy transfer with ΔEdown = 32.6(T/298 K)1.7 cm-1 in He. The low- and high-pressure limit rate coefficients are k1,0 = 3.2 × 10-4(T/298)-5.81exp(-12 770/T) cm3 s-1 and k1,∞ = 1.4 × 1013(T/298)0.06exp(-10 010/T) s-1, with median uncertainty of ∼12% over the range of experimental conditions used here. Extrapolation to atmospheric conditions yields k1(298 K, 760 Torr) = 1.1+1.5-1.1 × 10-3 s-1. For CD2OO, MESMER calculations result in ΔEdown = 39.6(T/298 K)1.3 cm-1 in He and a small decrease in the calculated barrier to decomposition from 81.0 kJ mol-1 to 80.1 kJ mol-1. The fitted rate coefficients for CD2OO are k2,0 = 5.2 × 10-5(T/298)-5.28exp(-11 610/T) cm3 s-1 and k2,∞ = 1.2 × 1013(T/298)0.06exp(-9800/T) s-1, with overall error of ∼6% over the present range of temperature and pressure. The extrapolated k2(298 K, 760 Torr) = 5.5+9.2-5.5 × 10-3 s-1. The master equation calculations for CH2OO indicate decomposition yields of 63.7% for H2 + CO2, 36.0% for H2O + CO and 0.3% for OH + HCO with no significant dependence on temperature between 400 and 1200 K or pressure between 1 and 3000 Torr.

Experimental investigations of potassium chemistry in premixed flames

Quantitative potassium species concentrations have been measured in alkali-seeded premixed methane-air flames of different stoichiometry. Potassium chloride (KCl) and hydroxide (KOH) were measured by broadband UV absorption spectroscopy and laser-induced photofragmentation fluorescence, while atomic potassium was measured using tuneable diode-laser spectroscopy. In addition, laser Rayleigh scattering was employed for temperature measurements. Investigations were made for different alkali-seeding levels and chlorine loads resulting in KCl/KOH concentrations up to ∼30ppm and concentrations of K atoms at ppm levels. Experimental results were compared with predictions from a chemical mechanism used in a homogenous reactor model. The observed trends, decrease in KCl and KOH concentrations and increase in K-atom concentrations with flame equivalence ratio, were well reproduced in simulations and are compared with results reported in literature. In addition, possibilities using the experimental methods for further investigations of alkali-related combustion phenomena and detailed model validation of alkali chemistry are discussed.

Absolute OH density measurements in an atmospheric pressure dc glow discharge in air with water electrode by broadband UV absorption spectroscopy

Spatially resolved absolute OH radical density measurements are performed in an atmospheric pressure glow discharge generated in ambient air with water electrode by broadband UV absorption spectroscopy. The radial distributions of OH density and gas temperature are obtained for the positive column, anode and cathode regions both for water-cathode and water-anode discharges. It is found that for both polarities of the water electrode the radial profiles of the ground state OH density and gas temperature are significantly broader than the total discharge emission intensity and the emission intensity originating from OH() only. Exceptional large OH densities exceeding 1023 m−3 are found. The OH kinetics are discussed in detail.

Plasma–wall interactions during silicon etching processes in high-density HBr/Cl2/O2 plasmas

