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Notes: The collisional behavior of NCO[X˜(0,n,0)] in specific vibronic states in the gas phase has been investigated in the time-domain by laser-induced fluorescence (LIF) on transitions within the system NCO(A˜ 2Σ+–X˜ 2Π). The NCO radical was generated by the infrared multiphoton dissociation (IRMPD) of phenyl isocyanate (PhNCO) by means of a TEA-CO2 laser operating on the 9R24 line at λ=9.25 μm with subsequent monitoring of the vibronic levels of the X˜ state, characterized by Renner–Teller coupling, in the presence of N2, O2, NO, CO2, N2O, SO2, and PhNCO itself. The states probed were as follows: (0010)2Π3/2, (0010)2Π1/2, (0100)μ2Σ+, (0120)2Δ5/2, (0120)2Δ3/2, (0210)μ2Π3/2,1/2, (0230)2Φ7/2, and (0230)2Φ5/2. Various pairs of spin–orbit states were found to be tightly coupled kinetically. Thus, the time-evolution of the pairs of vibronic states (0010)2Π3/2 and (0010)2Π1/2; (0120)2Δ5/2 and (0120)2Δ3/2; (0230)2Φ7/2 and (0230)2Φ5/2 were found to be equal, yielding an effective local equilibrium within these spin–orbit components within experimental error. Further, states such as NCO(0100) and NCO(0120) were characterized by relatively long decay profiles in the presence of molecules such as CO2 and O2 where the contribution of rotational quenching to the overall decay process could be neglected. By contrast, NCO(0210) and NCO(0230) were removed on significantly faster time scales on collision with SO2. In the absence of extensive information required for solving the large set of coupled differential kinetic equations, albeit reduced in number of those states strongly coupled kinetically, such as a detailed knowledge of the nascent state distributions in NCO following IRMPD, not necessarily Boltzmann in character, the vibronic states were taken to behave independently as the most practical approach to this study. Absolute second-order rate data for the collisional quenching of NCO in the vibronic states (0010), (0100), (0120), (0210), and (0230) by the above molecular species are reported. No clear selection rules are apparent except for the low propensity rule ΔK=2 within the same vibronic state, i.e., μ 2Σ+(0100)–2Δ5/2(0120) and 2Π3/2,1/2(0210)–2Φ7/2(0230). This is presumed to reflect the role in the collisional interaction of the oscillating dipole in the vibronic state, facilitating ΔK=1, whereas ΔK=2 would involve the quadrupole which is smaller. It is found that the data for (V–V) and (V–T) energy transfer correlate best with the attractive part of the potential curves between the collision partners using the established Parmenter–Seaver plots, yielding well depths [(εMM/kB)1/2] for the vibronic states NCO[μ 2Σ+(0100), 2Δ5/2(0120), 2Π3/2,1/2(0210), and 2Φ7/2(0230)], significantly larger than those of the closed shell collision partners and equal within experimental error. The data are also considered in terms of a multipolar attractive force model involving a collision complex where a sensible correlation is found between the computed and observed collision cross sections for O2, N2, CO2, N2O, and SO2 assuming no change in the multipoles with vibrational state. © 1997 American Institute of Physics.

Notes: We present a kinetic study of the reactions of ground-state sodium atoms with the molecules CH3F, CH3Cl, CH3Br, HCl, and HBr at elevated temperatures (537-966 K). Na(32S1/2) was generated by the pulsed irradiation of various sodium halide vapors and monitored by time-resolved atomic resonance absorption of the unresolved D-lines at λ = 589 nm [Na(32PJ) ← Na(32S1/2)] in the “single-shot mode.” The photoelectric signals were amplified without distortion, captured, and digitized in a transient recorder interfaced to a microcomputer for data analysis. Absolute second-order rate constants were measured at various temperatures in each case, yielding the following Arrhenius parameters (kRX = A exp(-E/RT), errors 1σ): which constitute, to the best of our knowledge, the first direct measurements of these quantities. The reaction between Na and HBr demonstrated anomalous behaviour which is discussed in terms of potential surfaces that have been calculated previously for this type of collisional process. The data are compared with analogous results for Na + CF4, CF3Cl, and CF3Br and with single-temperature measurements on diffusion flames.

Notes: A 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: TextReactantkR/cm3 molecule-1 s-1 (300 K)C2F4(1.9 ± 0.1) × 10-10C2Cl4(10.6 ± 0.5) × 10-10CH2CF2(4.3 ± 0.2) × 10-10CHClCCl2(7.9 ± 0.4) × 10-10C6H6(4.8 ± 0.3) × 10-10C6F6(4.9 ± 0.3) × 10-10C6HF5(5.0 ± 0.3) × 10-10C6H2F4(4.4 ± 0.2) × 10-10C6H5 - CH3(5.5 ± 0.3) × 10-10C6F5 - CF3(5.4 ± 0.3) × 10-10These 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.

Notes: We 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;): TextRkR/cm3 molecule-1 s-1C2F41.6 ± 0.2 × 10-10C2Cl49.9 ± 1.7 × 10-10CH2CF24.0 ± 0.6 × 10-10CHClCCl27.0 ± 1.1 × 10-10CF3CH—CH24.6 ± 0.5 × 10-10C6H64.4 ± 1.0 times; 10-10C6F64.4 ± 0.6 × 10-10C6HF54.6 ± 1.3 × 10-10C6H2F43.9 ± 0.8 × 10-10C6F5 - CF35.1 ± 0.6 × 10-10These 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.