Experimental and theoretical study of distribution of O2 molecules over vibrational levels in O2(a 1Δg)–I mixture

Laboratory Studies of Vibrational Excitation in O2( a1Δg, v) Involving O2, N2, and CO2

Collisional removal of electronic energy from O₂ in the low-lying a¹Δ_g state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O₂( a¹Δ_g, v = 1-3) in collisions with O₂ and CO₂. Ozone photodissociation in the 200-310 nm Hartley band is the source of O₂( a, v), and resonance-enhanced multiphoton ionization is used to probe the vibrational-level populations. Deactivation of the a( v = 1-3) levels in collisions with O₂ at 300 K is fast, with rate coefficients of (5.6 ± 1.1) × 10^-11, (3.6 ± 0.4) × 10^-11, and (1.9 ± 0.4) × 10^-11 cm³ s^-1 (2σ) for v = 1, 2, and 3, respectively. The relaxation process appears to involve a near-resonant electronic energy transfer pathway analogous to that observed in vibrationally excited O₂( b¹Σ_g⁺). With CO₂ collider gas, the removal rate coefficient at 300 K is (1.8 ± 0.4) × 10^-14 and (4.4 ± 0.6) × 10^-14 cm³ s^-1 (2σ) for v = 1 and 2, respectively. Despite the small mole fraction of O₂ in the atmospheres of Mars and Venus, O₂ is at least as important as CO₂ in the final stages of collisional relaxation within the O₂ vibrational-level manifold.

On the dissociation of I2 by O2(a1Delta): Pathways involving the excited species I2(A'3Pi2u,A3Pi(1u)), I2(X1sigma,upsilon), and O2(a1Delta,upsilon)

Kinetic studies were carried out to explore the role of the excited species I(2)(A(') (3)Pi(2u),A (3)Pi(1u)), I(2)(X (1) summation operator,upsilon), and O(2)(a (1)Delta,upsilon) in the dissociation of I(2) by singlet oxygen. A flow tube apparatus that utilized a chemical singlet oxygen generator was used to measure the I(2) dissociation rate in O(2)(a (1)Delta)/I(2) mixtures. Vibrationally excited I(2)(X) is thought to be a significant intermediate in the dissociation process. Excitation probabilities (gamma(upsilon)) for population of the upsilonth I(2)(X) vibrational level in the reaction I(2)(X)+I((2)P(1/2))-->I(2)(X,upsilon>10)+I((2)P(3/2)) were estimated based on a comparison of calculated populations with experimentally determined values. Satisfactory agreement with the experimental data [Barnault et al., J. Phys. IV 1, C7/647 (1991)] was achieved for total excitation probabilities partitioned in two ranges, such that Gamma(25</=upsilon</=47)= summation operator(upsilon=25) (47)gamma(upsilon) approximately 0.1 and Gamma(15</=upsilon</=24)= summation operator(upsilon=15) (24)gamma(upsilon) approximately 0.9. A multipathway I(2) dissociation model was developed in which the intermediates are I(2)(A(') (3)Pi(2u),A (3)Pi(1u)) and I(2)(X,upsilon). It was shown that the iodine dissociation process passes predominantly through the I(2)(A(') (3)Pi(2u),A (3)Pi(1u)) intermediate. These states are populated by collisions of I(2) with vibrationally excited O(2)(a (1)Delta,upsilon) at the initiation and the chain stages, when the mole fraction of I(2) is small (eta(I(2) )<1%). For higher I(2) concentrations (eta(I(2) )>/=1%) the excited states are populated in the chain stage by collisions of I(2)(X,15</=upsilon</=24) with O(2)(a (1)Delta).

Spin-orbit coupling in O2(v)+O2 collisions. II. Quantum scattering calculations on dimer states involving the XΣg−3, aΔg1, and bΣg+1 states of O2

