Time-resolved Atomic Resonance Absorption Spectroscopy Research Articles

Kinetic investigation of the reaction of ground state lead atoms Pb(6 3P 0) with alkyl bromides at elevated temperatures by time-resolved atomic resonance absorption spectroscopy

We present a kinetic investigation of the reactions between ground state lead atoms Pb(6p 2( 3P 0)) and a range of alkyl bromides RBr in the temperature range 640 – 760 K. Pb(6 3P 0) was generated by the pulsed irradiation of the PbBr 2 vapour in equilibrium with the solid at the elevated temperatures and monitored by time-resolved atomic resonance absorption spectroscopy at λ = 283.3 nm (Pb(7 3P 1°) ← Pb(6 3P 0)). Absolute second-order rate constants k RBr for reaction were measured at various temperatures, yielding the following Arrhenius parameters ( k RBr = A exp(− E/RT)) (errors 1σ): ▪ For the molecules 1-bromobutane and 1,3-dibromopropane rate data could be determined only at single temperatures, yielding k C 4H 9Br = (2.3 ± 0.2) × 10 −15 cm 3 molecule −1 s −1 ( T = 639 K) and k C 3H 6Br 2 = (3.3 ± 1.0) × 10 −14 cm 3 molecule −1 s −1 ( T = 740 K). The activation energies are compared with reaction endothermicities and used to show that the bond energy D(PbBr) is not more than 2.5 eV. The rate data extrapolated to room temperature indicate that the previously reported values of k RBr at that temperature are too high and presumably result from the effect of secondary reactions of Pb(6 3P 0) with photofragments. The diffusion of Pb(6 3P 0) in helium is studied in detail leading to a value of D(Pb(6 3P 0)−He) = 0.48 ± 0.03 cm 2 s −1, extrapolated to standard temperature and pressure. To the best of our knowledge, the present measurements constitute the first characterization of absolute rate data for atomic abstraction reactions by lead atoms at elevated temperatures.

Kinetic study of the absolute rate constant for the reaction between Na + N 2O in the temperature range 349–917K by time-resolved atomic resonance absorption spectroscopy at λ = 589 nm [formula omitted] following pulsed irradiation

We present a kinetic study of the reaction between atomic sodium and nitrous oxide in the temperature range 349–917K. Na(3 2S 1 2 ) was generated by pulsed irradiation of NaI vapor in the presence of N 2O and excess helium buffer gas, and monitored photoelectrically in the “single-shot” mode by time-resolved atomic resonance absorption spectroscopy of the unresolved double at λ = 589 nm ( Na(3 2P J) ← Na(3 2S 1 2 ) ). The photoelectric signals representing resonance absorption were amplified without distortion, captured and digitized in a transient recorder, and interfaced to a microcomputer for kinetic analysis. Absolute second-order rate constants for the reaction (I) Na+N 2O→NaO+N 2 were measured at 349, 390, 473, 540, 730, and 917K, leading to the following Arrhenius form: k 1=(1.9±0.3)×10 −10 X exp(−12.5±0.6 kJ mol −1/RT) X cm 3 molecule −1 s −1 . This result is compared with rate constants measured at single temperatures using diffusion flames at T = ca. 535K and a fast flow reactor at T = ca. 330K where, in both cases, atomic densities of Na(3 2S 1 2 ) are at least a factor of 10 3 greater than employed in the present investigation and where the effect of secondary reactions of NaO is significant. Finally, reaction (1) is considered in terms of the potential energy surfaces involved in reaction on the basis of C 5 symmetry in the “least symmetrical complex” and the weak spin orbit coupling approximation.

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.

Determination of the absolute second-order rate constant for the reaction Na + O3? NaO + O2

The absolute second-order rate constant for the reaction Na + O3→NaO + O2(k1) has been determined by time-resolved atomic resonance absorption spectroscopy at λ= 589 nm [Na(32PJ)â†� Na(32S1/2)] following pulsed irradiation, coupled with monitoring of O3 by light absorption in the ultra-violet; this yields k1(500 K)= 4(+4,–2)× 10–10 cm3 molecule–1 s–1, resolving large differences for various estimates of this important quantity used in modelling the sodium layer in the mesosphere.

