Sergy Yu. Grebenshchikov

Theoretical investigation of the temperature dependence of the O+O2 exchange reaction

The exchange reaction 16O+18O2→16O18O+18O and, in particular, its dependence on the transition state region is investigated by classical trajectories on three potential energy surfaces, all based on high-level electronic structure calculations. The first one is the original potential recently constructed by Siebert, Schinke, and Bittererova [Phys. Chem. Chem. Phys. 3, 1795 (2001)]; it has a very small barrier above the O+O2 asymptote. The second potential is a modification of the first one in that the transition state region is adjusted according to new electronic structure calculations on higher levels of theory; it has a small barrier below the asymptote. The third potential is obtained by artificially removing this barrier. The variation of the exchange reaction cross section with collision energy and the magnitude of the thermal rate constant at and below room temperature depend drastically on the shape of the potential at intermediate distances. The second potential, which is believed to represent th...

van der Waals states in ozone and their influence on the threshold spectrum of O3(X1A1). I. Bound states.

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−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.

Analysis of the highly excited vibrational dynamics of HCP using a high-order Fermi resonance Hamiltonian

Based on an ab initio potential energy surface, the features of the quantum spectrum of HCP have been recently discussed in terms of the periodic orbits of the exact classical Hamiltonian [J. Chem. Phys. 107, 9818 (1997)]. In particular, it was shown that the abrupt change in the bending character of the states at the lower end of the Fermi polyads, at about 15 000 cm−1 above the origin, can be ascribed to a classical saddle node bifurcation. The purpose of the present article is to show that the use of a very accurate Fermi resonance Hamiltonian, which was derived very recently from high-order perturbation theory [J. Chem. Phys. 109, 2111 (1998)] can provide a still deeper insight into the highly excited vibrational motion. The principal advantages of the resonance Hamiltonian compared to the exact one rely on the remaining good quantum numbers and classical action integrals, which enable one to consider HCP as a formal one-dimensional system parametrized by the polyad number i and the number v3 of quant...

Isotope dependence of the lifetime of ozone complexes formed in O + O2 collisions

Results of classical trajectory calculations are presented for the lifetime of ozone complexes formed in collisions of O atoms with O2 diatoms. The average trajectory lifetimes for the complexes 16O3, 16O18O2 and 18O16O2 are significantly different. The difference decreases with increasing collision energy and is shown to be due to the difference in the quantum mechanical zero-point energies of reactants and products, which is incorporated in a phenomenological way in the classical calculations.

Resonance spectrum and dissociation dynamics of ozone in the 3B2 electronically excited state: experiment and theory.

The rovibrational spectrum assigned to the low-lying (3)B(2) electronic state of ozone is measured with intracavity laser absorption spectroscopy. The experimental results are interpreted by means of quantum dynamical calculations on a global ab initio potential energy surface. The observed spectrum is shown to originate from the vibrational ground state in the local minimum of the (3)B(2) potential. The spectrum of short-lived resonance states in this local minimum is analyzed. Additionally, the global minimum of the surface is shown to lie in the dissociation channel in the van der Waals region. This region supports a short sequence of weakly bound vibrational states.

Ab Initio Quantum Mechanical Study of the O(1D) Formation in the Photolysis of Ozone between 300 and 330 nm

Spin-allowed production of O((1)D) in the near-UV photolysis of ozone is studied using ab initio potential energy surfaces and quantum mechanics. The O((1)D) quantum yield, reconstructed from the absolute cross sections for eight initial vibrational states in the ground electronic state, is shown to agree with the measurements in a broad range of photolysis wavelengths and temperatures. Relative contributions of one- and two-quantum stretching and bending initial excitations are quantified, with the contribution of the antisymmetric stretch being dominant for lambda < 330 nm. Large scale structures in the low-resolution quantum yield are shown to reflect excitations in the high-frequency short bond stretch in the upper electronic state. Spin-forbidden contribution to the O((1)D) quantum yield at wavelengths lambda > 320 nm is estimated using ab initio energies of the triplet states and their spin-orbit couplings.

Signatures of a conical intersection in photofragment distributions and absorption spectra: Photodissociation in the Hartley band of ozone

Photodissociation of ozone in the near UV is studied quantum mechanically in two excited electronic states coupled at a conical intersection located outside the Franck-Condon zone. The calculations, performed using recent ab initio PESs, provide an accurate description of the photodissociation dynamics across the Hartley/Huggins absorption bands. The observed photofragment distributions are reproduced in the two electronic dissociation channels. The room temperature absorption spectrum, constructed as a Boltzmann average of many absorption spectra of rotationally excited parent ozone, agrees with experiment in terms of widths and intensities of diffuse structures. The exit channel conical intersection contributes to the coherent broadening of the absorption spectrum and directly affects the product vibrational and translational distributions. The photon energy dependences of these distributions are strikingly different for fragments created along the adiabatic and the diabatic paths through the intersection. They can be used to reverse engineer the most probable geometry of the non-adiabatic transition. The angular distributions, quantified in terms of the anisotropy parameter β, are substantially different in the two channels due to a strong anticorrelation between β and the rotational angular momentum of the fragment O2.

Comment on 'Rate coefficients for photoinitiated NO2 unimolecular decomposition: energy dependence in the threshold regime' [Chem. Phys. Lett. 358 (2002) 71]

Abstract Recently, Wittig and co-workers have published rate coefficients k ( E ) for the unimolecular decomposition of photoinitiated NO 2 close to the dissociation threshold [Chem. Phys. Lett. 358 (2002) 71]. They found out that k ( E ) for low angular momentum J exhibits a strong increase within 25 cm −1 of the reaction threshold. The authors emphasize that their experimental results are surprising and cannot be understood quantitatively on the basis of current theory on NO 2 . In this Comment we demonstrate that recent quantum mechanical calculations of NO 2 resonances on a global 3D-potential energy surface can indeed explain their data close to the dissociation threshold as well as for larger excess energies.

Energies and spatial features for the rotationless bound states of 4He3+ (2Σg+): A cationic core from helium cluster ionization

Ab initio quantum calculations have been carried out on the helium ionic trimer. The potential energy surface is accurately fitted, especially in the vicinity of the three equivalent minima. The spectrum of bound states for the zero angular momentum is computed and analyzed in detail. Energies and wave functions reveal several interesting features related to the fact that He3+ represents one of the few homonuclear ionic trimers that are linear in their ground vibrational state. At low energies, the triply degenerate eigenfunctions are localized at the potential minimum. With growing excitation energy, however, the wave functions exhibit stronger spatial delocalization.