Atomic Resonance Absorption Spectroscopy Research Articles

High-temperature oxidation of dispersed soot particles by O atoms

The reaction of O atoms with soot particles dispersed in 10 to 20 ppm N 2 O/Ar mixtures was studied at high temperatures behind reflected shock waves. Under the present conditions ( T >1900 K), N 2 O represents a fast thermal O-atom source, because it decomposes rapidly to form O atoms which in turn react with the dispersed soot particles. The progress in the heterogeneous reaction was determined by time-resolved measurements of O-atom concentration using the highly sensitive atomic resonance absorption spectroscopy. Besides the absorption of the OI radiation ( λ =130.5 nm) by O atoms, the absorption signal includes also particle light extinction. A separation of both influences is possible by performing additional experiments on pure soot/Ar aerosols. Laser light extinction measurements using an Ar + laser ( λ =488.0 nm) were performed simultaneously providing a relationship between the particle extinction at both wavelengths, which allows us to determine the pure O-atom absorption in aerosols. The reaction conditions were chosen such that the total amount of carbon is much higher than the total amount of atomic oxygen: that is, the reaction conditions are pseudo-first-order. A rate coefficient for the consumption of O atoms due to soot particle oxidation was determined. CO was identified to be the main reaction product. To determine the heterogeneous reaction probability from the O-atom consumption rate, it is essential to measure the reactive particle surface. This quantity can be determined from the soot volume fraction available from the laser light extinction.

C 2D 5I dissociation and D+CH 3 → CH 2D+H at high temperature: Implications to the high-pressure rate constant for CH 4 dissociation

The shock tube technique with H- and D-atom atomic resonance absorption spectrometry detection has been used to study the thermal decomposition of C 2 D 5 I and the reaction, CH 3 +D → CH 2 D+H over the temperature ranges 924–1370 and 1294–1753 K, respectively. First-order rate constants for the thermal decomposition of C 2 D 5 I can be expressed by the Arrhenius equation, log k C2D51 = (10.397± 0.297)−(7700±334 K)/ T , giving k C2D51 =2.49 × 10 10 exp(−17,729 K/ T ) s −1 . The branching ratio between product channels, C 2 D 5 +I and C 2 D 4 +DI, was also determined. These results coupled with the fast decomposition of C 2 D 5 radicals were then used to specify [D] t in subsequent kinetics experiments with CH 3 where [CH 3 ] 0 was prepared from the concurrent thermal decomposition of CH 3 I. Within experimental error, the rate constants for reaction 1 were found to be temperature independent with k 1 = (2.20±0.22) × 10 −10 cm 3 molecule −1 s − . The present data have been combined with earlier lower temperature determinations and the joint database has been examined with unimolecular rate theory. The implications of the present study can be generalized to supply a reliable value for the high-pressure limiting rate constant for methane dissociation.

High‐temperature kinetics of some Si‐ and Cl‐containing ceramic precursors

AbstractThis article summarizes our recent experimental investigations of high‐temperature kinetics of Si‐ and Cl‐containing precursor molecules relevant to chemical vapor synthesis and ceramic processing. Reaction systems using SiCl4 and SiHCl3 highly diluted in argon, which were studied in a shock tube using the combination of thermal pyrolysis and laser flash photolysis methods, are described. In situ concentrations of the atomic species Si, Cl, and H were measured simultaneously using the atomic resonance absorption spectroscopy. The measured properties were sensitive to a limited number of elementary reactions, which could be analyzed in terms of rate coefficients. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 741–754, 2001

Rate constants for H + CH4, CH3 + H2, and CH4 dissociation at high temperature

AbstractThe Laser Photolysis‐Shock Tube technique coupled with H‐atom atomic resonance absorption spectrometry has been used to study the reaction, H + CH4 → CH3 + H2, over the temperature range, 928–1697 K. Shock‐tube studies on the reverse of this reaction, CH3 + H2 → H + CH4, using CH3I dissociation in the presence of H2 yielded H‐atom formation rates and rate constants for the reverse process over the temperature range, 1269–1806 K. These results were transformed (using well‐established equilibrium constants) to the forward direction. The combined results for H + CH4 can be represented by an experimental three parameter expression, k = 6.78 × 10−21 T3.156 exp(−4406 K/T) cm3 molecule−1 s−1 (348–1950 K) that was evaluated from the present work and seven previous studies. Using this evaluation, disagreements between previously reported values for the dissociation of CH4 could be reconciled. The thermal decomposition of CH4 was then studied in Kr bath gas. The dissociation results agreed with the earlier studies and were theoretically modeled with the Troe formalism. The energy transfer parameter necessary to explain both the present results and those of Kiefer and Kumaran (J Phys Chem 1993, 97, 414) is, −〈ΔE〉all/cm−1 = 0.3323 T0.7. The low temperature data on the reverse reaction, H + CH3 (in He) from Brouard et al. (J Phys Chem 1989, 93, 4047) were also modeled with the Troe formalism. Lastly, the rate constant for H + CH4 was theoretically calculated using conventional transition state theory with Eckart tunneling corrections. The potential energy surface used was from Kraka et al. (J Chem Phys 1993, 99, 5306) and the derived T‐dependence with this method agreed almost perfectly with the experimental evaluation. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 669–684, 2001