Temperature dependence of the absolute third-order rate constant for the reaction between Na + O2+ N2over the range 571–1016 K studied by time-resolved atomic resonance absorption spectroscopy

We present a detailed investigation of the absolute third-order reaction rate constant (k1) for the process Na + O2+ N2→ NaO2+ N2(1) measured over the temperature range T= 571–1016 K. Na(3 2S1/2) was generated by the pulsed irradiation of NaI vapour and monitored in the presence of O2 and N2 in the ‘single-shot’ time-resolved mode by atomic resonance absorption spectroscopy of the unresolved D-line doublet at λ= 589 nm [Na(3 2P1/2,3/2)â†� Na(3 2S1/2)]. A new experimental system has been specially designed for this temperature-dependent investigation. The diffusion coefficient, D(Na–N2), has been measured in this temperature range, demonstrating a dependence on T1.52±0.24 and yielding a value of D(Na–N2) at s.t.p. of 0.15 ± 0.05 cm2 s–1. A limited body of data from similar measurements in neon yields D(Na–Ne) at s.t.p. = 0.19 ± 0.04 cm2 s–1. The data for k1(Na + O2+ N2) within the temperature range 571–1016 K are described by the form k1=(1.11 ± 0.08)× 10–23T–2.47±0.14 cm6 molecule–2 s–1. However, for extrapolation to temperatures outside the measured regime the formalism of the unimolecular rate theory of Troe, with a dependence of 〈ΔE〉 on T–0.7, has been quantitatively employed to yield k1(200 K) and k1(2000 K)= 8.8 × 10–30 and 6.8 × 10–32 cm6 molecule–2 s–1, respectively. The extrapolated rate data are of particular relevance to the role of reaction (1) in the measosphere and in flames. A limited body of rate data is reported for the effect of neon in the analogue of reaction (1) yielding k1(Na + O2+ Ne)=(3.5 ± 0.2)× 10–31 and (2.7 ± 0.1)× 10–31 cm6 molecule–2 s–1 at T= 726 and 844 K, respectively.

Chemical kinetics in combustion systems: The specific effect of energy, collisions, and transport processes

Experimental result on the effect of selective translational and vibrational excitation of reactants in elementary combustion reactions using laser photolysis and time-resolved atomic line resonance absorption, laser-induced fluorescence and CARS spectroscopy are compared with the results of quasiclassical trajectory studies on ab initio potential surfaces and thermal rate parameters. Experimental data on collisional energy transfer from vibrationally highly excited polyatomic molecules and the pressure and temperature dependence of product formation in elementary combustion reactions with complex formation are discussed. Models for the prediction of transport coefficients in multicomponent mixtures, and recent model calculation and experiments on the coupling of chemical reactions and transport effects are presented.

Quantum yield measurements of Cl(3 2P [formula omitted]) and Cl(3 2P [formula omitted]) in the photolysis of C 1 chlorofluorocarbons determined by atomic resonance absorption spectroscopy in the vacuum UV

We describe measurements of the relative quantum yields of electronically excited and ground state chlorine atoms generated following the broad band pulsed irradiation of the C 1 chlorofluorocarbons CF 3Cl, CF 2Cl 2, CFCl 3 and CCl 4. Cl((3p) 5, 2P 1 2 ) and Cl((3p) 5, 2P 3 2 ) produced by the pulsed irradiation of these Freons in the presence of excess helium buffer gas were monitored photoelectrically by time-resolved atomic resonance absorption spectroscopy in the vacuum UV using signal averaging. The two atomic resonance transitions employed to construct concentration-time profiles for Cl(3 2P 3 2 ) and Cl(3 2P 1 2 ) were those at λ = 134.72 nm (Cl(3p) 4(4s) 1, 2P 3 2 ) ← Cl((3p) 5, 2P 3 2 )) and λ = 136.34 nm (Cl((3p) 4(4s) 1, 2P 3 2 ) ← Cl((3p) 5, 2P 1 2 )) respectively and hence involved a common upper state which was specifically required for the analysis. The higher energized spin—orbit state Cl(3 2P 1 2 ), which is 882 cm −1 above the Cl(3 2P 3 2 ) ground state, decayed on a short time scale (about 0 – 500 μs) before the onset of Boltzmann equilibrium. This can be contrasted with the longer-lived decay of Cl(3 2P 3 2 ) (about 0 – 10 ms) which was monitored in a regime where both spin—orbit states were Boltzmann equilibrated. Computerized extrapolation of the decay traces of I tr at these two wavelengths for the resonance transitions to t = 0 yielded the following relative quantum yields (φ = [Cl(3 2P 1 2 )] t = 0 / [Cl(3 2P 3 2 )] t = 0 ) which were found to be sensibly independent of the initial Freon concentration: CF 3Cl, 6.2 ± 2.3; CF 2Cl 2, 3.0 ± 1.1; CFCl 3, 3.8 ± 1.3; CCl 4, 3.7 ± 1.3. Whilst these results indicate population inversions in all cases, it is considered highly unlikely that these molecules could form the basis of atomic photodissociation lasers operating on the transition Cl(3 2P 1 2 ) → Cl(3 2P 3 2 ) + hv (882 cm −1) on account of the very low Einstein coefficient ( A nm = 0.012 s −1) and the highly efficient collisional quenching of the 2P 1 2 state by the Freons themselves. The present results are compared with analogous results for bromides and iodides reported previously.

