Repetitive Pulsed Irradiation Research Articles

Influence of repetitive pulsed laser irradiation on the surface characteristics of an aluminum alloy in the melting regime

We have investigated the influence of repetitive near-infrared (NIR) pulsed laser shots in the melting regime on the surface characteristics of an aluminum 6061-T6 alloy. Characteristics of interest include surface morphology, surface roughness, and surface hardness in the melted zone as well as the size of the melted zone. For this study, the proper pulse energy for inducing surface melting at one shot is selected using numerical simulations that calculate the variation in temperature at the laser beam spot for various input pulse energies in order to find the proper pulse energy for raising the temperature to the melting point. In this study, 130 mJ was selected as the input energy for a Nd:YAG laser pulse with a duration of 5 ns. The size of the melted zone measured using optical microscopy (OM) increased logarithmically with an increasing shot number. The surface morphology observed by scanning electron microscopy (SEM) clearly showed a re-solidified microstructure evolution after surface melting. The surface roughness and hardness were measured by atomic force microscopy (AFM) and nano-indentation, respectively. The surface roughness showed almost no variation due to the surface texturing after laser shots over 10. The hardness inside the melted zone was lower than that outside the zone because the β″ phase was transformed to a β phase or dissolved into a matrix.

Collisional quenching of electronically excited germanium atoms, Ge[4p 2( 1S 0)], by small molecules investigated by time-resolved atomic resonance absorption spectroscopy

The collisional quenching of electronically excited germanium atoms, Ge[4p 2( 1S 0)], 2.029 eV above the 4p 2( 3P 0) ground state, has been investigated by time-resolved atomic resonance absorption spectroscopy in the ultraviolet at λ = 274.04 nm [4 d( 1 P 1 0) ← 4 p 2( 1 S 0)] . In contrast to previous investigations using the ‘single-shot mode’ at high energy, Ge( 1S 0) has been generated by the repetitive pulsed irradiation of Ge(CH 3) 4 in the presence of excess helium gas and added gases in a slow flow system, kinetically equivalent to a static system. This technique was originally developed for the study of Ge[4p 2( 1D 2)] which had eluded direct quantitative kinetic study until recently. Absolute second-order rate constants obtained using signal averaging techniques from data capture of total digitised atomic decay profiles are reported for the removal of Ge( 1S 0) with the following gases ( k R in cm 3 molecule −1 s −1, 300 K): Xe, 7.1 ± 0.4 × 10 −13; N 2, 4.7 ± 0.6 × 10 −12; O 2, 3.6 ± 0.9 × 10 −11; NO, 1.5 ± 0.3 × 10 −11; CO, 3.4 ± 0.5 × 10 −12; N 2O, 4.5 ± 0.5 × 10 −12; CO 2, 1.1 ± 0.3 × 10 −11; CH 4, 1.7 ± 0.2 × 10 −11; CF 4, 4.8 ± 0.3 × 10 −12; SF 6, 9.5 ± 1.0 × 10 −13; C 2H 4, 3.3 ± 0.1 × 10 −10; C 2H 2, 2.9 ± 0.2 × 10 −10; Ge(CH 3) 4, 5.4 ± 0.2 × 10 −11. The results are compared with previous data for Ge( 1S 0) derived in the single-shot mode where there is general agreement though with some exceptions which are discussed. The present data are also compared with analogous quenching rate data for the collisional removal of the lower lying Ge[4p 2( 1D 2)] state (0.883 eV), also characterized by signal averaging methods similar to that described here.

Kinetic investigation of the collisional behavior of ground state atomic carbon, C(2p2(3PJ)), with halogenated olefins and aromatic compounds studied by time‐resolved atomic resonance absorption spectroscopy in the vacuum ultra‐violet