Kinetic measurements of fluorine, bromine and iodine atom-abstraction reactions at elevated temperatures by ground-state atomic rubidium, Rb(5 2S 1/2), studied by time-resolved laser-induced fluorescence

The kinetics of a range of halogen atom-abstraction reactions undergone by atomic rubidium in its electronic ground state, Rb(5 2S 1/2), have been investigated by time-resolved laser-induced fluorescence at elevated temperatures. Rb(5 2S 1/2) was generated in an excess of halogenated reactant and He buffer gas by pulsed photolysis of rubidium halide vapours. Two separate experimental systems were employed operating at the Rydberg transition at λ=420.2 nm {Rb[6p( 2P 3/2)]→Rb[5s( 2S 1/2)]} and at the shorter wavelength component of the spin-orbit resolved D-line doublet transition at λ=780.0 nm {Rb[5s5p( 2P 3/2)]→Rb[5s( 2S 1/2)]}. Second-order rate constants k RX for various F, Br and I abstraction reactions have been measured for essentially single-temperature conditions. These rate constants, which represent a new body of absolute rate data for Rb( 2S 1/2), are compared with the values reported hitherto by time-resolved atomic resonance absorption spectroscopy and with those previously obtained for other alkali metal atoms using time-resolved techniques.

Kinetics and Mechanism for the Oxidation of CS2 and COS at High Temperature

Abstract The rate constants for the thermal decompositions of CS2 and COS were investigated by measuring the time profiles of S atom using atomic resonance absorption spectroscopy behind the incident shock of CS2/Ar or COS/Ar mixtures, and the rate constants were determined to be k = (4.0 ± 0.1) × 1014 exp (−295 ± 20 kJ/RT) cm3 mol−1 s−1, T: 2200–2900 K and k = (1.4 ± 0.3) × 1015 exp (−290 ± 25 kJ/RT) cm3 mol−1 s−1, T: 2000–3200 K, respectively. The oxidation mechanism of COS was also investigated behind the incident shock of COS/O2/Ar mixtures and the rate constant for the reaction S + O2 → SO + O was obtained to be k = (9.5 ± 0.7) × 1013 exp (−41 ± 28 kJ/RT) cm3 mol−1 s−1, T: 2200–2900 K. From a numerical analysis for time profiles of S atom behind the incident shock of CS2/O2/Ar mixtures, the rate constant for the reaction O + CS → CO + S was expressed as k = (3.2 ± 1.0) × 1013 cm3 mol−1 s−1, T: 2100–3000 K. Similarly, the rate constant for the reaction CS + O2 → COS + O was also determined to be k = (6.1 ± 0.6) × 1012 exp (−51.0 ± 68 kJ/RT) cm3 mol−1 s−1, T: 2000–2900 K by measuring the time profiles of O atom behind the reflected shock of CS2/O2/Ar mixtures. Ab initio calculations were carried out and the reaction pathways of CS + O2 → COS + O and CS + O2 → CO + SO were also investigated.