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 Ca(43PJ) studied by time-resolved emission, 43P1→ 41S0+hν(λ= 657.3 nm), following dye-laser excitation

We present a study of the collisional quenching of electronically excited calcium atoms, Ca[4s4p(3PJ)], 1.885 eV above the 4s2(1S0) ground state, by a range of added gases. Ca(4 3P1) was generated by repetitive pulsed dye-laser excitation at λ= 657.3 nm [Ca(4 3P1)â†� Ca(4 1S0)] of calcium vapour in equilibrium with solid calcium at elevated temperatures (1000 K) in a slow flow system, kinetically equivalent to a static system. Following rapid Boltzmann equilibration within the 4 3PJ manifold, the time-resolved forbidden emission, 4 3P1→ 4 1S0+hν(λ= 657.3 nm), was monitored in the presence of added gases using ‘pre-trigger photomultiplier gating’ and boxcar integration. The following absolute second-order quenching constants (kQ/cm3 molecule–1 s–1, 1000 K, errors 1σ) are reported: CO, 3.7 ± 0.5 × 10–13; NO, 3.2 ± 0.3 × 10–13; CO2, 3.1 ± 0.3 × 10–13; N2O, 6.5 ± 0.5 × 10–13; NH3, 2.5 ± 0.3 × 10–13; CH4, 5.4 ± 0.4 × 10–13; CF4, 1.9 ± 0.2 × 10–13; C2H2, 1.4 ± 0.2 × 10–13; C2H4, 2.3 ± 0.3 × 10–13; C6H6, 6.0 ± 0.2 × 10–12. Where possible these data are compared with those derived from other decay measurements in which Ca(4 3PJ) was generated by dye-laser excitation and monitored either via time-resolved atomic resonance absorption spectroscopy or by phase-shift techniques. The absolute rate constants are compared with analogous data for Mg(3 3PJ)(2.71 eV) and discussed, where appropriate, in terms of orbital correlation and in relation to data from atomic beams which describe chemiluminescence from product states.

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.

Kinetic investigation of the reaction between Na + O2+ M by time-resolved atomic resonance absorption spectroscopy

A kinetic investigation has been carried out on ground-state sodium atoms, Na(32S), generated by the ultraviolet pulsed irradiation of NaI vapour at the temperatures T= 724 and 844 K, and monitored by time-resolved atomic resonance absorption in the “single-shot” mode at λ= 589 nm (32PJâ†� 32S). In the presence of the gases He, N2 and CO2 alone, the atoms exhibit diffusional decay with removal at the walls of the vessel. The variation of the decay of Na (32S) as a function of the pressure of these gases, coupled with the use of the “long-time” solution of the diffusion equation for a cylinder, yields the following average diffusion coeffcients (corrected to s.t.p.): DNa–He= 0.25, DNa–N2= 0.24 ± 0.3 and DNa–CO2= 0.30 cm2 s–1. The third-order reactions between Na + O2+ M (M = He, N2 and CO2) were investigated at T= 724 and 844 K, the rate constants indicating no significant temperature variation. The following third-order absolute rate constants for these processes for the range T= 724–844 K are reported: k(M = He)=(6 ± 1)× 10–31 cm6 molecule–2 s–1, k(M = N2)=(1.0 ± 0.24)× 10–30 cm6 molecule–2 s–1, k(M = CO2)= 2 × 10–30 cm6 molecule–2 s–1, the datum for CO2 being reliable to within a factor of two. The data are compared with those from analogous measurements for other metal atom reactions of the type X + O2+ M and are found to be of comparable magnitude. By contrast, the present results are ca. three orders of magnitude greater than those reported hitherto from flame measurements.