AbstractA kinetic investigation is described of the reaction of ground state atomic carbon, C(2p2(3PJ)), monitored by time‐resolved atomic resonance absorption spectroscopy, with a wide range of halogenated olefins and aromatic compounds. Atomic carbon was generated by the repetitive pulsed irradiation (λ > ca. 160 nm) of C3O2 in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(23PJ) was then monitored photoelectrically by time‐resolved atomic resonance absorption in the vacuum ultra‐violet (λ = 166 nm, 33PJ ← 23PJ) with direct computer interfacing for data capture and analysis. The following absolute second‐order rate constants for the reactions of C(23PJ) with the following reactants are reported: Reactant kR/cm3 molecule−1 s−1 (300 K) C2F4 (1.9 ± 0.1) × 10−10 C2Cl4 (10.6 ± 0.5) × 10−10 CH2CF2 (4.3 ± 0.2) × 10−10 CHClCCl2 (7.9 ± 0.4) × 10−10 C6H6 (4.8 ± 0.3) × 10−10 C6F6 (4.9 ± 0.3) × 10−10 C6HF5 (5.0 ± 0.3) × 10−10 C6H2F4 (4.4 ± 0.2) × 10−10 C6H5—CH3 (5.5 ± 0.3) × 10−10 C6F5—CF3 (5.4 ± 0.3) × 10−10 These results, constituting the first reported body of absolute rate data for reactions of ground state carbon with these reactants, are compared with the analogous body of absolute rate data for atomic silicon in its Si(3p2(3PJ)) ground state, also determined hitherto by time‐resolved atomic resonance absorption spectroscopy and demonstrating similar kinetic behavior. © 1993 John Wiley & Sons, Inc.

Kinetic studies of the reactions of ground state atomic carbon, [formula omitted], with halogenated methanes investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet

A kinetic investigation is presented of the reactions of ground state atomic carbon, C(2p 2( 3P J)) , with the molecules methyl chloride, methyl bromide, methyl iodide, methyl fluoride, dichloromethane, chloroform and carbon tetrachloride. Atomic carbon was generated by the repetitive pulsed irradiation (λ > ca. 160 nm) of C 3O 2 in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(2 3 P J ) was then monitored photoelectrically by time-resolved atomic resonance absorption in the vacuum ultraviolet (λ = 166 nm, 3 3 P J ← 2 3 P J ) with direct computer interfacing for data capture and analysis. The following absolute second-order rate constants for the reactions of C(2 3 P J ) with the above collision partners are reported (errors 2σ): Reactant k R/ cm 3 molecule −1 s −1 (300 K) CH 3Cl (4.5 ± 0.3) × 10 −12 CH 3Br (5.2 ± 0.3) × 10 −11 CH 3I (5.3 ± 0.3) × 10 −11 CH 3F (1.3 ± 0.1) × 10 −12 CH 2Cl 2 (5.9 ± 0.3) × 10 −12 CHCl 3 (8.9 ± 0.5) × 10 −12 CCl 4 (1.5 ± 0.1) × 10 −11 These yield, to the best of our knowledge, the first reported body of absolute rate data for reactions of ground state carbon with these reactants. Although a large body of absolute rate data for atomic silicon in its Si(3P 2( 3P J)) ground state has now been determined directly in recent years by time-resolved atomic resonance absorption spectroscopy in the ultraviolet itself with a wide range of collision partners, there are no analogous data for these particular reactants with which the data for C(2 3 P J ) may be compared. Estimates of the small energy barriers consistent with the rate data for C(2 3 P J ) + CH 3 Cl, Br, I coupled with the bond dissociation energies of these polyatomic reactants in particular confirms the values of the bond dissociation energies for CCl, Br, I derived from mechanistic interpretation on C 2 Swann band emission from alkali metal-organic halide diffusion flames.

The Collisional Behaviour of Ground State Atomic Carbon, C(2p2 (3PJ)), with Unsaturated Cyclic Compounds Studied by Time‐Resolved Atomic Resonance Absorption Spectroscopy in the Vacuum Ultra‐Violet

AbstractWe present a kinetic investigation of the collisional behaviour of ground state atomic carbon, C(2p2 (3PJ)), with unsaturated cyclic compounds. Atomic carbon was generated by the repetitive pulsed irradiation (λ > ca. 160 nm) of C3O2 in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(23PJ) was then monitored photoelectrically by time‐resolved atomic resonance absorption in the vacuum ultra‐violet (λ = 166 nm, 33PJ←23PJ) with direct computer interfacing for data capture analysis. The following absolute second‐order rate constants for the reactions of C(23PJ) with the following cyclic collision partners are reported (errors 2σ), demonstrating rapid reaction in all cases: magnified image These results constitute the first reported body of absolute rate data for reactions of ground state carbon with these reactants. Whilst a large body of absolute rate data for atomic silicon in its Si(3p2 (3PJ)) ground state has now been determined directly in recent years by time‐resolved atomic resonance absorption spectroscopy in the ultra‐violet itself with a wide range of collision partners, including unsaturated reactants, there are no analogous data for these cyclic reactants. Comparison is made with rate data for C(23PJ) with benzenoid and fluorinated benzenoid collision partners, and the results also discussed in the context of nucleation and soot formation in flames.

