Mbar Total Pressure Research Articles

MOCVD of tantalum nitride thin films from TBTEMT single source precursor as metal electrodes in CMOS applications

Tantalum nitride films obtained by MOCVD using a tert-butylimido-tris-ethylmethylamido-tantalum (TBTEMT, Ta(NCMe 3)(NEtMe) 3) compound were physically and electrically investigated. Lowest resistivity of 2.5 mΩcm was obtained at 600 °C and 0.25 mbar total pressure. Deposition rate under these conditions was 4 nm/min, but step coverage was poor. Values of 50 to 75% are obtained at the side walls (10% of trench depth) and 25% at the bottom of a 5 μm deep trench with an aspect ratio of 2:1. Film analysis by XPS reveals Ta rich layers (e.g., Ta:N ratios of 1:0.85 for 400 °C and 1:0.6 for 650 °C deposition temperature, respectively) with a high oxygen contents of about 35 at.%. Oxygen contamination is due to both, continuous incorporation during film growth caused by leakage of the reactor and post-deposition oxidation. TaN electrodes deposited at 600 °C and 0.25 mbar reveal a metal work function of about 4.5 eV independent on thickness of TaN. However, thicker TaN layers show lower leakage current densities. Post-deposition annealing at 900 °C for 10 s does not cause changes in TaN layer thickness, resistivity, or oxide charge, though annealing in NH 3 atmosphere reduces O contamination. Leakage current densities of annealed MOS capacitors (900 °C, 10 s, NH 3) are reduced by nearly one order of magnitude, but leakage current characteristics are strongly degraded after post-deposition annealing. Metal work function is shifted towards mid gap and, in dependence on TaN layer thickness, values of 4.65 eV (90 nm TaN) to 4.79 eV (30 nm TaN) are obtained.

Production and reactivity of the hydroxyl radical in homogeneous high pressure plasmas of atmospheric gases containing traces of light olefins

A photo-triggered discharge has been used to study the production kinetic mechanisms and the reactivity of the hydroxyl radical in a N2/O2 mixture (5% oxygen) containing ethane or ethene for hydrocarbon concentration values in the range 1000–5000 ppm, at 460 mbar total pressure. The discharge (current pulse duration of 60 ns) has allowed the generation of a transient homogeneous non-equilibrium plasma, and the time evolution of the OH density has been measured (relative value) in the afterglow (up to 200 µs) by laser induced fluorescence (LIF). Experimental results have been explained using predictions of a self-consistent 0D discharge and plasma reactivity modelling, and reduced kinetic schemes for OH have been validated. It has been shown that recombination of H- and O-atoms, as well as reaction of O with the hydroperoxy radical HO2, plays a very important role in the production of OH radicals in the mixture with ethane. H is a key species for production of OH and HO2 radicals. As for ethane, O, H and HO2 are key species for the production of OH in the case of ethene, but carbonated radicals, following the partial oxidation of the hydrocarbon molecule by O, also play a non-negligible role. The rate constant for O- and H-atom recombination has been estimated to be 3 × 10−30 cm6 s−1 at near ambient temperature, consistent with LIF measurements on OH for both mixtures with ethane and ethene.

Small-volume, ultrahigh-vacuum-compatible high-pressure reaction cell for combined kinetic and in situ IR spectroscopic measurements on planar model catalysts

We describe the design and performance of a high-pressure reaction cell for simultaneous kinetic and in situ infrared reflection (IR) spectroscopic measurements on model catalysts at elevated pressures, between 10−3 and 103mbars, which can be operated both as batch reactor and as flow reactor with defined gas flow. The cell is attached to an ultrahigh-vacuum (UHV) system, which is used for sample preparation and also contains facilities for sample characterization. Specific for this design is the combination of a small cell volume, which allows kinetic measurements with high sensitivity under batch or continuous flow conditions, the complete isolation of the cell from the UHV part during UHV measurements, continuous temperature control during both UHV and high-pressure operation, and rapid transfer between UHV and high-pressure stage. Gas dosing is performed by a designed gas-handling system, which allows operation as flow reactor with calibrated gas flows at adjustable pressures. To study the kinetics of reactions on the model catalysts, a quadrupole mass spectrometer is connected to the high-pressure cell. IR measurements are possible in situ by polarization-modulation infrared reflection-absorption spectroscopy, which also allows measurements at elevated pressures. The performance of the setup is demonstrated by test measurements on the kinetics for CO oxidation and the CO adsorption on a Au∕TiO2∕Ru(0001) model catalyst film at 1–50 mbar total pressure.