Kinetic study of Mg(33Pj) by time-resolved emission, 33P1→ 31S0+hν(λ= 457.1 nm), following dye-laser excitation

A kinetic study of the low lying, optically metastable, electronically excited magnesium atom, Mg[3s3p(3PJ)], is presented. Mg(3 3P1) was generated by rapid, dye-laser pulse excitation at λ= 457.1 nm of magnesium vapour at 800 K in a slow-flow reactor, kinetically equivalent to a static system and monitored, following fast Boltzman equilibration of the close lying spin–orbit levels within the 3 3P0,1,2 manifold, by the subsequent “slow” spontaneous emission at the excitation wavelength, Mg(3 3P1)→ Mg(3 1S0)+hν. (I) The experimental procedure included repetitive dye-laser pulsing, photoelectric detection with “pretrigger photomultiplier gating”, boxcar integration and kinetic analysis of the resulting XY-output. The diffusional decay of Mg(3 3PJ) was studied in the presence of all the noble gases, He, Ne, Ar, Kr and Xe, together with detailed measurements of the mean radiative lifetime of transition (I)(at λ= 451.7 nm), which is reported as τe= 2.1 ± 0.5 ms. The collisional quenching of Mg(3 3PJ) has also been investigated, yielding second-order absolute rate constants (kQ/cm3 molecule–1 s–1, 800 K) for the following gases (errors 1σ): N2, 4.9 ± 0.2 × 10–13; H2, 7.3 ± 0.6 × 10–13; CO, 1.4 ± 0.1 × 10–12; O2, 3.3 ± 0.8 × 10–11; CH4, 1.1 ± 0.2 × 10–14; Kr, ⩽ 2.3 × 10–15; Xe, ⩽ 1.0 × 10–15. The results are compared, where possible, with previous measurements derived primarily from investigations of time-resolved atomic resonance absorption spectroscopy following pulse dye-laser excitation and from intensity measurements of the forbidden emission 3 3P1→ 3 1S0+hν in a flow–discharge system. The quenching rate constants are found to be in accord with the results of these earlier measurements.

Kinetic investigation of the third-order rate processes between K + O2+ M by time-resolved atomic resonance absorption spectroscopy

Absolute reaction rate constants (k3) for the third-order processes K + O2+ M → KO2+ M have been determined by time-resolved resonance absorption measurements on ground-state atomic potassium at λ= 768 nm (42PJâ†� 42S1/2) in the “single-shot mode” following the flash photolysis of KI vapour. The reactions were investigated at the temperatures T= 753 and 873 K, k3 exhibiting a small negative temperature dependence, fitted to the form k3=AT–1, and yielding the following results for the third bodies M = He, N2 and CO2: M k3/cm6 molecule–2 s–1, He (9.8 ± 1.5)× 10–28T–1, N2(1.7 ± 0.6)× 10–27T–1, CO2 4 x 10–27T–1. The rate constants, k3, for K + O2+ M are found to be greater than those for Na + O2+ M, obtained hitherto by time-resolved resonance absorption measurements, by approximately a factor of two. These results for both K and Na, obtained from monitoring the atoms following pulsed irradiation, are approximately three orders of magnitude greater than those derived previously from flame measurements. The rate measurements further demonstrated a kinetic component, second-order overall in [K] and [O2], which did not arise from direct O-atom abstraction but is tentatively attributed to a mechanism involving the species “O2–I” and which can readily be extracted in the analysis that leads to the quantitative determination of k3. The rates of diffusion of K(42S) in the gases He, N2 and CO2 were also studied in detail in this investigation and the resulting diffusion coefficients for this atom, as well as those for atomic sodium obtained previously using the “long-time solution” of the diffusion equation for a cylinder, are compared with diffusion coefficients for Na and K reported from measurements on the burnt gases (H2+ H2O + N2) of fuel-rich flames.

Laser induced photolysis of ozone in the visible: A kinetic study of O( 3P) by atomic absorption spectroscopy

Time-resolved atomic resonance absorption spectroscopy has been used to study the reactions of ground state oxygen atoms, O( 3P), formed in the photodissociation of ozone at 600 nm. A critical evaluation of the experimental procedure is made and problems arising from the mismatch of line profiles in atomic absorption spectroscopy are discussed. The resulting temperature dependence of the rate constant of the O—O 2 recombination reaction with N 2 as third collision partner is significantly different from that reported earlier and affects atmospheric model calculations.