The collisional behaviour of ground state atomic carbon, C(2p2(3Pj) with ethylene and acetylene investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet

Abstract We present a kinetic study of the collisional behaviour of ground state atomic carbon, C(2p2(3PJ) with the molecules C3O2, C2H4 and C2H2. Atomic carbon was generated by the repetitive pulsed irradiation (λ>ca. 160 nm of C3O2 in a slow flow system and then monitored photoelectrically by time-resolved atomic resonance absorption in vacuum ultraviolet (λ= 166 nm, 33PJ←23PJ) with direct computer interfacing for data capture and analysis. The following absolute second-order rate constants for the reactions of C(23PJ) with the molecules C3O2, C2H4 and C2H2 were obtained: kR/cm3 molecule−1 s−1(300 K); C3O2 (1.8 ±0.1) x 10−10, C2H4(2.0±0.1) x 10−10 and C2H4(2.1±0.1) x 10−10. The result for C3O2 is in accord with earlier measurements. Those for C2H4 and C2H2 yield values at least six orders of magnitude faster than initial estimates from earlier indirect measurements on a flow system.

Absolute rate data for the reactions of ground-state atomic carbon, C[2p2(3P J )], with alkenes investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet

The collisional behaviour of ground-state atomic carbon, C[2p2(3PJ)], monitored by time-resolved atomic resonance absorption spectroscopy, is reported for reactions with a wide range of alkenes. Atomic carbon was generated and studied in a modified vacuum ultraviolet experimental system similar to that employed hitherto by the repetitive pulsed irradiation (λ≳ 160 nm) of C3O2 in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(2 3PJ) was then monitored photoelectrically by time-resolved atomic resonance absorption in vacuum UV (λ= 166 nm, 3 3PJâ†� 2 3PJ) with direct computer interfacing for data capture and analysis. The following absolute second-order rate constants for the reactions of C(2 3PJ) with the following alkenes are reported: [graphic omitted] These results, constituting the first reported body of absolute rate data for reactions of ground-state carbon with alkenes, are compared with the analogous body of absolute rate data for atomic silicon in its Si[3p2(3PJ)] ground state, also determined hitherto by time-resolved atomic resonance absorption spectroscopy and demonstrating similar kinetic behaviour. The result for ethene is greater by at least six orders of magnitude than the previous estimate derived from indirect measurements on a flow system and is in accord with earlier considerations of the nature of the potential surface for collision. The result for reaction of C(2 3PJ) with C3O2[kR/cm3 molecule–1 s–1(300 K)=(1.8 ± 0.1)× 10–10] is found to be in accord with earlier measurements.

Absolute rate data for the collisional removal of Ge(4 3P 1) by olefins by time-resolved atomic resonance absorption spectroscopy

We present a kinetic study of the collisional removal of atomic germanium, Ge(4 3P 1), with a range of olefins. This atomic state was generated by the repetitive pulsed irradiation of Ge(CH 3) 4 in the presence of excess helium buffer gas in a slow flow system, kinetically equivalent to a static system, and monitored in the time-resolved mode, on time scales by which Boltzmann equilibration within GE(4 3P J ) had taken place, at λ = 259.3 nm (Ge(5s( 3P 1)) ← Ge(4p 2( 3P 1)). A new body of absolute second-order rate constants, k R, for collisional removal by the olefins, R, are reported ( T = 300 K, errors = 2 σ): R k R (×10 −10 cm 3 molecule −1 s −1) Ethylene 1.2 ± 0.2 Propylene 1.8 ± 0.2 cis-Butene 1.0 ± 0.4 1-Pentene 1.5 ± 0.6 1-Hexene 1.3 ± 0.6 Cyclohexene 2.5 ± 0.4 Cycloheptene 2.5 ± 0.4 1,5-Cyclo-octadiene 3.6 ± 0.2 These data, representing collisional removal of atomic germanium with efficiencies of those of the order of the collision numbers, are compared with analogous absolute data for Si(3 3P j ).