Diode laser based photoacoustic water vapor detection system for atmospheric research.

A wavelength modulated, distributed feedback diode laser based photoacoustic water vapor mixing ratio measuring system for atmospheric research applications is presented. Laser modulation parameters were optimized either at 180 or 500 mbar total pressure to enhance the system's sensitivity for low or high pressures (upper troposphere/lower stratosphere or biosphere exchange layer), respectively. A wavelength locking method was developed that ensured sub-picometer absolute (5 x 10(-7) relative) wavelength stability of the laser while consuming minimum additional measurement time. At the calibration of the system, correction factors for the pressure- and temperature-dependence of the photoacoustic signal were determined, which were in turn applied to the calculation of the water vapor mixing ratio from the measured signal during the test operation of the system. The introduced features resulted in reliable, sub-ppm-level water vapor detection even under abrupt gas pressure or temperature variations typical in open atmospheric applications.

FT–IR study of the ring-retaining products from the reaction of OH radicals with phenol, o-, m-, and p-cresol

The ring-retaining products of the OH-initiated degradation of phenol and o-, m-, and p-cresol in the presence of NO x have been investigated and their formation yields determined. The experiments were carried out in a large volume reactor at (298±2) K and 1000 mbar total pressure of synthetic air using FT–IR spectroscopy for the analysis of reactants and products. The products observed and their respective molar yields were: from phenol: 1,2-dihydroxybenzene (80.4±12.1)%, 1,4-benzoquinone (3.7±1.2)% and 2-nitrophenol (5.8±1.0)%; from o-cresol: 3-methyl-1,2-dihydroxybenzene (73.4±14.6)%, methyl-1,4-benzoquinone (6.8±1.0)% and 6-methyl-2-nitrophenol (6.8±1.5)%; from m-cresol: 3-methyl-1,2-dihydroxybenzene (68.6±13.4)%, 4-methyl-1,2-dihydroxybenzene (9.7±2.7)%, methyl-1,4-benzoquinone (11.3±2.5)%, 5-methyl-2-nitrophenol (4.4±1.5)% and 3-methyl-2-nitrophenol (4.3±1.6)% and from p-cresol: 4-methyl-1,2-dihydroxybenzene (64.1±11.3)% and 4-methyl-2-nitrophenol (7.6±2.2)%. Reaction pathways leading to the observed products are proposed and potential ramifications for the atmospheric reaction mechanisms of aromatic hydrocarbons are considered.

FT-IR product study on the photo-oxidation of dimethyl sulphide in the presence of NO x—temperature dependence

The products of the OH radical-initiated oxidation of dimethyl sulphide (DMS) have been investigated as a function of temperature (284, 295, and 306 K) and different initial NO x (NO+NO 2) concentrations: initial NO was varied between 434 and 2821 ppb and NO 2 between 135 and 739 ppb. The experiments were performed at 1000 mbar total pressure in synthetic air using the photolysis of H 2O 2 as the OH-radical source and FT-IR spectroscopy to monitor reactants and products. The major sulphur-containing products identified were SO 2, dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO 2), methane sulphonic acid (MSA), methane sulphonyl peroxynitrate (MSPN) and OCS. The variation of the product yields with temperature and NO x concentration are consistent with the occurrence of both addition and abstraction channels in OH radical-initiated oxidation of DMS. Distinct trends in the yields of the various products have been observed as a function of temperature, initial NO x conditions and also reaction time as NO is consumed in the system. Increasing the initial NO concentration was found to depress the DMSO, SO 2 and OCS formation yields and enhance those of DMSO 2, MSA and MSPN. The yield–time behaviour of DMSO 2 is supportive of a formation mechanism involving addition of O 2 to a (CH 3) 2SOH adduct, formed via the addition channel, followed by sequential reactions with NO and O 2. The mechanisms controlling the concentration–time profiles of the individual products under the present experimental conditions are discussed in detail and consideration is given to possible implications for the photo-oxidation of DMS under ambient conditions.

