The radiative lifetime of the A1II state of BH

Second-order polarization propagator calculations of the X¹Σ⁺ → A¹II transition moment as well as the radiative lifetime of the A¹II state of BH are reported. The calculated vibrational lifetimes are τ(v′ = 0) = 121 ns, τ(v′ = 1) = 129 ns, and τ(v′ = 2) = 137 ns. The τ(v′ = 0 lifetime agrees with the most recent experiment of τ(v′ = 0) = 125 ± 5 ns. We show that the electronic oscillator strength computed at the ground state equilibrium is rather different from the band absorption oscillator strength f₀₀, which demonstrates that theoretical electronic oscillator strengths should not be expected to agree with experimentally determined band oscillator strengths.     

Pressure saturation of SO2(3B1 0,0,0) lifetimes

Lifetime data have been obtained for the decay of SO₂(³B₁ 0,0,0) at 25°C over the pressure range of 1–762 torr. The ³B₁ state was populated by direct absorption to eliminate any possible complications in interpretation due to the participation of excited-singlet manifolds. At pressures greater than about 10 torr, the measured lifetimes are longer than predicted from low-pressure Stern–Volmer parameters. This deviation can be interpreted in terms of Freed's theory on collisionally induced intersystem crossing and provides unequivocal evidence to support earlier speculations that the lengthening of the lifetimes at high pressures is due to saturation in depopulation of the ³B₁ state.     

The lifetime of the A2Sigmau+ state of the N3 radical

The lifetime of the A 2Sigmau+ state of N3 has been determined from line broadening in the rotationally resolved laser-induced fluorescence (LIF) spectrum of the A 2Sigmau+-X 2Pig transition. N3 radicals, produced by fluorine atom abstraction from HN3, were probed with an intracavity doubled ring laser operating near 272 nm. Careful examination of the LIF spectrum indicates a significant Lorentzian component to the line shape due to a rapid predissociation in the A 2Sigmau+ state. The predissociation lifetime is found to be 132+/-21 ps for the 000 vibrational level and 64+/-10 ps for the 010 level. The short lifetime is consistent with the low intensity of the LIF signal especially for the 010-010 hot band.     

The temperature dependence and the mechanism of the SO2 (3B1) quenching reactions

The Arrhenius parameters have been determined for the SO₂(³B₁) quenching reaction (9), SO₂(³B₁) + M → (SO₂ ? M), for 21 different molecules as quenching partner M. The rate constants were calculated from phosphorescence lifetime measurements made over a range of reactant pressures and temperatures. Excitation of the SO₂ (³B₁) molecules was accomplished by two very different methods: (1) a 3829 Å laser pulse generated the triplet directly through absorption within the “forbidden” SO₂ (³B₁) → SO₂ (¹A₁) band; (2) a broadband Xe-flash system generated SO₂(³B₁) molecules and triplets were formed subsequently by intersystem crossing, SO₂(¹B₁) + M → SO₂(³B₁) + M. The measured rate constants were independent of the method of triplet formation employed. For the atmospheric gases, the activation energies (kcal/mole) were identical within the experimental error: N₂, 2.9 ± 0.4; 0₂, 3.2 ± 0.5; Ar, 2.8 ± 0.6; CO₂, 2.8 ± 0.4; CO, 2.7 ± 0.4; CH₄, 2.5 ± 0.6. This energy corresponds to the first region of the SO₂(³B₁) → SO₂(¹A₁) absorption spectra in which Brand and coworkers observe strong perturbations. It is suggested that the quenching in these cases results largely from the physical process involving potential energy surface crossing to another electronic state. Activation energies for SO₂(³B₁) quenching by the paraffinic hydrocarbons show a regular decrease in the series ethane, neopentane, propane, n-butane, cyclohexane, and isobutane, which parallels closely the decrease in C? H bond energies in these compounds. These and other data are most consistent with the dominance of chemical quenching in these cases. The rate constants for the olefinic and aromatic hydrocarbons and nitric oxide show only very small variations with temperature change, and they are near the kinetic collision number. These data support the hypothesis that quenching in these cases is associated with the formation of a charge-transfer complex and subsequent chemical interactions between the SO₂(³B₁) molecule and the π-system of these compounds.     