Measurement of absolute rate data for the reactions of ground state atomic silicon Si(33PJ) with acetylenes by time-resolved atomic resonance absorption spectroscopy

We present a kinetic study of ground state atomic silicon with acetylenic reactants. Si(3 3 P J ) was generated by the repetitive pulsed irradiation of SiCl 4 in the presence of excess helium buffer gas and the reactant R in a slow flow system, kinetically equivalent to a static system. The transient atom was then monitored by time-resolved atomic resonance absorption spectroscopy at λ = 252 nm (Si(4 3 P J ) ← Si(3 3 P J )) using signal averaging techniques. The following absolute second-order rate constants k R are reported ( T = 300 K; erros, 2σ): These rate constants constitute a new body of fundamental rate data for the atom. They are discussed within the context of nuclear recoil measurements in general for 31 Si and insertion reactions of silylenes. We also report a value of the diffusion coefficient of atomic silicon in helium of D 12 {Si(3 3 P J )−He} = 0.48 ± 0.04 (2σ) cm 2 s −1 at 1 atm ( T = 300 K).

Measurement of absolute rate data for the reaction of ground state atomic silicon, Si(33PJ), with halogenated olefinic and aromatic compounds by time‐resolved atomic resonance absorption spectroscopy

AbstractWe present a kinetic study of the reaction of ground state silicon atoms with halogenated unsaturated organic compounds (R). Si(33PJ) was generated by the repetitive pulsed irradiation of SiCl4 in the presence of excess helium buffer gas and the reactant R in a slow flow system, kinetically equivalent to a static system. The ground state atom was monitored by time‐resolved atomic resonance absorption spectroscopy at λ = 252 nm [Si(43PJ) ← Si(33PJ)] on time scales by which the optically metastable tates,Si(31D2) and Si(31S0) had relaxed to the 3P state, using signal averaging methods. Computerized fitting of the resulting atomic decay traces in the presence of the various reactants, R, yielded the following new body of absolute second‐order rate constants (kR, T = 300 K, errors = 2sigma;): R kR/cm3 molecule−1 s−1 C2F4 1.6 ± 0.2 × 10−10 C2Cl4 9.9 ± 1.7 × 10−10 CH2CF2 4.0 ± 0.6 × 10−10 CHClCCl2 7.0 ± 1.1 × 10−10 CF3CHCH2 4.6 ± 0.5 × 10−10 C6H6 4.4 ± 1.0 times; 10−10 C6F6 4.4 ± 0.6 × 10−10 C6HF5 4.6 ± 1.3 × 10−10 C6H2F4 3.9 ± 0.8 × 10−10 C6F5—CF3 5.1 ± 0.6 × 10−10 These data are compared, where appropriate, with analogous data for unsaturated hydrocarbon organic compounds. They are also discussed within the general context of nuclear recoil measurements involving 31Si.

Kinetics of SiF(X2Πr) by time-resolved molecular resonance absorption spectroscopy following the reaction of Si(33PJ, 31D2, 31S0), generated by pulsed irradiation, with fluorinated compounds

We present an investigation of the ground and first excited states of SiF(X2Πr and A2Σ+) leading to the first measurements of absolute second-order rate constants for this molecule. SiF(X2Πr) was produced from the reaction of Si(33PJ, 31D2, 31S0) with molecules of the type RF, generated by the repetitive pulsed irradiation of SiCl4 in the presence of an excess of helium buffer gas in a slow-flow system. SiF(X2Πr) was monitored by time-resolved molecular resonance absorption at λ= 436.8 nm [SiF(A 2Σ+)â†� SiF(X2Πr), (0,0)] using signal-averaging techniques. The growth of SiF derived from the absorption profile measured in the presence of F2, SiF4 and SF6 was in accord with previously reported rate data for the removal of Si(3 3PJ), Si(3 1D2) and Si(3 1S0). The decay components of the SiF(X2Πr) derived from the profiles yielded the following absolute second-order rate constants for reaction with RF: [graphic omitted] F-atom abstraction being highly exothermic in all cases and demonstrating similar reactivity to atomic silicon itself. We also report time-resolved chemiluminescence measurements on SiF(A2Σ+→X2Πr) at λ= 436.8 nm using photon-counting techniques. The emission decay profiles are consistent with the removal of atomic silicon and indicate the correlation of SiF(A–X) chemiluminescence as a spectroscopic marker technique for kinetic studies of atomic silicon in specific electronic states. Finally, from the results of the present investigation and previously reported absolute rate data for the removal of atomic silicon, we suggest the design for the future construction of a pulsed molecular electronic chemical laser based on the transition SiF(a4Σ–)→ SiF(A2Σ+)+hν(1.4 µm) generated by the reaction of excited states of Si(3 1D2 and 3 1S0) with F2.

