David Husain

Kinetic investigation of electronically excited oxygen atoms, O(21D2), by time-resolved attenuation of atomic resonance radiation in the vacuum ultra-violet. Part 2.—Collisional quenching by the atmospheric gases N2, O2, CO, CO2, H2O and O3.

The electronically excited oxygen atom. O(21D2), has been studied in absorption by time-resolved attenuation of the atomic resonance radiation at λ= 115.2 nm (31Do2→ 21D2). Absolute quenching coefficients for the removal of O(21D2) by the gases N2, O2, CO, CO2, H2O and O3 are reported.These collisional processes are discussed within the context of the spin and orbital symmetry of reactants and products. A detailed comparison with previous laboratory and atmospheric measurements of these processes is presented.

Kinetic investigation of ground state carbon atoms, C(23PJ)

A kinetic study of C(23PJ), generated by the repetitive pulsed irradiation of carbon suboxide in the Schumann region, is presented. The ground state carbon atoms were monitored photoelectrically by resonance absorption at λ≈ 166 nm in the time-resolved mode. The resulting photoelectric signals were translated into kinetic data by means of rapid response precision logarithmic circuitry, signal averaging and computerised analysis of the raw data. Rate data for the removal of C(23PJ) in the presence of the added reactant gases H2, O2, NO, N2O, CO2, H2O and C3O2 are reported and these are compared with those derived from other studies involving vacuum ultra-violet absorption spectroscopy, principally “single-shot” flash photolysis experiments involving oscilloscopic recording of attenuation of resonance radiation and plate photometry. The advantages of the present method, premitting detailed investigation of the atomic decays, are demonstrated.

Kinetic study of electronically excited oxygen atoms, O(21D2), by time-resolved atomic absorption spectroscopy in the vacuum ultra-violet (λ=115.2 nm, O(31D02←21D2))

Abstract Electronically excited oxygen atoms, O(2 1 D 2 ), generated by the pulsed irradiation of ozone in the Hartley continuum, have been observed by absorption spectroscopy using time-resolved attenuation of resonance radiation in the vacuum ultra-violet (O(3 1 D 0 2 )←O(2 1 D 2 ), λ=115.2 nm). The concentration profile of O(2 1 D 2 ) has been monitored photoelectrically following repetitive pulsed photolysis and the kinetic decay characteristics of the excited atom determined using signal averaging techniques. The rate constants for the collisional removal of O(2 1 D 2 ) were found to be as follows (300°K): k O 3 = (3.5±0.3)×10 −10 cm 3 molecule −1 sec −1 , k He = ⩽ 1.5×10 −15 cm 3 molecule −1 sec −1 .

Kinetic study of electronically excited nitrogen atoms, N(22DJ, 22PJ), by attenuation of atomic resonance radiation in the vacuum ultra-violet

The collisional behaviour of electronically excited nitrogen atoms, N(22DJ,22PJ), has been investigated by atomic absorption spectroscopy using time-resolved attenuation of resonance radiation in the vacuum ultra-violet. The optically metastable atoms have been generated by the repetitive, pulsed irradiation of nitrous oxide in the Schumann region and the resulting photoelectric pulses representing the concentration profiles of the atomic states have been signal averaged. This system has involved the use of a rapid response logarithmic signal converter, capable of measuring directly in (I0/Itr) for a weak, fast decaying signal. This has permitted quenching rate constants to be determined with an accuracy considerably improved upon those obtained previously from “single-shot” measurements. Deactivation rate constants are reported for N(22DJ) and N(22PJ) with the gases H2, N2, O2, CO, NO, CO2 and N2O and compared with those from earlier determinations.

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.

The collisional behaviour of Cl(3p5(2P12) by time-resolved attenuation of atomic resonance radiation in the vacuum ultraviolet

Abstract We present a study of the collisional behaviour of chlorine atoms in the 2P 1 2 spin orbit state, 881 cm−1 above the 3P 3 2 level in the 3p5 ground state configuration. Cl(32P 1 2 ) was generated by the repetitive, pulsed irradiation of CCl4 and monitored photoelectrically in absorption by time-resolved attenuation of atomit resonance radiation in the vacuum ultraviolet a λ = 136.34 nm [3p44s(2P 3 2 ) ← 3p5(2P0 1 2 )]. The method includes a sophisticated signal averaging system and will be of general applicability to the kinetic study of this important atomic state, one that is of considerable current interest but that has hitherto been the subject of very limited investigation by direct observation. Absolute rate constants (kQ, cm3 molecule−1, s−1, 300 K) are reported for collisional quenching by the gases CCl4 [(2.0 ± 0.2) × 10−10], Cl2[(4.5 ± 0.4) × 10−11], O2[2.1 ± 0.5) × 10−11] and He [(3.8 ± 0.6) × 10−15].

Kinetic investigation of the reactions of OH(X2Π) with the hydrogen halides, HCl, DCl, HBr and DBr by time-resolved resonance fluorescence (A2∑+–X2Π)

The reaction rates of ground-state OH radicals, X2Π(v″= 0), with the molecules CH4, CO, HCl, DCl, HBr and DBr have been investigated in the gas phase by time-resolved resonance fluorescence at λ= 307 nm [OH(A2∑+–X2Π, (0, 0)]. The OH radicals were generated photochemically by the repetitive vacuum ultraviolet photolysis of water vapour in a flow system, kinetically equivalent to a static system, using a high-intensity, magnetically pinched pulsed light source, and optically excited to the A2∑+ state by means of a resonance source operating in the standard orthogonal arrangement. The resulting time-resolved resonance fluorescence signals were monitored by means of a highly sensitive detection system employing pre-trigger photomultiplier gating, photon counting and signal averaging. Absolute second-order rate constants (k/cm3 molecule–1 s–1, 300 K) were obtained for the reactions of OH with the above molecules as follows: CH4(7.66 ± 0.64)× 10–15 CO (1.46 ± 0.12)× 10–13, HCl(6.66 ± 0.52)× 10–13, DCl (3.48 ± 0.30)× 10–13, HBr (6.01 ± 0.32)× 10–12 and DBr (2.05 ± 0.14)× 10–12, the quoted errors being 2σ. The rate constants for CH4 and CO constituted a kinetic test of the system, being in accord with accepted values for these quantities. The data for the hydrogen halides are compared, where possible, with previously determined rates obtained from monitoring OH directly by various methods. To the best of our knowledge, the absolute rate constant for the reaction with DBr has not been reported hitherto from direct monitoring of OH.