Experimental lifetime of the metastable 5D 3/2 state in Ba+

A discrepancy exists between theoretical and experimental lifetimes of the metastable 5D _3/2 state in Ba⁺. In order to redertermine that lifetime, we probe the population of the metastable 5D _3/2 state of a Ba⁺ ion cloud stored in a Paul ion trap in the presence of He buffer gas as the function of time delay after pulsed laser excitation of this state. The measured decay rates at different buffer gas pressures are extrapolated to zero pressure and we obtain a radiative decay time of 48.0±5.9 s. This is not in agreement with theoretical predictions of about 80 s, but reduces the discrepancy from a previously reportet experimental value of 17.5 s. If the possibility of finestructure mixing to an adjacent 5D _5/2 level by collisions is considered our value represents a lower limit of the radiative lifetime for the 5D _3/2 state.     

Solvent effects on the pure radiative lifetime of ovalene

The pure radiative lifetime of ovalene was measured in a series of solvents. The effect of S₁–S₂ energy gap reduction by the solvent on the pure radiative lifetime was studied and could be fitted to the theoretical calculations. Several questions are raised concerning the validity of the theory at small inter-electronic energy gaps.     

The SO2(3B1) photosensitized isomerization of cis- and trans-1,2-dichloroethylene

The photolysis of SO₂ at 3712 Å in the presence of the 1,2-dichloroethylenes has been investigated at 22deg;C. The data are consistent with the SO₂(³B₁) photosensitized isomerization of the 1,2-dichloroethylene isomer. A kinetic treatment of the initial quantum yield data was consistent with the formation of a polarized charge-transfer intermediate whenever SO₂(³B₁) molecules and one of the 1,2-dichloroethylene isomers collide which ultimately decays unimolecularly to the cis-isomer with a probability of 0.70 ± 0.26 and to the trans-isomer with a 0.37 ± 0.16 probability. Quenching rate constants for removal of SO₂(³B₁) molecules by cis- and trans-1,2-dichloroethylene have been estimated from quantum yield data and from laser excited phosphorescence lifetimes using an excitation wavelength of 3130 Å. Estimates of the quenching rate constant (units of 1./mole ± sec) are for the cis-isomer, (1.63 ± 0.71) × 10¹⁰, quantum yield data, and (2.44 ± 0.11) × 10¹⁰, lifetime data; and for the trans-isomer,(2.59 ± 0.09)×10¹⁰, lifetime data, and (2.35 ±0.89) × 10¹⁰, quantum yield data. An experimentally determined photostationary composition,[cis-C₂Cl₂H₂]/[trans-C₂Cl₂H₂] = 1.8 - 0.1, was in good agreement with a value of 2.00 - 1.15 which was predicted from rate constants derived in this study.     

Quenching rate constants for the removal of SO2(3B1) by various fluorinated olefins

A competitive technique employing the SO₂(³B₁) photosensitized isomerization of cis-C₂F₂H₂ to trans-C₂F₂H₂ in the presence of selected fluorinated olefins has been used at 3712 Å and 22°C to determine the quenching rate constants of the reaction \documentclass{article}\pagestyle{empty}\begin{document}${\rm SO}_{\rm 2} ({}^3B_1){\rm M}\mathop \to \limits^{k_{_4}}$\end{document} removal. With P_So2 = 25.4 torr and P_cis-C₂F₂H₂ = 0.239 torr Stern–Volmer plots for M = C₂H₄, C₂H₂F, 1,1-C₂F₂H₂, C₂F₄, and C₃F₆ yielded k₄ (units of 10¹⁰ l./mole · sec) values of 5.29 ± 0.16, 4.21 ± 0.53, 1.92 ± 0.23, 0.575 ± 0.060, and 0.0335 ± 0.0027, respectively. The results were consistent with the ability of an olefin to quench SO₂(³B₁) being inversely proportional to the polarizability of the olefin's π bond and the effect can be clearly noted as each H atom in C₂H₄ is individually replaced by an F atom.     