Absolute third-order rate constants for the recombination reactions between alkali-metal and iodine atoms and the measurement for Rb + I + He

We present a direct measurement of the absolute third-order rate constant for the reaction Rb + I + He → RBI + He (1) which is considered within the context of the recombination of alkali-metal atoms and atomic iodine in general. I(5 2P3/2) was generated by the repetitive pulsed irradiation of RbI in a flow system with He, and monitored by time-resolved atomic resonance fluorescence in the vacuum u.v. at λ= 178.3 nm {I[5p4 6s(2P3/2)]–I[5p5(2P3/2)]} using photon counting. Rb(52S1/2) was monitored in the steady mode by atomic resonance fluorescence at λ= 421 nm [Rb(6 2PJ)–Rb(52S1/2)] using phase-sensitive detection. These measurements yield the value of k1(T= 540 K)=(3.34 ± 0.67)× 10–31 cm6 atom–2 s–1. We also report an estimate of the rate constant for the reaction I(5 2P3/2)+ Rb2→ RbI + I (3) of k3(T= 540 K)= 4.1 × 10–10 cm3 molecule–1 s–1, in accord with previous measurements for Br + K2 derived from molecular beams. The recombination reaction was modelled using trajectory calculations with Monte Carlo averaging, coupled with the Landau–Zener formalism for the covalent and ionic surfaces, the latter being constructed using the Rittner potential. The calculated result was in good accord with the experimentally measured result and further employed for extrapolation of k1 to T= 2400 K. Detailed theoretical consideration of the ‘fall-off’ region indicated that the present measurements have been carried out in the region of third-order kinetics. Direct experimental measurements of the recombination between Cs, Rb, K + I + He have now been reported by the present method, together with calculations of these rate constants that are in good accord with experiment. The analogous reactions for the lighter elements, Na and Li, lie beyond the temperature range accessible by the present experimental system, and hence for these we report analogous trajectory calculations only. The overall body of data may be summarised as follows: [graphic omitted] This set of rate constants emphasises the small temperature dependences for all five recombination processes, a direct result of the recombination dynamics occurring on an ionic surface in each case.

The collisional behaviour of atomic silicon in the electronic states Si(3p 2( 3P J, 1D 2, 1S 0)) with SF 6 by time-resolved atomic resonance absorption spectroscopy

We present a kinetics study of the three states of atomic silicon arising from the overall ground state configuration, namely Si(3p 2( 3P J )), Si(3p 2( 1D 2)) (0.781 eV) and Si(3p 2( 1(S 0)) (1.909 eV) with the molecule SF 6. These atoms, generated by the repetitive pulsed irradiation of SiCl 4 in the presence of SF 6 and helium buffer gas, were monitored photoelectrically by time-resolved atomic resonance absorption spectroscopy at the following wavelengths: λ = 251.6 nm (Si(3 3P 1) ← Si(3 3P J )), λ = 288.16 nm (Si(4 1P 1) ← Si(3 1D 2) and λ = 390.53 nm (Si(4 1P 1) ← Si(3 1S 0)). Of the three states, Si(3 1D 2) showed marked effects due to cascading from Si(3 1S 0) on collision with SF 6. The following absolute rate constants k (cm 3 molecule −1 s −1) (300 K) are obtained: k(Si(3 3P J ) + SF 6) = (3.2 ± 0.5) × 10 −11, k(Si(3 1D 2) + SF 6) = (8.4 ± 0.6 × 10 −11 and k(Si(3 1S 0) + SF 6) = (2.0 ± 0.5) × 10 −10. These are compared with results for the removal of atomic silicon with other fluorinated compounds that have been investigated as collision partners, namely F 2 and CF 4. Subsequent production of SiF in various electronic states is considered as is the feasibility of constructing a laser operating on the transition SiF(a 4Σ −) → SiF(A 2Σ +) + hv (1.4 μm) and resulting from the reaction of Si(3 1D 2) and Si(3 1S 0) generated photochemically from SiCl 4 in the single-shot pulsed mode and in the presence of fluorine-containing molecules.

