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The two-body dissociation reactions of the dication C2H2+2, initiated via double ionization of acetylene molecules by photons in the energy range 31.9-50.0 eV, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The angular distributions and kinetic energy of product ions, measured in the 31.9-50.0 eV energy range, exhibit significant differences for the three leading dissociation reactions with respect to a previous investigation carried out at a fixed energy of 39.0 eV, providing thus new information on the dynamical evolution of the system. The analysis of the results indicates that such dissociation reactions occur with a different mechanism. In particular, the symmetric dissociation in two CH+ ions is characterized by different dynamics, and the anisotropy of the angular distribution of ionic products increases with photon energy in a more pronounced way than the other two reactions. Moreover, the kinetic energy distribution of the symmetric dissociation reaction exhibits several components that change with photon energy. The new experimental findings cast light on the microscopic evolution of the system and can provide a laboratory reference for new theoretical calculations on specific features of the multidimensional potential energy surface, namely, the structure, energy and symmetry of dication states, the electronic state of dissociation products, energy barriers and their dependence on the geometry of the intermediate state. [ABSTRACT FROM AUTHOR]

Energy transfer from O(1D) to O2 is the main source of O2(b1∑g+) in vibrational levels v = 0 and 1 in the Earth's thermosphere. Knowledge of the relative yields for O2(b1∑g+) production in v = 0 and 1 is essential for a reliable interpretation and modeling of the O2 atmospheric band emissions (b1∑g+-X3∑g- ) from these two vibrational levels. We report laboratory measurements of the relative yields at room temperature. In the experiments, O2(b1∑g+,v=0,1) is generated by O(1D) + O2 collisions following partial photodissociation of O2 at 157.6 nm. O2(b1∑g+,v=0,1) emission detection is used to monitor the temporal evolution of the vibrational level populations. The measured fractional yield for v = 1 is 0.8±0.1, in contrast with the results of previous studies that indicated dominant O2(b1∑g+,v=0,1)production. A revision is warranted of the values used for these relative yields in atmospheric models. [ABSTRACT FROM AUTHOR]

Laboratory measurements are reported of the rate coefficient for collisional removal of O2(X3Σg-, υ = 1) by O(3P), and the rate coefficients for removal of O2(a1Δg, υ = 1) by O2, CO2, and O(3P). A two-laser method is employed, in which the pulsed output of the first laser at 285 nm photolyzes ozone to produce oxygen atoms and O2(a1Δg, υ = 1), and the output of the second laser detects O2(a1Δg, υ = 1) via resonance-enhanced multiphoton ionization. The kinetics of O2(X3Σg-, υ = 1) + O(3P) relaxation is inferred from the temporal evolution of O2(a1Δg, υ = 1), an approach enabled by the rapid collision-induced equilibration of the O2(X3Σg-, υ = 1) and O2(a1Δg, υ = 1) populations in the system. The measured O2(X3Σg-, υ = 1) + O(3P) rate coefficient is (2.9 ± 0.6) × 10-12 cm3 s-1 at 295 K and (3.4 ± 0.6) × 10-12 cm3 s-1 at 240 K. These values are consistent with the previously reported result of (3.2 ± 1.0) × 10-12 cm3 s-1, which was obtained at 315 K using a different experimental approach [K. S. Kalogerakis, R. A. Copeland, and T. G. Slanger, J. Chem. Phys. 123, 194303 (2005)]. For removal of O2(a1Δg, υ = 1) by O(3P), the upper limits for the rate coefficient are 4 × 10-13 cm3 s-1 at 295 K and 6 × 10-13 cm3 s-1 at 240 K. The rate coefficient for removal of O2(a1Δg, υ = 1) by O2 is (5.6 ± 0.6) × 10-11 cm3 s-1 at 295 K and (5.9 ± 0.5) × 10-11 cm3 s-1 at 240 K. The O2(a1Δg, υ = 1) + CO2 rate coefficient is (1.5 ± 0.2) × 10-14 cm3 s-1 at 295 K and (1.2 ± 0.1) × 10-14 cm3 s-1 at 240 K. The implications of the measured rate coefficients for modeling of atmospheric emissions are discussed. [ABSTRACT FROM AUTHOR]