Reactions of ground state atomic calcium, Ca[4s 2( [formula omitted])], investigated by time-resolved atomic resonance absorption spectroscopy at elevated temperature

Absolute second-order rate data at elevated temperature are reported for reactions of ground state calcium atoms, Ca[4s 2( 1 S 0 )], generated by broad band pulsed photodissociation of CaI 2 vapour in the presence of various reactants and excess helium buffer gas and monitored by time-resolved atomic resonance absorption spectroscopy at λ=422.673 nm ( Ca[4 p( 1 P 1)]← Ca[4 s 2( 1 S 0)] ). Unlike most previous time-resolved atomic resonance absorption measurements on ground state atomic calcium at this wavelength, complete decay profiles were recorded digitally in the ‘single-shot mode’ in the time-domain using resonance radiation from a highly intense microwave-powered sealed atomic emission source. The following second-order rate constants for the reactions of Ca( 1 S ) are reported: k R (cm 3 s −1 per molecule) (873 K), 1-C 3H 7Cl (3.5±0.1)×10 −12; 1-C 4H 9Cl (2.5±0.2)×10 −12; 1-C 5H 11Cl (2.2±0.4)×10 −12; 1-C 6H 13Cl (2.8±0.3)×10 −12; CHF 2Cl (4.8±0.2)×10 −12; and CH 3I (16.3±3.0)×10 −12 (errors 1 σ). These data are compared with analogous rate data for reactions of atomic calcium with other halides. Estimates are also reported for diffusion coefficients for Ca( 1 S 0 ) in various buffer gases, D 12 (cm 2 s −1) (STP): He, 0.47±0.05; Kr, 0.14±0.02; Xe, 0.09±0.02; and N 2, 0.26±0.02. These are found to be in sensible accord with analogous values of D 12 ( Ba( 1 S 0)+ He , Kr and Xe) derived hitherto by laser methods.

A High Temperature Chemical Kinetics Study of the Reaction: OH+Ar = H+O+Ar by Atomic Resonance Absorption Spectrophotometry

The OH radical dissociation has been studied behind shock waves in the temperature range of 2950-3700 K at total pressures of about 220-310 kPa by Atomic Resonance Absorption Spectroscopy using mixtures of H2 and O2 highly diluted in Ar. The OH decomposition was followed by monitoring the time dependent O concentration in the post shock reaction zone. In our experimental conditions this reaction is very sensitive to O-atom profiles. The rate constant k−2 of the termolecular recombination reaction between H, O and Ar as collision partner was calculated from OH dissociation rate constant k2 measurements and the equilibrium constant. The results expressed in the simple Arrhenius form are: with an uncertainty of ± 30 %, which are in good agreement with the estimate of Tsang and Hampson in 1986. No experimental results were reported by earlier investigators.

Collisionally induced intramultiplet mixing of Sr(5 3P J) metastable states by He, Ar and Sr ground state atoms

Intramultiplet collisionally induced mixing within the Sr[5s5p( 3P 0,1,2)] manifold is investigated by time-resolved laser-induced fluorescence (LIF) following the pulsed dye-laser generation of Sr( 3P 1) of the electronic ground state {Sr[5s5p( 3P 0,1,2)]←Sr[5s 2( 1S 0)], λ=689.26 nm} at elevated temperatures. The population profiles of the three spin–orbit states were individually monitored by LIF as well as that of Sr( 3P 1) by spontaneous emission at the resonance wavelength. A kinetic model is employed that enables the process of spontaneous emission from Sr( 3P 1) to be isolated initially and characterised by experiment. Particular emphasis is placed on the modelling procedure itself in which the separate kinetic component due to spontaneous emission and the positions of the maxima in the 3P 0 and 3P 2 population profiles constitute severe constraints on the model. The collisional components within the model are reduced to three rate constants where pairs of J states are connected in this context by detailed balance. Thus k 10 and k 12, and, by detailed balance, k 01 and k 21 are quantified at various temperatures and pressures to yield the absolute value of these collision properties for Sr( 3P J ) with He and Ar. Rate data for collisionally induced intramultiplet mixing in Sr( 3P J ) by Sr( 1S 0) itself is also reported found to proceed at close to unit collisional efficiencies in all cases. Thus, at elevated temperatures, variations in atomic profiles are dominated by the differing vapour pressures of atomic strontium. Estimates of the activation energies associated with k 10 and k 12 for the noble gases observed against this large competing background are found to be of the order of the spin–orbit splittings. The model overall is found to be insensitive to k 02 and k 20 whose magnitudes are small by comparison with those for the collisional rate data connecting adjacent J states. Whilst collisional processes for He are some two orders of magnitude more efficient than those for Ar, all of these are seen to be ‘adiabatic’, in contrast with the gas kinetic rate constants of ground Sr, considered to be ‘sudden’ in character. The results are compared with analogous data derived by atomic resonance absorption spectroscopy following pulsed generation of Sr( 3P 1) and are considered in the context of theoretical calculations employing quantum close coupling calculations.