The SO2(3B1) photosensitized isomerization of cis- and trans-1,2-difluoroethylene

Quantum yield measurements for the SO₂(³B₁) photosensitized isomerization of cis-1,2-difluoroethylene have been made at 3712 Å and 22°C. The [SO₂]/[cis-C₂F₂H₂] ratio was varied from 47.4 to 455 and the quantum yield measurements over this variation of concentration ratios were consistent with a mechanism in which SO₂(³B₁) molecules and the cis isomer form a collision intermediate which decomposes with a probability of 0.42 ± 0.17 and 0.58 ± 0.17 of producing trans- and cis-1,2-difluoroethylene, respectively. When SO₂ was subjected to prolonged irradiations in the presence of initially either pure cis- or pure trans-1,2-difluoroethylene, a photostationary composition, [cis]/[trans] = 1.0 ± 0.2, was obtained. The rate constant at 22°C for removal of SO₂(³B₁) molecules by cis-1,2-difluoroethylene was estimated to be (1.72 ± 0.72) × 10¹⁰ 1./mole · sec.     

A kinetic study of the chemistry of the SO2 (3B1) reactions with cis- and trans-2-butene

The chemical reactions of SO₂(³B₁) molecules with cis- and trans-2-butene have been studied in gaseous mixtures at 25°C by excitation of SO₂ within the SO₂(³B₁) → SO₂(+, ¹A₁) ‘forbidden’ band using 3500–4100-Å light. The initial quatum yields of olefin isomerization were determined as a function of the [SO₂]/[2-butene] ratio and added gases, He and O₂. The kinetic treatment of these data suggests that there is formed in the SO₂(³B₁) quenching step with either cis- or trans-2-butene, some common intermediate, probably a triplet addition complex between SO- and olefin. It decomposes very rapidly to form the 2-butene isomers in the ratio [trans-2-butene]/[cis-2-butene] = 1.8. In another series of experiments SO₂ was excited using a 3630 ± 1-Å laser pulse of short duration, and the SO₂(³B₁) quenching rate constants with the 2-butenes were determined from the SO₂(³B₁) lifetime measurements. The rate constants at 21°C are (1.29 ± 0.18) × 10¹¹ and (1.22 ± 0.15) × 10¹¹ l/mole·sec with cis-2-butene and trans-2-butene, respectively, as the quencher molecule. Within the experimental error these quenching constants equal those derived from the quantum yield data. Thus the rate-determining step in the isomerization reaction is suggested to be the quenching reaction, presumably the formation of the triplet SO₂-2-butene addition complex. In a third series of experiments using light scattering measurements, it was found that the aerosol formation probably originates largely from SO₃ and H₂SO₄ mist formed following the reaction SO₂(³B₁) + SO₂ → SO₃ + SO(³Σ^?). Aerosol formation from photochemically excited SO₂-olefin interaction is probably unimportant in these systems and must be unimportant in the atmosphere.     

Radiative lifetime determination for the 3s4p-1P1 state of magnesium using pulsed level-crossing spectroscopy

The natural radiative lifetime of the 3s4p ¹ P ₁ level of neutral magnesium was measured by pulsed level-crossing spectroscopy at the short excitation wavelength of 202.6 nm. We obtained =13.4(5) ns for this state, strongly deviating from a previous beam-foil result, but in good agreement with a multi-configuration Hartree-Fock calculation. A discussion of the applicability of the pulsed Hanle effect method is given.     

Unsymmetrical 1B2 (ππ1) state of ozone

Using ab initio SCF CI calculations it is shown that the ππ1 singlet state of ozone has an unsymmetrical equilibrium geometry with one long and one short OO bond. A similar behavior has been experimentally demonstrated on the parent SO₂ molecule. The double-minimum potential energy surface of this state results from an avoided crossing involving (1) the singlet ππ1 state itself which is stabilized by an unsymmetrical stretching of the two bonds and (2) a lower lying (nπ1)² doubly excited state which is destabilized by the same distortion.     