We report a laboratory measurement of the rate coefficient for the collisional removal of O2(X3Σg-,υ=1) by O(3P) atoms. In the experiments, 266-nm laser light photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O2(a1Δg) that is rapidly converted to O2(X3Σg-,υ=1–3) in a near-resonant electronic energy-transfer process with ground-state O2. In parallel, a large amount of O(1D) atoms is generated that promptly relaxes to O(3P). Under the conditions of the experiments, only collisions with the photolytically produced O(3P) atoms control the lifetime of O2(X3Σg-,υ=1), because its removal by molecular oxygen at room temperature is extremely slow. Tunable 193-nm laser light monitors the temporal evolution of the O2(X3Σg-,υ=1) population by detection of laser-induced fluorescence near 360 nm. The removal rate coefficient for O2(X3Σg-,υ=1) by O(3P) atoms is (3.2±1.0)×10-12 cm3 s-1 (2σ) at a temperature of 315±15 K (2σ). This result is essential for the analysis and correct interpretation of the 6.3-μm H2O(ν2) band emission in the Earth’s mesosphere and indicates that the deactivation of O2(X 3Σg-,υ=1) by O(3P) atoms is significantly faster than the nominal values recently used in atmospheric models. [ABSTRACT FROM AUTHOR]

Vibrational relaxation of O2(X 3Σg-,υ=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 1Δg) that is rapidly converted to O2(X 3Σg-,υ=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 3Σg-,υ=2,3) population via laser-induced fluorescence detected near 360 nm. The rate coefficients for the vibrational relaxation of O2(X 3Σg-,υ=2,3) in collision with O2 are 2.0-0.4+0.6×10-13 cm3 s-1 and (2.6±0.4)×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. [ABSTRACT FROM AUTHOR]

The temperature dependence of the collisional removal of 02 in the v= 3 level of the b&sup1 ∑[sup +][sub g] state by N[sub 2], O[sub 2], and CO[sub 2] was investigated at room temperature and below. Measurements on the v = 2 level with the colliders, O[sub 2], N[sub 2]O, Ar, and He are also reported. For v= 3 removal by O[sub 2], the trend of sharply decreasing loss rate coefficients with increasing v is seen to continue. For example, at 200 K the rate coefficient for collisional removal of O[sub 2] from the v= 3 level by O[sub 2] is some three orders of magnitude smaller than that for the v= 1 level. We argue that the mechanism of the deactivation is electronic-electronic (E-E) energy transfer. Observation of emission from a broad range pf O[sub 2](b¹ ∑[sup +][sub g] vibrational levels in the terrestrial nightglow, recently discovered in astronomical sky spectra, show less than an order of magnitude difference in population between the more highly populated v = 3 level and the less populated v= 1 level. The implications of these two observations on the vibrational-level-specific atmospheric sources of vibrationally excited molecules in the b ¹∑[sup +][sub g] electronic state are explored. [ABSTRACT FROM AUTHOR]

Crossed-beam and beam-gas experiments on the reaction Ba+HI→BaI+H have been performed, in which the most probable collision energy ranges from 3 to 17 kcal/mol. The results, combined with previous experimental studies on this reaction system, show a remarkable collision energy dependence. Between low and high collision energies, a transition occurs in the intensity, width, and peak location of the product vibrational and rotational population distributions. The onset of this transition is estimated to occur at approximately 5 kcal/mol. For collision energies smaller than 5 kcal/mol, the product vibrational distribution is bell shaped and peaks at v=12. For collision energies larger than 5 kcal/mol, a second maximum appears at v=0 in the vibrational distribution. The rotational distributions of the crossed-beam experiments are extremely narrow but broaden at lower collision energies. As the collision energy is increased above 5 kcal/mol, the BaI rotational excitation is very near the energetic limit, and the maximum for the BaI(v=0) rotational population distribution moves from J=415.5 to J=538.5. In contrast, below the transition onset, the maximum remains unchanged around J=420.5. Moreover, the peaks of the BaI(v=1) and BaI(v=2) rotational distributions appear at successively lower J values, as expected from energy conservation arguments. The nature of the kinematic constraints for this reaction allows the determination of the opacity functions for the production of the BaI product in a specific vibrational level v. Detailed analysis of the collision energy dependence of the specific opacity functions offers insight into the role of conservation of energy and angular momentum in influencing this reaction. At low collision energies, the maximum reactive impact parameter, bmax, is determined by an angular momentum (centrifugal) barrier. At collision energies larger than 5 kcal/mol, conservation of energy dictates the value of bmax. These two processes are... [ABSTRACT FROM AUTHOR]