The dynamics of collisional deactivation of O(2)(X (3)Sigma(g) (-),v=20-32) by O(2)(X (3)Sigma(g) (-),v(')=0) is investigated in detail by means of quantum-mechanical calculations. The theoretical approach involves ab initio potential energy surfaces correlating to the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) states of O(2) and their corresponding spin-orbit couplings [F. Dayou, M. I. Hernandez, J. Campos-Martinez, and R. Hernandez-Lamoneda, J. Chem. Phys. 123, 074311 (2005)]. Accurate Rydberg-Klein-Rees potentials are included in order to improve the description of the vibrational structure of the fragments. The calculated Boltzmann-averaged depletion probabilities display a dependence with v in good agreement with experimental measurements. The onset of the vibrational-to-electronic (V-E) depletion mechanism is noticeable for v>/=26, and it is due to energy transfer to both a (1)Delta(g) and b (1)Sigma(g) (+) states of the diatom. For O(2)(X (3)Sigma(g) (-),v=28), a further and sharp increase in the removal probabilities is caused by a near degeneracy with the O(2)(b (1)Sigma(g) (+),v=19) vibrational state. Analysis of the temperature dependence of the Boltzmann-averaged probabilities indicates a transition from the vibrational-to-translational to the V-E energy transfer regime, which can be traced back to the behavior of the inelastic probabilities as functions of kinetic energy. Furthermore, branching ratios for outcomes through the three different electronic states show a strong propensity towards populating a unique vibrational level within each electronic state. These results provide supported evidence that spin-orbit couplings account for a large portion of the "dark channel" reported in total depletion measurements. New insight for further experimental and theoretical investigations is also given.

Spin-orbit effect in the energy pooling reaction O2(aΔ1)+O2(aΔ1)→O2(bΣ1)+O2(XΣ3)

Five-dimensional nonadiabatic quantum dynamics studies have been carried out on two new potential energy surfaces of S(2)((1)A(')) and T(7)((3)A(")) states for the title oxygen molecules collision with coplanar configurations, along with the spin-orbit coupling between them. The ab initio calculations are based on complete active state second-order perturbation theory with the 6-31+G(d) basis set. The calculated spin-orbit induced transition probability as a function of collision energy is found to be very small for this energy pooling reaction. The rate constant obtained from a uniform J-shifting approach is compared with the existing theoretical and experimental data, and the spin-orbit effect is also discussed in this electronic energy-transfer process.

Intermolecular Potential of the O2−O2 Dimer. An ab Initio Study and Comparison with Experiment

Accurate intermolecular potentials for the lowest three multiplet states of O2-O2 dimer have been produced on the basis of ab initio calculations. The quintet potential was taken from previous highly correlated CCSD(T) calculations. In this work, we perform MRCI calculations, with large basis sets including bond functions, of the singlet and triplet states, which are of multireference character. As expected the size inconsistency and lack of higher order excitations limit the accuracy of the MRCI potentials specifically in describing the long range interactions. We show that the Heisenberg Hamiltonian provides an accurate representation of the exchange interactions in this system and this enables us to combine the accurate CCSD(T) potentials with the MRCI spin-exchange parameter to obtain accurate singlet and triplet potentials. The reliability of these potentials is tested by computing integral cross sections and comparing them with the detailed experimental study of the Perugia group, with excellent results. More interestingly, comparison with the experimentally derived potential shows important discrepancies for some angular orientations including that corresponding with the global minima, indicating the need for further work, both theoretical and experimental, to clarify their origin.

Spin-orbit coupling in O2(υ)+O2 collisions: I. Electronic structure calculations on dimer states involving the XΣg−3, aΔg1, and bΣg+1 states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.

Vibrational energy transfer in O2(X 3sigma(g)-, upsilon=2,3) + O2 collisions at 330 K

Vibrational relaxation of O2(X 3sigma(g)-, upsilon=2,3) by O2 molecules is studied via a two-laser approach. Laser radiation at 266 nm photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O2(a 1delta(g)) that is rapidly converted to O2(X 3sigma(g)-, upsilon=2,3) in a near-resonant adiabatic electronic energy-transfer process involving collisions with ground-state O2. The output of a tunable 193-nm ArF laser monitors the temporal evolution of the O2(X 3sigma(g)-, upsilon=2,3) population via laser-induced fluorescence detected near 360 nm. The rate coefficients for the vibrational relaxation of O2(X 3sigma(g)-, upsilon=2,3) in collision with O2 are 2.0(-0.4)(+0.6) x 10(-13) cm3 s(-1) and (2.6+/-0.4) x 10(-13) cm3 s(-1), respectively. These rate coefficients agree well with other experimental work but are significantly larger than those produced by various semiclassical theoretical calculations.