Vertical Distribution of PH 3 in Saturn from Observations of Its 1-0 and 3-2 Rotational Lines

Far-infrared Fourier-transform spectrometer measurements of the 1-0 and 3-2 PH 3 transitions in Saturn's disk near 267 and 800 GHz (8.9 and 26.7 cm −1), respectively, were analyzed simultaneously to derive a global mean profile for the PH 3 vertical mixing ratio between 100 and 600 mbar total pressure. The far-infrared spectrum is relatively free from spectral interlopers, suffers minimal absorption or scattering by atmospheric particulates, and contains intrinsically weak PH 3 lines that are sensitive to a range of atmospheric depths. The combined spectra are inconsistent with a uniform tropospheric mixing ratio, even with a stratospheric cutoff. They are consistent with a volume mixing ratio of PH 3 that drops from 1.2×10 −5 at 645 mbar pressure to a value of 4.1×10 −7 at 150 mbar pressure, a decrease that is linear in log abundance vs log pressure. The mixing ratio could drop even more quickly at atmospheric pressures below 150 mbar and still be consistent with the data. The mixing ratio may well remain constant with depth for pressures above 630 mbar. The maximum PH 3 mixing ratio in this model is consistent with a [P]/[H] ratio in the deep atmosphere that is about a factor of 10 higher than solar composition. Such a model is consistent with rapid mixing up to the radiative–convective boundary and transport by, for example, vertical waves just above this boundary. In the best fitting model, the eddy diffusion coefficient is ∼10 4 cm 2 near 630 mbar, and it must increase with altitude. The predominant PH 3 loss mechanisms are direct photolysis by UV radiation and scavenging by H atoms produced by the photolysis.

FT-IR product study of the photo-oxidation of dimethyl sulfide: Temperature and O2 partial pressure dependence

The products of the OH-radical initiated oxidation of dimethyl sulfur (DMS) have been investigated under NOx free conditions using the photolysis of H2O2 as the OH radical source and FT-IR spectroscopy to monitor reactants and products at 1000 mbar total pressure (N2+O2). The products were investigated as a function of the temperature (284, 295 and 306±2 K) and the O2 partial pressure (20, 200 and 500 mbar). Dimethyl sulfoxide (DMSO), SO2 and methane sulfonic acid (MSA: CH3SO3H) were the major sulfur-containing products and evidence has been found for the formation of methane sulfinic acid (MSIA: CH3S(O)OH) in considerable yield. The variation of the formation of DMSO and SO2 with temperature and O2 partial pressure is consistent with a mechanism involving both addition and abstraction channels. The molar formation yields of DMSO support that a major fraction of the OH-DMS adduct formed in the addition channel reacts with O2 to form DMSO. Due to the uncertainty in the literature rate coefficients used for the corrections in the formation yield of DMSO the possibility of other channels cannot be completely excluded. High overall molar formation yields of SO2 have been measured, suggesting that further oxidation of the products in both the addition and abstraction channels result mainly in SO2 production under the NOx free conditions employed in the experiments. Consideration is made of the implications of the results for the atmospheric oxidation of DMS under conditions of low NOx such as occur in the remote marine boundary layer.

Kinetic study of the reaction of OH with a series of acetals at 298 � 4 K

Rate coefficients have been measured at 298 ± 4 K and 1000 mbar total pressure for the reactions of OH with a series of symmetrical acetals (ROCH2OR, R = C1 to C4) using a relative kinetic technique. The investigations have been performed in a laboratory photoreactor and also in the large outdoor EUPHORE simulation chamber facility in Valencia, Spain. The following rate coefficients (in units of 10−11 cm3 molecule−1 s−1) have been obtained: dimethoxy methane (R = CH3), 0.49 ± 0.02; diethoxy methane (R = CH3CH2), 1.84 ± 0.18; di-n-propoxy methane (R = CH3CH2CH2), 2.63 ± 0.49; di-iso-propoxy methane (R = (CH3)2CH), 3.93 ± 0.48; di-n-butoxy methane (R = CH3CH2CH2CH2), 3.47 ± 0.42; di-iso-butoxy methane (R = (CH3)2CHCH2), 3.68 ± 0.57; di-sec-butoxy methane (R = CH3CH2C(CH3)H), 4.68 ± 0.05. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 797–803, 1999

Absolute and Relative Rate Constants for the Reactions CH3C(O)O2 + NO and CH3C(O)O2 + NO2 and Thermal Stability of CH3C(O)O2NO2