We have investigated the production and loss kinetics of SiClX radicals during silicon gate etching processes in HBr/Cl2/O2 plasmas. The absolute concentrations of SiClX (X = 0–2) radicals have been measured by broad band UV absorption spectroscopy at different O2 gas flow rates in the process gas mixture, and at different RF powers injected in the plasma. At the same time, the chemical composition of the layer deposited on the reactor walls has been investigated by x-ray photoelectron spectroscopy analysis. Without O2 in the plasma, the reactor walls stay clean because the silicon containing compounds redeposited on them (from Si, SiCl, Si+ and SiCl+ precursors) are subsequently etched by Cl atoms and recycled back into the plasma as SiCl2 and SiCl4 volatile species. Hence, the reactor walls are a region of production for these species, leading to their high concentrations in the gas phase. The introduction of O2 gas into the plasma results in the oxidation of the silicon chloride radicals resident on the surfaces and in the appearance of a silicon oxychloride layer on the reactor walls, whose deposition rate increases rapidly with the O2 flow. As a consequence, the production rate of SiCl2 by the reactor walls decreases because a part of the silicon containing species redeposited on the reactor walls is oxidized and incorporated in the silicon oxychloride film instead of being recycled back into the plasma as SiCl2 and SiCl4. Finally, a simple zero-dimensional model is built to predict the densities of SiClX radicals from the measured densities of SiClX+1 radicals and ions. The comparison between the calculated and measured densities at different RF powers allowed us to conclude that SiCl and Si radicals are produced both in the gas phase by electron impact dissociation of SiCl2 (SiCl) radicals, and at the reactor walls by the neutralization and reflection of approximately 50% of the SiCl+ (Si+) ions impinging on these surfaces. At the same time, SiCl and Si are estimated to be lost (adsorption and abstraction reactions) on the reactor walls with a probability ranging between 0.2 and 1.

CF and CF2 radical kinetics and transport in a pulsed CF4 ICP

The kinetics of CF and CF2 radicals (in their electronic ground states) was investigated using space and time resolved laser induced fluorescence (LIF) in an inductively coupled discharge in CF4 gas at 33 mTorr, both in the steady state and in pulsed plasmas. The gas temperature was determined from the rotationally resolved LIF excitation spectra of the CF radical, reaching about 800 K in the centre of the chamber. The densities were put on an absolute scale with the aid of broad-band UV absorption spectroscopy. The density profiles were used to deduce the radical diffusive transport, and therefore the localized (gas-phase and surface) net production or destruction rates. However, when gas heating is significant, the radical transport becomes more complex due to gas density gradients and thermo-diffusion. In the afterglow, gas convection induced by gas cooling and contraction can play a significant role. The CF and CF2 axial fluxes estimated from measured density and temperature profiles indicate that these species are emitted from the top and bottom surfaces, and are destroyed in the gas phase at high rates (≈900 s−1 and 1500 s−1, respectively): for CF2 this process appears to be electron-impact induced. In the afterglow, the CF2 density at the reactor centre increases abruptly, reaching four times the steady-state value after 3 ms, followed by a slow decay (100 s−1). This phenomenon is the combined result of the return of CF2-rich gas as the central region cools and contracts, along with gas-phase reactions producing CF2. The central CF density shows only a slight initial increase in the afterglow, followed by a rapid decay (600 s−1), due to gas-phase chemical reactions, e.g. with F2 or CF3, converting it to CF2, processes which are also likely to be important in the steady state.

Kinetics of the gas phase HO2 self-reaction: Effects of temperature, pressure, water and methanol vapours

The kinetics of the gas phase HO2 self-reaction have been studied using flash photolysis of Cl2/CH3OH/O2/N2 mixtures coupled with time-resolved broadband UV absorption spectroscopy. The HO2 self-reaction rate coefficient (HO2 + HO2 --> H2O2 + O2 (R1)) has been determined as a function of temperature (236 < T < 309 K, at 760 Torr) and pressure (100 < p < 760 Torr, at 296 K). In addition, the effects of water vapour ((0-6.0) x 10(17) molecules cm(-3), 254 < T < 309 K at 760 Torr, 400 < p < 760 Torr at 296 K) and methanol vapour ((0.06-4.7) x 10(17) molecules cm(-3), 254 < T < 309 K, at 760 Torr) on the rate coefficient have been characterised. The observed rate coefficient, k1, was found to exhibit a negative temperature dependence with both pressure dependent and pressure independent components, in agreement with previous studies. Furthermore, the rate coefficient k1 was found to be enhanced in the presence of elevated H2O or CH3OH concentrations, as reported previously. This study reports the most extensive characterisation of the rate coefficient k1 as a function of T, p, [H2O] and [CH3OH]. The present results indicate that k1 is higher at low temperatures, and that enhancement of k1 by H2O is greater, than has been reported previously. The pressure dependence of k1 at ambient temperature is in good agreement with previous studies. The rate enhancement by CH3OH reported here is in good agreement with previous studies at ambient temperatures but is smaller at low temperatures than the most recent previous investigation suggests. The rate coefficient k1 is adequately parameterised by: k1(760 Torr) = {(1.8 +/- 0.8) x 10(-14) exp((1500 +/- 120)/T/K)} x {1 + (2.0 +/- 4.9) x 10(-25) [H2O] exp((4670 +/- 690)/T/K)} x (1 + (0.56 +/- 1.00) x 10(-21) [CH3OH] exp((2550 +/- 500)/T/K)} cm(-3) molecule(-1) s(-1), where [H2O] and [CH3OH] are in molecules cm(-3). Errors are 1 sigma, and statistical only. The atmospheric implications of these results are briefly discussed.