High temperature reaction of Sn(3P0) atoms with O2 based on Sn- and O-concentration measurements

The reaction of Sn(3P0) atoms with O2 has been studied over the temperature range 1300 to 2250 K and the total density range 2.5 × 1018 to 5.4 × 1018 molecules cm−3 by using a shock tube equipped for atomic resonance absorption spectroscopy (ARAS). Tetramethyltin was used as a precursor of Sn(3P0) atoms. The overall rate coefficient was determined from the pseudo-first-order decay of Sn(3P0) atoms to be k(Sn + O2) = 10−9.41 ± 0.03 exp[ − (11.5 ± 1.1) kJ mol−1/RT] cm3 molecule−1 s−1 (error limits at the two standard deviation level). This result is in good agreement with previous data of Fontijn and Bajaj (J. Phys. Chem., 1996, 100, 7085), but about twice that of Zaslonko and Smirnov (Kinet. Catal., 1980, 21, 602). To examine the product channel of the Sn(3P0) + O2 reaction experimentally, O-atom ARAS measurements were performed. Two types of O-profiles were found, one reaching a peak around 40 μs and the other displaying a two-step increase, depending on the composition of the mixtures. When the reaction channel for forming O(3P) atoms was assumed, numerical calculation was found to reproduce both these measured profiles, demonstrating that the main products are SnO(X1Σ+) and atomic O(3P).

Measurements of the Rates of Reactions of Ground State Atomic Calcium, Ca[4s2(1S0)], with Alkyl Bromides at Elevated Temperature by Time-Resolved Atomic Resonance Absorption Spectroscopy [Ca(41P1 - 41S0); λ = 422.7 nm

We present a kinetic study of reactions of ground state atomic calcium, Ca[4

Photolysis of Ketene at 193 nm and the Rate Constant for H + HCCO at 297 K

The 193 nm photolysis of ketene was studied by measuring the amount of atomic hydrogen produced when very dilute ketene/Ar and ketene/H2 mixtures were irradiated by a single pulse from an ArF excimer laser. Absolute concentrations of atomic hydrogen were monitored over a time interval of 0−2.5 ms by using Lyman-α atomic resonance absorption spectroscopy (ARAS). Four different photodissociation channels of ketene were identified: H2CCO + hν gives (a) CH2(3B1) + CO; (b) CH2(1A1) + CO; (c) HCCO + H; and (d) C2O(b1Σ+) + H2. The quantum yields for each channel were measured as φa = 0.628, φb = 0.193, φc= 0.107, and φd = 0.072, respectively. To explore the secondary chemistry that occurred when using higher pressure H2CCO/Ar mixtures, a mechanism was constructed that used well-documented reactions and, for most processes, rate constants that had already been accurately determined. Modeling studies using this mechanism showed the [H] profile to be determined largely by the rate of the reaction H + HCCO → CH2 + ...

Thermal Decomposition of Tin Tetrachloride Based on Cl- and Sn-Concentration Measurements

The kinetics of the thermal decomposition of tin tetrachloride has been studied experimentally and theoretically. Ab initio MO calculations showed that SnCl4 finally decomposed into Sn(3P) and four chlorine atoms through four subsequent Sn−Cl bond dissociation channels. Two sets of kinetic experiments were performed using a shock tube equipped with atomic resonance absorption spectroscopy (ARAS). Chlorine atoms were first measured over the temperature range of 1250−1700 K and the total density range of 1.7 × 1018 to 8.9 × 1018 molecules cm-3. The rate coefficient for the initial reaction step, SnCl4 (+M) → SnCl3(2A1) + Cl (+M) (eq 1a), was found to be in the falloff region fairly close to the low-pressure limit under the present conditions. The second-order rate coefficient based on the Cl-atom measurements was determined to be k1a2nd = 10-5.37±0.62 exp[−(285 ± 18) kJ mol-1/RT] cm3 molecule-1 s-1 (error limits at the 2 standard deviation level). The second group of experiments was carried out by detecting tin atoms over the temperature range of 2250−2950 K and at a total density of 3.2 × 1018 molecules cm-3. The second-order rate coefficients for the subsequent reaction steps: SnCl2(1A1) (+M) → SnCl(2Π) + Cl (+M) (eq 3a) and SnCl(2Π) (+M) → Sn(3P) + Cl (+M) (eq 4a) were obtained to be k3a2nd = 10-8.36±0.86 exp[−(310 ± 42) kJ mol-1/RT] cm3 molecule-1 s-1 and k4a2nd = 10-9.50±0.78 exp[−(265 ± 40) kJ mol-1/RT] cm3 molecule-1 s-1, respectively. The Rice−Ramsperger−Kassel−Marcus (RRKM) calculations including variational transition state theory were also applied for reactions 1a and 3a. Structural parameters and vibrational frequencies of the reactants and transition states required for the RRKM calculations were obtained from the ab initio MO calculations. Energy barriers of the reactions, E0's, which are the most sensitive parameters in the calculations, were adjusted until the RRKM rate coefficients matched the observed ones. These fittings yielded E0,1a = 326 kJ mol-1 for reaction 1a and E0,3a = 368 kJ mol-1 for reaction 3a, in good agreement with the Sn−Cl bond dissociation energies of SnCl4 and SnCl2, demonstrating that the experimental data for k1a and k3a were theoretically reasonable and acceptable.