A pulse-radiolysis system was used to measure absolute rate constants for the reactions of CH3C(O)O2 radicals with NO and NO2 at 295 K and 1000 mbar total pressure of SF6. When the rate of formation and decay of NO2 using its absorption at 400.5 and 452 nm were monitored, the rate constants k(CH3C(O)O2 + NO) = (2.0 ± 0.3) × 10-11 and k(CH3C(O)O2 + NO2) = (1.0 ± 0.2) × 10-11 cm3 molecule-1 s-1 were determined. Long path-length Fourier transform infrared spectrometers were used to study the rate-constant ratio k(CH3C(O)O2 + NO)/k(CH3C(O)O2 + NO2) in 6−700 Torr total pressure of N2 diluent at 243−295 K. At 295 K in 700 Torr of N2 diluent k(CH3C(O)O2 + NO)/k(CH3C(O)O2 + NO2) = 2.07 ± 0.21. The results are discussed in the context of the atmospheric chemistry of acetylperoxy radicals.

Absolute rate constants for F + CH3CHO and CH3CO + O2, relative rate study of CH3CO + NO, and the product distribution of the F + CH3CHO reaction

Using a pulse-radiolysis transient UV–VIS absorption system, rate constants for the reactions of F atoms with CH3CHO (1) and CH3CO radicals with O2 (2) and NO (3) at 295 K and 1000 mbar total pressure of SF6 was determined to be k1=(1.4±0.2)×10−10, k2=(4.4±0.7)×10−12, and k3=(2.4±0.7)×10−11 cm3 molecule−1 s−1. By monitoring the formation of CH3C(O)O2 radicals (λ>250nm) and NO2 (λ=400.5nm) following radiolysis of SF6/CH3CHO/O2 and SF6/CH3CHO/O2/NO mixtures, respectively, it was deduced that reaction of F atoms with CH3CHO gives (65±9)% CH3CO and (35±9)% HC(O)CH2 radicals. Finally, the data obtained here suggest that decomposition of HC(O)CH2O radicals via CC bond scission occurs at a rate of <4.7×105 s−1. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 913–921, 1998

Rate Constants of the Reactions of CHCl (X̃1A′) with Ethylene, Propylene, and Acetylene

AbstractThe rate constants of the reactions of CHCl (X̃1A′) with C2H2 (k4), C2H4 (k5) and C3H6 (k6) were measured as a function of temperature. Experiments were carried out under quasistationary conditions at around 18.5 mbar total pressure. It is shown that excimer laser photolysis of CF2 = CHCl at λPH = 193 nm is a suitable source for CHCl (X̃1A′) generation. The following rate constants were obtained from logarithm plots: k4 = (4.2±0.1)·1012 (T/295K)−(0.79±0.06) cm3 mol−1 s−1 k5 = (3.2±0.2)·1012 (T/295 K)−(1.52±0.12) cm3 mol−1 s−1 k6 = (8.3±1.4)·1012 (T/295 K)−(1.39±0.05) cm3 mol−1 s−1 The reactivity of CHCl (X̃1A′) is compared to those of other carbenes.

Atmospheric Chemistry of CH2BrCl: Kinetics and Mechanism of the Reaction of F Atoms with CH2BrCl and Fate of the CHBrClO• Radical

The reaction of F atoms with CH2BrCl was studied using a pulsed radiolysis technique and found to proceed via two channels: direct hydrogen abstraction and adduct formation. The adduct exists in equilibrium with F atoms and CH2BrCl. The equilibrium constant K4 = [Adduct]/([CH2BrCl][F]) = (4.2 ± 1.0) × 10-16 cm3molecule-1. In 1000 mbar total pressure of SF6 at 294 ± 2 K, the yield of the adduct is 68 ± 11% and the overall rate constant for reaction of F atoms with CH2BrCl is k4 (1000 mbar) = (3.6 ± 0.9) × 10-11 cm3 molecule-1 s-1. The adduct reacts with NO with a rate constant of (2.09 ± 0.41) × 10-11 cm3 molecule-1 s-1. The atmospheric fate of the alkoxy radical CHBrClO is Br atom elimination, which occurs at a rate estimated to be >2 × 106 s-1 in 700 Torr of air at 295 K. Relative rate techniques were used to measure k(Cl + CH2BrCl) = (4.2 ± 0.5) × 10-13 and k(F + CH2BrCl → CHBrCl + HF) = (1.2 ± 0.4) × 10-11 cm3 molecule-1 s-1. These results are compared to the available literature data.