Identification of halogen containing radicals in silicon etching plasmas and density measurement by UV broad band absorption spectroscopy

Silicon etching by-products play a major role in silicon gate etching processes because they are the precursors to SiOClX passivation layer formation on feature sidewalls and on reactor walls. As a result, the understanding and modelling of these processes relies on the knowledge of the absolute value of SiClX (SiBrX) fluxes to the surfaces exposed to the plasma. However, only a few experimental data on the absolute concentration of SiClX etch byproducts are available in the literature. In this work, we show that broad band UV absorption spectroscopy can be used to measure the absolute densities of SiClX (X = 0–2), SiFX (X = 1–2) and SiO species, as well as to detect SiBr molecules. In addition, a new absorption spectrum has been observed and attributed to the SiClF radical. We have used a state of the art silicon gate etching process operating in a low pressure high density HBr/Cl2/O2/CF4 plasma etching a 200 mm diameter silicon wafer to produce the different radicals.The absorption bands of these species are all located in the 200–400 nm spectral region. Absorbances of 10−4 could be detected with a good signal to noise ratio in a few seconds exposure time using a Xe arc light source and a photodiode array detector. Details of the experimental procedure used to measure absorption spectra and to extract absolute concentrations from these spectra are presented, including a discussion concerning the influence of the monochromator spectral resolution on the absolute density determination for atomic species.

Optical and electrical diagnostics of fluorocarbon plasmaetching processes

This article reviews recent work concerning the role of CF and CF2 radicals in etching and polymerization processes occurring in capacitively coupled radio-frequency plasmas in fluorocarbon gases used for the selective etching of SiO2 layers in microelectronic device fabrication. Laser-induced fluorescence (LIF) was used to determine time-resolved axial concentration profiles of these species in continuous and pulse-modulated CF4 and C2F6 plasmas. Calibration techniques, including broad-band UV absorption spectroscopy, were developed to put the LIF measurements on an absolute scale. A novel technique was used to determine the ion flux to the reactor walls in these polymerizing environments. The mass distribution of the ions arriving at the reactor walls was determined using a quadrupole mass spectrometer. It was found that CFx radicals are produced predominantly by the reflection of neutralized and dissociated CFx+ ions at the powered electrode surface. When the fluorine atom concentration is high, the CFx radicals are destroyed effectively by recombination catalysed by the reactor walls. When the fluorine atom concentration is lowered, the CF2 concentration rises markedly, and it participates in gas-phase oligomerization processes, forming large CxFy molecules and, after ionization, large CxFy+ ions. These species appear to be the true polymer precursors. This mechanism explains the well known correlation between high CF2 concentrations, polymer deposition and SiO2 over Si etch selectivity.

Absolute radical densities in etching plasmas determined by broad-band UV absorption spectroscopy

Broad-band UV absorption spectroscopy was used to determine radical densities in reactive gas plasmas generated in a 13.56 MHz capacitively coupled parallel plate reactor. Five radical species were detected: , CF, AlF, and . Absolute (line-integrated) densities were determined in and plasmas, as were the vibrational and rotational temperatures in the latter case. In plasmas the CF radical was also detected, along with the etch products AlF (from the Al powered electrode) and (when an Si substrate was present). The fraction that comprises of the total etch products was estimated. Finally, the dimer was detected in an plasma in the presence of an Si substrate. This simple technique allows absolute concentrations of many key reactive species to be determined in reactive plasmas, without the need to analyse the complex rotational spectra of these polyatomic molecules.