Thermal rate constants over thirty orders of magnitude for the I+H 2 reaction

I-atom atomic resonance absorption spectrometry has been used to measure thermal rate constants for the I+H 2 → HI+H reaction between 1755 and 2605 K in reflected shock waves. Measured rate constants can then be expressed by k 1=(3.92±1.15)×10 −9 exp(−21 398±658 K/ T) cm 3 molecule −1 s −1. Combining these results with earlier values, the rate behavior over the combined temperature range, 230 K⩽ T⩽2605 K, is: k 1=(4.52±0.34)×10 −10 exp(−17 070±34 K/ T) cm 3 molecule −1 s −1. This result holds over ∼30 orders of magnitude and is discussed in terms of bimolecular rate theory.

Direct observation of the rate of H-atom formation in the thermal decomposition of Ortho-, Meta-, and Para-xylene behind shock waves between 1300 and 1800 K

The rate of H-atom formation in the thermal decomposition of o-, m- , and p -xylene was studied behind reflected shock waves between 1300 and 1800 K and at pressures of 1.6 to 4 bar by using atomic resonance absorption spectroscopy at 121.6 nm. For the thermal decomposition of p -xylene, m -xylene, and o -xylene p -CH 3 C 6 H 4 CH 3 → p -CH 3 C 6 H 4 CH 2 +H (R1) m -CH 3 C 6 H 4 CH 3 → m -CH 3 C 6 H 4 CH 2 +H (R2) o -CH 3 C 6 H 4 CH 3 → o -CH 3 C 6 H 4 CH 2 +H (R3) the following rate constants were determined: k 1 =1×10 16 exp(−380 kJ mol −1 /RT) s −1 , k 2 =1×10 16 exp(−382 kJ mol −1 /RT) s −1 , and k 3 =1×10 16 exp(−378 kJ mol −1 /RT) s −1 . The rate constants are all close to the high-pressure limit, and the accuracy was estimated to be 30%. The H-atom-concentration time profiles were modeled using a simple mechanism in which the parallel C-C bond split, and secondary reactions have been taken into account. We found that within our experimental conditions, the C-C bond scission contributes about 15% to the overall decomposition rate constant of the xylenes.

A shock tube study of the reaction of Si atoms with HCl

The reaction of Si atoms with HCl was studied behind reflected shock waves at temperatures between 1500 and 2850 K and pressures around 1.4 bar. Atomic resonance absorption spectroscopy (ARAS) was applied for time-resolved measurement of Si and Cl atoms. Si atoms were produced by the thermal decomposition of SiH4 and Si2H6, respectively. HCl being present in excess causes a rapid consumption of Si atoms, which follows a pseudo-first-order rate law. The reaction channel and the rate coefficient for the reaction of Si atoms with HCl was determined to be:

High-temeprature investigations on pyrolytic reactions of propargyl radicals

The thermal decomposition and bimolecular recombination of propargyl was studied behind reflected shock waves. In the shock-tube experiments, 3-iodo-propyne was employed as a thermal source for the propargyl radical (H2CCCH). For a different series of experiments, the temperature ranged from 1100 to 2100 K at pressures between 1.5 and 2.2 bar. Initial concentrations of the propargyl radical ranged from 0.6 ppm to 210 ppm, diluted in argon. Atomic resonance absorption spectrometry was used to record the temporal concentration profiles of H atoms during the pyrolysis of propargyl under very low concentration conditions. By gas chromatography-mass spectroscopy analysis of the (postshock) residual gas, different product channels were identified. For the unimolecular decomposition of propargyl R3: C3H3→C3H2+H, a rate expression of k1=5.2E12 exp, (−39480/T) s−1 was derived.