Absolute third-order rate constant for the reaction between Rb + OH + He determined by time-resolved molecular resonance-fluorescence spectroscopy, OH (A2Σ+–X2Π), coupled with steady atomic resonance-fluorescence measurements, Rb(62PJ–52S1/2)

We present the first direct measurements of the absolute third-order rate constant (k2) for the reaction Rb + OH + He → RbOH + He. OH(X2Π) was generated from the repetitive pulsed irradiation of H2O vapour through CaF2 optics in the presence of an excess of atomic rubidium, derived from a heat-pipe oven, and helium buffer gas. The decay of ground-state hydroxyl radical was monitored by OH(A–X) resonance fluorescence following optical excitation in a flow system, kinetically equivalent to a static system, using signal averaging. Atomic rubidium was monitored in the steady mode via the Rydberg transition, Rb(6 2PJ–5 2S1/2), at λ= 421 nm using phase-sensitive detection. The following value for k2 was obtained: k2=(8.8 ± 1.3)× 10–31 cm6 molecule–2 s–1(T= 490 K). This was extrapolated to high temperatures (2000 K) using the unimolecular reaction rate theory developed by Troe and incorporating, in a quantitatively detailed manner, the effects of hindered rotation on the low-frequency bending modes of RbOH. The extrapolations for the previous rate measurements we have reported for the analogous reactions involving Na and K have been reanalysed to include the effects of hindered rotation in the same manner, yielding the following forms for the recombination rate constants (kR): [graphic omitted]. The rate data extrapolated to 2000 K are also considered in terms of the role of alkali-metal hydroxides and alkali-metal atoms in flame inhibition.

Measurement of the absolute third-order rate constant for the reaction between Cs + I + He by time-resolved atomic resonance fluorescence monitoring of iodine atoms in the vacuum ultraviolet region {I[6s(2PJ)]–I[5p5(2P3/2)]} coupled with steady atomic resonance fluorescence on atomic caesium [Cs(72PJ)– Cs(62S1/2)] in the visible region

We present a direct kinetic measurement of the absolute third-order rate constant for the reaction Cs + I + He → CsI + He. (1) I(52P3/2) was generated by the repetitive pulsed irradiation of CsI produced in a flow system from the reaction of CH3I in the presence of excess atomic caesium derived from a heat-pipe oven. The ground-state iodine atom was monitored by time-resolved atomic resonance fluorescence at λ= 178.3 nm {I[5p46s(2P3/2)]–I[5p5(2P3/2)]} using photon counting in the vaccum ultraviolet region. Cs(62S1/2) was monitored by steady atomic resonance fluorescence of the Rydberg doublet at λ= 457 nm [Cs(72PJ)–Cs(62S1/2)] using phase-sensitive detection. We report a measured absolute third-order rate constant of k1(T= 491 K)=(7.9 ± 1.2)× 10–31 cm6 atom–2 s–1, which we believe to be the first measurement of its kind. We also report an estimate of the rate constant for the reaction I + Cs2→ CsI + Cs (3) of k3(T= 491 K)≈ 2 × 10–10 cm3 molecule–1 s–1, which is found to be in accord with previous kinetic measurements on I + Na2 and Br + K2. The crossing between the covalent and the ionic surface, described by the Rittner potential function, is found to take place at 17.3 Å, and hence the recombination is governed by the large impact parameters involved on collision. Consideration of the splitting between the two surfaces, coupled with the Landau–Zener formalism and Monte Carlo calculations of trajectories on those surfaces, yields a good quantitative account of the observed kinetic behaviour, not only of the present recombination measurements but also of shock-tube data that have been reported for the dissociation of CsI at elevated temperatures (2400 K). In view of this, despite the clear third-order kinetic behaviour observed here for reaction (1), detailed theoretical consideration of the ‘fall-off’ pressure regime found to satisfy both the present measurements and the shock-tube data leads to the preferred value of k1(T = 491 K)= 9.1 × 10–31 cm6 atom–2 s–1(±25%). The combination of the recombination and dissociation-rate measurements and the modelling calculations lead to the temperature dependence of the form k1(491 < T/K < 2400)= 4.1 × 10–30T–0.24 cm6 atom–2 s–1.