Predissociation lifetime of the 1σ g 2 2σ g diabatic state of He 2 + : a one-channel approach

Summary This work is concerned with the application of a one-channel model to obtaining predissociation lifetimes and transition rates in a system of crossing diabatic states. The calculation focuses on the first shape resonance of the 1 _g ² 2 _g diabatic state of He ₂ ⁺ , which is relatively stable with respect to tunneling. This resonance predissociates as a result of the 1 _g ² 2 _g state being crossed by the 1 _g 1 _u ² dissociative diabatic state near the resonance level. We have estimated its predissociation lifetime to be of the order of 10^–11 s.     

Lifetime of the B3Σ−u state of S2

Using the single-photon time correlation method, we have determined the lifetime of the S₂(B³Σ^?_u) state from the decay rate of the fluorescence at 370 nm. A lifetime of 45.0 ± 0.6 ns was measured, and the cross section for fluorescence quenching by S₂ as found to be 81.3 ± 4.7 A². A slight dependence of the lifetime on the wavelength of the excitation source over the range of 280 to 337 nm was observed.     

The radiative lifetime of metastable CO (a 3Pi, v=0)

We present a combined experimental and theoretical study on the radiative lifetime of CO in the a (3)Pi(1,2), v=0 state. CO molecules in a beam are prepared in selected rotational levels of this metastable state, Stark-decelerated, and electrostatically trapped. From the phosphorescence decay in the trap, the radiative lifetime is measured to be 2.63+/-0.03 ms for the a (3)Pi(1), v=0, J=1 level. From the spin-orbit coupling between the a (3)Pi and the A (1)Pi states a 20% longer radiative lifetime of 3.16 ms is calculated for this level. It is concluded that coupling to other (1)Pi states contributes to the observed phosphorescence rate of metastable CO.     

Radiative lifetime for v = 1 and v = 2 ground state NO+ ions

Radiative lifetimes of vibrationally excited ions in their electronic ground state have been measured for the first time. A nonoptical technique has been developed, involving ion cyclotron resonance trapping in conjunction with chemical monitoring of the energy content of the ions. For NO⁺(X ¹Σ⁺) ions, the measured lifetimes of 95 ± 15 ms for v = 1 and 46 ± 10 ms for v = 2 are in excellent agreement with the values obtained from the ab initio calculations of Werner and Rosmus.     

Solid state NMR study of acid sites formed by adsorption of SO3 onto gamma-Al2O3

Detailed structure of Br?nsted acid sites on the surface of SO3/Al2O3 catalyst has been proposed based on 1H/27Al TRAPDOR NMR results and the acidity of the catalyst has also been characterized by NMR probe molecules.     

Radiative lifetimes for the B1piu state of the Na2 molecule

A study of the radiative lifetimes calculation of the Na2 B1piu state is presented. RKR electronic potentials are considered. The studied vibrational levels are for v' = 0-33 (B1piu) and v" = 0-65 (chi1sigmag+). The rotation is considered for values of J' = 1-225 (B1piu). The Einstein emission coefficients are calculated for the specified B1piu rovibrational levels (for Q line and R, P lines, for all ground state vibrational levels). With the inverse of Einstein emission coefficients sum, the radiative lifetimes are calculated. These calculated lifetimes are in good agreement with the experimental and previously calculated (with RKR potentials) lifetimes, but now great extension of considered rovibrational levels is considered. The bound-free contribution is irrelevant for Na2 lifetimes of the B1piu state. The perturbation between Na2 B1piu and alpha1sigmau+ states is considered.     

电子激发态SO2(A^1A2, B^1B1)分子的碰撞猝灭动力学研究

用四倍频YAG激光(266nm)把SO2分子从电子基态X^1A1激励到电子激发态A^1A2和B^1B1的高振动耦合区, 通过检测自发辐射SO2(B^1B1→X^1A1)的时间分辨信号, 测定了室温(290K)下SO2(A^1A2, B^1B1)被He, 氯代甲烷分子和某些烷烃分子猝灭的速率常数。此外, 还从碰撞配合物模型出发, 对SO2(A^1A2, B^1B1)的猝灭机制进行了讨论。