REMPI as a tool for studies of OH radicals in catalytic reactions

+½ O2→H2O on polycrystalline Pt foils has been studied by detection of desorbing OH radicals using the Resonance Enhanced Multiphoton Ionization, REMPI, technique. The measurements were performed at catalyst temperatures of 1000–1400 K and a total pressure below 10-4 mbar. The studies of OH desorption by REMPI were achieved using a two-photon excitation D2Σ-–X2Π (1–0), followed by one-photon ionization. The ions were detected in a time-of-flight mass spectrometer, TOF-MS, in order to avoid interference from non-resonantly ionized molecules. By applying TOF-MS, a simultaneous non-resonant ionization and detection of H2, O2 and H2O was achieved. Recorded REMPI spectra were compared with spectra simulated using known molecular constants. The kinetics of the reaction derived from the measurements were compared with what was obtained in earlier LIF detection of OH, performed at higher total reactant pressure using the A–X transition. REMPI TOF-MS is shown to be a complement to LIF for reaction studies below 5×10-4 mbar total pressure, where LIF is too insensitive for quantification. The reaction kinetics was found to be in agreement with a theoretic model and previous LIF studies.

Absolute rate constants for the self reactions of HO2, CF3CFHO2, and CF3O2 radicals and the cross reactions of HO2 with FO2, HO2 with CF3CFHO2, and HO2 with CF3O2 at 295 K

The kinetics of the self-reactions of HO2, CF3CFHO2, and CF3O2 radicals and the cross reactions of HO2 with FO2, HO2 with CF3CFHO2, and HO2 with CF3O2 radicals, were studied by pulse radiolysis combined with time resolved UV absorption spectroscopy at 295 K. The rate constants for these reactions were obtained by computer simulation of absorption transients monitored at 220, 230, and 240 nm. The following rate constants were obtained at 295 K and 1000 mbar total pressure of SF6 (unit: 10−12 cm3 molecule−1 s−1): k(HO2+HO2)=3.5±1.0, k(CF3CFHO2+CF3CFHO2)=3.5±0.8, k(CF3O2+CF3O2)=2.25±0.30, k(HO2+FO2)=9±4, k(CF3CFHO2+HO2)=5.0±1.5, and k(CF3O2+HO2)=4.0±2.0. In addition, the decomposition rate of CF3CFHO radicals was estimated to be (0.2–2)×103 s−1 in 1000 mbar of SF6. Results are discussed in the context of the atmospheric chemistry of hydrofluorocarbons. © 1997 John Wiley & Sons, Inc.

Non-linear behaviour in the NOH2 reaction over Rh(111)

The dynamic behaviour of the NOH2 reaction over Rh(111) was studied using mass spectrometry and Auger electron spectroscopy in the 10−7–10−5 mbar total pressure range for NO:H2 ratios between 1:5 and 1:30. On cooling the sample in a flow of the gases, an increase in the rate of formation of N2 occurred between 500 and 450 K, which indicated the presence of an autocatalytic step in the reaction sequence. A heat-cool cycle of the sample actually produced a large hysteresis in the formation of all products: N2, NH3 and H2O. This type of behaviour was also observed under similar conditions over the Rh(533) surface, as were regular macroscopic rate oscillations. During these oscillations, N2 production was out of phase with that of NH3 and H2O. The Rh(533) surface consists of four-atom wide (111) terraces separated by (100) steps. The steps seem to play an important role in the synchronisation of the oscillations, as only irregular macroscopic rate oscillations could be observed over Rh(111) under these conditions. Furthermore, the introduction of microscopic defects on the Rh(111) surface by means of Ar+ ion sputtering led to an increase in the synchronisation of the oscillations, these defects are presumably playing a similar role to the steps on the Rh(533) surface.

FTIR Kinetic and Mechanistic Study of the Atmospheric Chemistry of Methyl Thiolformate

Some aspects of the atmospheric chemistry of methyl thiolformate (CH3SCHO), a recently detected intermediate in the oxidation of dimethyl sulfide, have been investigated at 298 K and 1000 mbar total pressure in large reaction chambers using long path in situ FTIR absorption spectroscopy for the analysis. Rate coefficients of (1.11 ± 0.22) × 10-11 and (5.80 ± 0.80) × 10-11 cm3 molecule-1 s-1 have been determined for its reaction with OH radicals and Cl atoms, respectively. The UV spectrum of CH3SCHO has been measured in the range 220−355 nm and a lower limit of 5.4 days determined for its atmospheric photolytic lifetime. Detailed product analyses have made for the OH and Cl initiated photooxidation of CH3SCHO. Strong SO absorption bands observed in both systems are tentatively assigned to CH3SOCHO in the OH system and to CH3SOCl in the Cl system. The first gas-phase spectra of CH3SCl and CH3SOCl are also presented. The results are discussed with respect to the atmospheric chemistry of CH3SCHO and possible ...