Kinetics of SiHCl 3 thermal decomposition based on Cl and Si atom measurements

Cl and Si concentration measurements were performed in highly diluted SiHCl3/Ar mixtures. The reflected shock method in combination with sensitive atomic resonance absorption spectroscopy was applied. In the literature, the initial decomposition step of SiHCl3 has been suggested to be S i H C l 3 ⇌ k 1 S i C l 2 + H C l (R1) but this has not yet been established. At 1730 K≤T≤3300K, 0.8 bar≤p≤4.0 bar, and mixtures of 0.5 to 5 ppm SiHCl3 in argon, a strongly temperature-dependent formation of Cl atoms was observed. The signals were evaluated in terms of the rate coefficient k2: S i C l 2 + M ⇌ k 2 S i C l + C l + M k 2 = 5.4 −2.3 +3.8 × 10 15 exp ⁡ [ − ( 39 , 300 ± 1200 ) K / T ] c m 3 m o l − 1 s − 1 (R2) Si2H6 is known to be a rapid silicon source at T≥1550 K. In highly diluted Si2H6/Ar mixtures, a very fast consumption of the initial Si atom concentrations was measured when a few parts per million SiHCl3 was added. The signals obtained at 1570 K≤T≤2030 K and p=1.7 bar were kinetically analyzed and assigned to the reaction of Si atoms with SiCl2. S i + S i C l 2 ⇌ k 4 p r o d u c t s k 4 = 5.6 ± 0.97 × 10 14 c m 3 m o l − 1 s − 1 (R4) All results were verified and discussed based on computer simulations.

Initiation in H 2/O 2: Rate constants for H 2+O 2→H+HO 2 at high temperature

The reaction between H2 and O2 has been studied in a reflected shock tube apparatus between temperatures of 1662–2097 K and pressures of 400–570 torr with Kr as the diluent gas. O atom atomic resonance absorption spectrometry (ARAS) was used to observe absolute [O]t under conditions of low [H2]0 so that most secondary reactions were negligible. Hence, the observed [O]t was the direct result of the rate controlling reaction between H2 and O2. Three different reactions were considered, but experimental and ab initio theoretical results both indicated that the process, H2+O2→H+HO2, is the most probable reaction. After rapid HO2 dissociation, O atoms are then instantaneously produced by H +O2→O+OH. Using the ab initio result, conventional transition state theoretical calculations (CTST) with tunneling corrections give the expression k1th=1.228×10−18 T2.4328 exp(−26,926 K/T) cm3 molecule−1 s−1, applicable between 400 and 2300 K. This theoretical result agrees with the present experimental determinations and those at lower temperature, derived from earlier work on the reverse reaction.

Collisional removal of atomic carbon, C[2p 2( 3P J)], by aldehydes and ketones, investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet

A kinetic study is presented of the collisional removal of ground state atomic carbon, C[2p 2( 3P J )], with various aldehydes and ketones in the gas phase following pulsed irradiation. The atomic carbon was generated by the photolysis of C 3O 2 ( λ > ca. 160 nm) in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system, and monitored photoelectrically by time-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet at λ = 166 nm (3 3P J ← 2 3P J ) using signal averaging techniques. Absolute second-order rate constants ( k R/cm 3 molecule −1 s −1, 300 K) for the removal of C(2 3P J ) with these reactants were found to be as follows: formaldehyde 6.2 ± 0.3 × 10 −10; acetaldehyde 5.4 ± 0.3 × 10 −10; propionaldehyde 4.1 ± 0.3 × 10 −10; n-butyraldehyde 6.6 ± 0.3 × 10 −10; pentanal 4.6 ± 0.2 × 10 −10; hexanal 5.3 ± 0.4 × 10 −10; acetone 5.9 ± 0.3 × 10 −10; butanone 5.1 ± 0.2 × 10 −10; 2-pentanone 3.8 ± 0.2 × 10 −10; and 3-pentanone 4.6 ± 0.1 × 10 −10. No significant monotonic variation is thus observed in the rate data within these series of collisional processes where, from the similarity in the observed results, it is concluded that reaction is dominated by attack on the carbonyl group. The large values of these rate constants indicate that reactions of C(2 3P J ) with aldehydes and ketones, some of which have been observed by radio frequency spectroscopy in interstellar clouds and considered to be generated initially by hot atom reactions, are sufficiently rapid to be included in modelling of the interstellar medium.