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The effect of zero-point energy differences (ΔZPE) between the possible fragmentation channels of highly excited O3 complexes on the isotope dependence of the formation of ozone is investigated by means of classical trajectory calculations and a strong-collision model. ΔZPE is incorporated in the calculations in a phenomenological way by adjusting the potential energy surface in the product channels so that the correct exothermicities and endothermicities are matched. The model contains two parameters, the frequency of stabilizing collisions ω and an energy dependent parameter Δdamp, which favors the lower energies in the Maxwell–Boltzmann distribution. The stabilization frequency is used to adjust the pressure dependence of the absolute formation rate while Δdamp is utilized to control its isotope dependence. The calculations for several isotope combinations of oxygen atoms show a clear dependence of relative formation rates on ΔZPE. The results are similar to those of Gao and Marcus [J. Chem. Phys. 116, 137 (2002)] obtained within a statistical model. In particular, like in the statistical approach an ad hoc parameter η≈1.14, which effectively reduces the formation rates of the symmetric ABA ozone molecules, has to be introduced in order to obtain good agreement with the measured relative rates of Janssen et al. [Phys. Chem. Chem. Phys. 3, 4718 (2001)]. The temperature dependence of the recombination rate is also addressed. [ABSTRACT FROM AUTHOR]

Threshold spectra of several isotopomers of ozone are studied using accurate quantum mechanical calculations and an ab initio potential energy surface. Shallow van der Waals minima in the dissociation channels, separated from the deep main wells by an 80 cm[sup -1] high barrier, are shown to accommodate long progressions of assignable states. As a result, dense vibrational spectrum of ozone near dissociation is dominated by van der Waals-type states for all studied isotope compositions. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The isotope dependence of the O+O[sub 2] exchange reaction is investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface. The measurements confirm the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic [sup 18]O+[sup 16]O[sub 2]→[sup 18]O[sup 16]O+[sup 16]O and the endothermic [sup 16]O+[sup 18]O[sub 2]→[sup 16]O[sup 18]O+[sup 18]O reaction between 233 and 353 K: k[sub 8]=(3.4±0.6)×10[sup -12] (300 K/T)[sup 1.1±0.5] cm[sup 3] s[sup -1] and k[sub 6]=(2.7±0.4)×10[sup -12] (300 K/T)[sup 0.9±0.5] cm[sup 3] s[sup -1]. In addition, the ratio of these two rates, R, has been measured with comparatively higher precision. It is 1.27±0.04 at 300 K and also shows a distinct negative temperature dependence. Four types of classical trajectory calculations are performed in order to interpret the experimental result. They differ by the way in which the quantum mechanical zero-point energy of the reactants and the differences of zero-point energies between reactants and products, ΔE[sub ZPE]≈±22 cm[sup -1], are phenomenologically incorporated. Only calculations which account for ΔE[sub ZPE] are successful in reproducing the temperature dependence of R. This emphasizes that ΔE[sub ZPE] is an important factor in low-temperature ozone kinetics. At energies slightly above threshold a dynamical effect is found which additionally favors the exothermic over the endothermic reaction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A clear explanation for an anomalous isotope effect in ozone formation is given in terms of the energy transfer mechanism, where the metastable states of ozone are formed first, and then stabilized by collisions with other atoms. Unusual nonstatistical properties of metastable states spectra discovered earlier [J. Chem. Phys. 118, 6298 (2003)] are incorporated into the kinetics model, where different metastable states are treated as different species, and the stabilization step is treated approximately. The population of the ozone metastable states builds up and decays through three possible O[SUB2] + O channels. When different isotopes of oxygen are involved the three channels become open at different energies because of the differences in the quantum zero-point-energies (ΔZPE) of the different O[SUB2] molecules. The spectrum of metastable states is anomalously dense below the DZPE threshold and these states are accessible only from the lower entrance channel. Also, these low-lying metastable states are stabilized very efficiently (by collisions with third body) because they are energetically close to the bound O[SUB3] states. Such processes significantly enhance the formation rates of ozone isotopologues through the lower channels over the formation rates through the upper channels. Numerical results obtained for J = 0 give isotope effects in the right direction and of the right order of magnitude. Consideration of J>0 should improve the comparison with experiment. [ABSTRACT FROM AUTHOR]

We present new calculations of the branching ratios betweenthevarious electronic and isotopic photodissociation channels of ozone.Special emphasis is placed on the isotopic/isotopologue differencesbecause the contribution of the ozone photodissociation to the oxygenisotope and ozone isotopologue enrichments or fractionations is importantfor atmospheric applications. These branching ratios, which dependon photon energy, have been calculated with a full quantum mechanicalwavepacket propagation approach: the multiconfiguration time-dependentHartree (MCTDH) method. Five ozone isotopologues are considered: threesymmetric, 16O3(noted 666), 16O17O16O (676), and 16O18O16O (686); two asymmetric, 16O217O (noted 667) and 16O218O (668).The 668 and 667 asymmetric isotopologues can dissociate into either66 + 8 or 68 + 6 for 668 and into 66 + 7 or 67 + 6 for 667. In theranges of the Chappuis and Hartley bands, the dissociation is veryfast and electronic and isotopic branching ratios are obtained fromthe wavepacket fluxes through complex absorbing potentials (CAPs)located perpendicular to the dissociation channels of the potentialenergy surfaces (PESs) of the A 1B1(Chappuis)and B 31A′ (Hartley/Huggins) electronic states.In the range of the Huggins band the dissociation is much slower andthe isotopic branching ratios of 667 and 668 asymmetric isotopologues,(e.g; 668 → 66 + 8 or 86 + 6) are obtained from the ratiosof two partial absorption cross sections corresponding to the selectiveexcitation of one or the other of the two isomers of Cssymmetry, which dissociate respectively into 66+ 8 and 86 + 6. We find that the photodissociation of the 668 asymmetricisotopologue favors the 68 + 6 channel with a propensity varying between52% (Hartley) and 54% (Huggins) as a function of the photon energy.The electronic branching ratios to the singlet channel (O3+ hυ → O(1D) + O2(1Δ)) are all close to 90% above ≈32 000cm–1. Below this energy, the singlet channel isenergetically closed and only the triplet channel (O3+ hυ → O(3P) + O2(3Σ)) is open. These branching ratios are required tocalculate the photolysis rates of each ozone isotopologue, which inturn contribute to the atomic oxygen and the ozone isotopic enrichmentsin the atmosphere. [ABSTRACT FROM AUTHOR]

The 0 + 02 exchange reaction is a prerequisite for the formation of ozone in Earth's atmosphere. We report here state-to-state differential and integral cross sections for several O + O2 isotope- exchange reactions obtained by dynamically exact quantum scattering calculations at collision energies relevant to atmospheric conditions. These reactions are shown to be highly nonstatistical, evidenced by dominant forward scattering and deviation of the integral cross section from the statistical limit. Mechanistic,analyses revealed that the nonstatistical channel is facilitated by short- lived osculating resonances. The theoretical results provided an in-depth interpretation of a recent molecular beam experiment of the exchange reaction and shed light on the initial step of ozone recombination. [ABSTRACT FROM AUTHOR]

© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]