Non-linear behaviour in the NO–H2reactions over single crystals and field emitters of some Pt-group metals

Field emission microscopy studies of the NO–H2 reactions on Pt, Rh and Ir field emitters in the 10–7 to 10–4 mbar total pressure range exhibit non-linear behaviour. In the cases of Ir and Rh field emitters, sustained isothermal oscillations in the form of periodic changes in the emission current from various planes present on the tips were observed. In the case of Ir, a bright wave initiated at the Ir(510) surface and moved to the (100) and surrounding planes. In the case of Rh, a bright wave initiated at the Rh(533) surface moved around the (111) plane essentially parallel to the steps around that plane. Subsequently, in both cases, the emission from these bright planes decreased slowly, until the cycle began again. The dynamic behaviour of the NO–H2 reactions over a number of selected large single-crystal surfaces of Pt, Rh and Ir, as studied by mass spectrometry and Auger electron spectroscopy, showed a rich variety of non-linear behaviour. The reaction over Pt(100) and various (111)-terraced Rh planes with (100) steps showed sustained isothermal oscillations in product formation when the temperature was held constant on the cooling branch of a heat–cool heating cycle. In both cases, N2 and NH3 were formed, but for Rh, formation of these products occurred out of phase with each other, whereas for Pt, it occurred in phase. The Ir(100) surface showed a hysteresis in N2 production. Models for the observed non-linear behaviour and the differences in behaviour between Pt, Rh and Ir will be discussed.

A spectrokinetic study of CH2I and CH2IO2 radicals

AbstractThe UV absorption spectrum and kinetics of CH2I and CH2IO2 radicals have been studied in the gasphase at 295 K using a pulse radiolysis UV absorption spectroscopic technique. UV absorption spectra of CH2I and CH2IO2 radicals were quantified in the range 220–400 nm. The spectrum of CH2I has absorption maxima at 280 nm and 337.5 nm. The absorption cross‐section for the CH2I radicals at 337.5 nm was (4.1 ± 0.9) × 10−18 cm2 molecule−1. The UV spectrum of CH2IO2 radicals is broad. The absorption cross‐section at 370 nm was (2.1 ± 0.5) × 10−18 cm2 molecule−1. The rate constant for the self reaction of CH2I radicals, k = 4 × 10−11 cm3 molecule−1 s−1 at 1000 mbar total pressure of SF6, was derived by kinetic modelling of experimental absorbance transients. The observed self‐reaction rate constant for CH2IO2 radicals was estimated also by modelling to k = 9 × 10−11 cm3 molecule−1 s−1. As part of this work a rate constant of (2.0 ± 0.3) × 10−10 cm3 molecule−1 s−1 was measured for the reaction of F atoms with CH3I. The branching ratios of this reaction for abstraction of an I atom and a H atom were determined to (64 ± 6)% and (36 ± 6)%, respectively. © 1994 John Wiley &amp; Sons, Inc.

Reaction of atomic oxygen with some simple alkenes. Part 1.—Low-perssure studies on reactions involving ethene, propene and (E)-but-2-ene

The reaction of atomic oxygen O(3P) with ethene, propene and (E)-but-2-ene has been studied between 10 and 50 mbar total pressure using the alkene as both reactant and bath-gas. NO2 was used as the source of atomic oxygen, and the decay of NO2, which is sensitive to the radical products from the reaction of O atoms with alkenes, was followed by laser-induced fluorescene. Branching ratios for total radical production were obtained for the propene and (E)-but-2-ene systems. The radical branching ratio for the propene system was observed to decrease with pressure by a factor of about two over the range measured, falling from 0.64 at 10 mbar to 0.36 at 40 mbar total pressure. The radical branching ratio for the (E)-but-2-ene system also decreased by a factor of two, falling from a value of 0.22 at 10 mbar to 0.11 at 30 mbar. The results for the ethene system were inconclusive, in that only a range for the branching ratios for production of CH3+ HCO and H + CH2CHO could be obtained. These findings were confirmed by extensive modelling of the results.