Determination of the absolute third-order rate constant for the reaction between Na + OH + He by time-resolved molecular resonance-fluorescence spectroscopy, OH(A2∑+–X2Π), coupled with steady atomic fluorescence spectroscopy, Na(32PJ– 32S1/2)

We present the first direct measurement of the absolute third-order rate constant for the reaction Na + OH + He → NaOH + He. OH(X2Π) was generated by the repetitive pulsed irradiation of H2O vapour through CaF2 optics in the presence of He and an excess of atomic sodium derived from a heat-pipe oven and comprising part of a slow-flow system, kinetically equivalent to a static system. Ground-state OH was monitored by time-resolved molecular resonance fluorescence at λ= 307 nm [OH(A2∑+–X2Π), (0,0)] following optical excitation using pre-trigger photomultiplier gating, photon counting and signal averaging. Na(3 2S1/2) was monitored in the steady mode using atomic resonance fluorescence at λ= 589 nm [Na(3 2PJ)–Na(3 2S1/2)] coupled with phase-sensitive detection. The following absolute third-order rate constant (T= 653 K) was obtained: k1(Na + OH + He)=(1.07 ± 0.21)× 10–30 cm6 molecule–2 s–1. Quantitative extrapolation to high temperatures using the unimolecular reaction rate theory developed by Troe yields the result k1(Na + OH + He)=(4.7 ± 1.0)× 10–26T–1.65 cm6 molecule–2 s–1. Furthermore, modifications to these calculations for flame conditions (M) indicate that k1(Na + OH + M) lies in the range (1–5)× 10–31 cm6 molecule–2 s–1 at 2000 K. This is significantly different to the values of k1 derived from modelling in flames. The role of Na + OH + M in flames seeded with atomic sodium is considered in this paper.

Kinetic studies of the reactions of OH(X2Π) with hydrogen chloride and deuterium chloride at elevated temperatures by time-resolved resonance fluorescence (A2∑+–X2Π)

We present a kinetic study of the reactions of the OH radical with the molecules HCl and DCl over the temperature range 300–700 K in order to investigate the ‘intermediate’ region between room-temperature measurements and flame investigations. Ground-state OH radicals were generated by the repetitive pulsed irradiation of H2O vapour in a flow system, kinetically equivalent to a static system, and monitored in the time-resolved mode by resonance fluorescence at λ= 307 nm [OH(A2∑+→X2Π), (0, 0)] following optical excitation. Decay profiles of the OH radical in the presence of HCl and DCl were constructed following pre-trigger photomultiplier gating, photon counting, signal averaging and computerised analysis. A specially constructed high-temperature stainless-steel reactor, particularly designed for photon-counting measurements on a high-temperature system and critical to the extension of previous rate measurements of such reaction of OH by ca. 250 K, is described in detail with particular emphasis for procedures for minimising large background photon counts normally encountered in this type of system. The resulting rate data obtained in this investigation for the range 300–700 K may be summarised in the Arrhenius forms: OH (X2Π)+ HCl: kR=(2.94 ± 0.48)× 10–12 exp[–(446 ± 32)/T] cm3 molecule–1 s–1, OH (X2Π)+ DCl: kR=(4.04 ± 0.74)× 10–12 exp[–(718 ± 33)/T) cm3 molecule–1 s–1.