Radiative lifetimes, diffusion and energy pooling of Sr(5s5p(3PJ)) and Sr(5s4d(1D2)) studied by time-resolved atomic emission following pulsed dye-laser excitation

Abstract A study is presented of the kinetic behaviour of the optically metastable states Sr(5s5p(3PJ)) and Sr(5s4d(1D2)), each separately generated by pulsed dye-laser excitation at λ = 689.3 nm (Sr(5s5p(3P1))←Sr(5s2(1S0))) and λ = 496.1 nm (Sr(5s4d(1D2))←Sr(5s2(1S0))) in the presence of helium buffer gas. Radiative and diffusional loss of both metastable states in helium, including the effect of radiation trapping for Sr(5s5p(3PJ)), have been investigated as a function of temperature and pressure by time-resolved emission measurements. We report the following radiative lifetimes: τe(Sr(3P1)) = (19.6−0.5+0.6) μs for fluorescence via the spin-forbidden transition to the ground state; and τ(Sr(1D2)) = (412−9+10) μs for radiative loss to all lower-lying states. Both values are compared with the results of previous experimental work and theoretical calculations as appropriate. When expressed in the form D12 ∝ Tn, the measured temperature dependence of the diffusion coefficients D12(Sr(3PJ)-He) and D12(Sr(1D2-He) yielded n = 1.59±0.51 and n = 1.83±0.89, respectively. Rate constants for self-quenching of Sr(3PJ) and Sr(1D2) atoms by ground-state Sr(1S0) vapour are estimated to be kSr = (2.9−6.4)×10−14 cm3 atom−1 s−1 and k′Sr = (3.4±0.9)×10−14 cm3 atom−1 s−1 respectively. Whilst an upper limit of kHe⩽(2.6±0.6)×10−15 cm3 atom−1 s−1 is determined for collisional quenching of Sr(3PJ) atoms by He, no such effect could be observed for Sr(1D2) atoms. Energy pooling processes involving Sr(5s5p(3PJ)) atoms to yield the higher-lying 5s5p(1P1), 5s4d(1D2) and 5s6s(3S1) states are also investigated in the time domain, where the mechanisms for pooling formation are established from a quantitative comparison of the time dependence of atomic emission from the energy store and pooled states. For Sr(5s4d(1D2)), fluorescence from some 22 energy-pooled atomic states is reported, mostly arising from bimolecular collisions between two Sr(1D2) atoms with subsequent emission of radiation. Approximate relative yields into all these pooled states, estimated from extrapolation of their time-dependent decay profiles coupled with appropriate corrections for Einstein coefficients and experimental sensitivity, indicate that energy pooling collisions constitute a small “bleed-off” from the store states which does not affect their time profiles. Finally, quantum yields for emission from Sr(5s5p(3PJ)) and Sr(5s4d(1D2)) atoms and branching ratios into the 3PJ levels following laser excitation of the 1D2 state are reported. These are shown to be quantitatively consistent with a mechanism based on removal of both excited states by radiative decay and diffusion, as seen by reference to previous calculated Einstein coefficients for the relevant radiative transitions. Using experimental data reported in an earlier publication, quantum yields for emission from Ca(4s4p(3PJ)) and Ca(4s3d(1D2)) atoms and the branching ratio for production of Ca(4s4p(3PJ)) following laser excitation of Ca(4s3d(1D2)) are also determined, from which a mechanism analogous to that for atomic strontium is established.

Kinetic Investigation of the Reactions of Ground State Atomic Carbon, C(2p2(3PJ)), with Acetylenes by Time-Resolved Atomic Resonance. Absorption Spectroscopy in the Vacuum Ultra-Violet

a range of acetylenic collision partners and with 1,3-Butadiene. Atomic carbon was generated by the repetitive pulsed irradiation (I > ca. 160 nm) of C302 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 timeresolved atomic resonance absorption in vacuum ultra-violet ( = 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 acetylenes are presented using the previously reported rate data for C(23Pj) + C302:

A kinetic study of lead atoms, Pb(63P0), by atomic absorption spectroscopy

Abstract A highly sensitive experimental method is described for the kinetic study of lead atoms in the lowest spin orbit component of the ground state configuration. Pb(63P0) is generated by the pulsed irradiation of lead tetraethyl and monitored photoelectrically in absorption by time-resolved attenuation of atomic resonance radiation at λ = 283.3 nm {Pb[7s(3P01) → 6p2(3P0]}. In the presence of the gases He, CO, CO2, N2O, CH4, C2H4 and C2H2, the decay of the ground state atom is slow and second order rate data are reported principally as upper limits for the absolute reaction rate constants. With the gases NO and O2, overall third order kinetics are observed for the removal of Pb(63P0), yielding the following third order rate constants: Download : Download full-size image Further, an estimate for the diffusion coefficient of the atom in helium is reported asDPb(63P0) − He ⋍ 0.045 cm2 s−1 at one atmosphere.