Collisional quenching of electronically excited chlorine atoms, Cl[3p 5(2 P 1/2)], by atmospheric gases studied by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet

The collisional quenching of electronically excited chlorine atoms, Cl[3p5(2P1/2)], has been studied by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet at λ= 136.34 nm {Cl[3p4 4s(2P3/2)]â†� Cl[3p5(2P01/2)]}. Cl(3 2P1/2) was generated by the repetitive pulsed irradiation of CCl4 in the presence of excess helium buffer gas. The excited atom was then monitored photoelectrically using signal averaging in the short-time domain before the onset of Boltzmann equilibrium. The following absolute second-order rate constants for the removal of Cl(3 2P1/2), 882 cm–1 above the 3 2P3/2 ground state, are reported (kQ/cm3 molecules–1 s–1, 300 K, errors 1 σ) for a wide range of gases of atmospheric interest: N2, (6.3 ± 1.0)× 10–13; CO2, <5 × 10–13; O2, (2.3 ± 0.3)× 10–11; N2O, (3.7 ± 0.6)× 10–13; H2, <6 × 10–13; H2O, (2.6 ± 0.5)× 10–12; HCl, (1.1 ± 0.1)× 10–12; CH4, (3.9 ± 0.8)× 10–12; CO, ca. 6 × 10–12. These rate data are compared with the collisional-quenching rate constants for the other gases that we have reported previously using this technique which is, at present, the only practical method for monitoring Cl(3 2P1/2) out of Boltzmann equilibrium for fundamental reasons discussed in this paper. General considerations of the chemistry of a Boltzmann system of Cl(3 2PJ) and the roles of the specific spin–orbit states, Cl(3 2P1/2) and Cl(3 2P3/2), are presented with special reference to CH4 and the atmospheric gases in general. Relative rate data for the quenching of the transient precursor, generated from the photolysis of CCl4 and from which Cl(3 2P1/2) is derived, are also presented for most of the gases studied.

Collisional behaviour of atomic silicon in specific electronic states, Si(3 3 P J , 3 1 D 2, 3 1 S 0), with molecular fluorine studied by time-resolved atomic resonance absorption spectroscopy

We present a kinetic study of atomic silicon in the three low-lying electronic states arising from the overall 3p2 ground-state configuration, namely Si(3 3PJ)(0 eV), Si(3 1D2)(0.781 eV) and Si(3 1S0)(1.909 eV). Si(3 3PJ, 3 1D2 and 3 1S0) were generated by the repetitive pulsed irradiation of SiCl4 in the presence of an excess of helium buffer gas in a slow-flow system, kinetically equivalent to a static system. The photochemically generated transient atoms were monitored photoelectrically by time-resolved resonance absorption at λ= 251.6 nm [Si(4 3PJâ†� 3 3PJ)], λ= 288.16 nm [Si(4 1P1â†� 3 1D2)] and λ= 390.53 nm [Si(4 1P1â†� 3 1S0)] using pre-trigger photomultiplier gating with signal averaging. The decays of these atoms were investigated in the presence of molecular fluorine, leading to the following absolute second-order rate constants (k2): [graphic omitted]

Collisional quenching of electronically excited chlorine atoms Cl((3p) 5, 2P [formula omitted]) by C 1 chlorofluorocarbons

A kinetic investigation of the collisional behaviour of the low-lying electronically excited chlorine atom Cl((3p) 5, 2P 1 2 ), located 882 cm −1 above the Cl((3p) 5, 2P 3 2 ) ground state, is reported. Cl(3 2P 1 2 ) was generated by the repetitive pulsed irradiation of each of the C 1 chlorofluorocarbons CF x Cl 4− x in a gaseous flow system kinetically equivalent to a static system and was monitored photoelectrically by time-resolved atomic resonance absorption spectroscopy in the vacuum UV at λ = 136.34 nm (Cl((3p) 4(4s), 2P sol3 2 ) ← Cl((3p) 5, 2P 1 2 0)) using signal averaging methods. The following absolute second-order quenching rate constants k Q for the collisional removal of Cl(3 2P 1 2 ) by the freons are reported ( k Q in centimetres cubed per molecule per second; 300 K; errors, 2σ): CCl 4, (2.1 ± 0.6) × 10 −10; CFCl 3, (3.1 ± 0.6) × 10 −10; CF 2Cl 2, (2.1 ± 0.4) × 10 −10; CF 3Cl, (2.2 ± 0.4) × 10 −10; CF 4, (1.5 ± 0.4) × 10 −10. These results, which are close to the unit collisional efficiencies, are compared where possible with previous measurements and estimates. This work is considered to be a basis for future work on the collisional removal of Cl(3 2P 1 2 ) by various gases in the polluted stratosphere.