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We investigate the contributions of the lowest two (1)A" states in the UV photodissociation of N(2)O employing three-dimensional potential energy surfaces and transition dipole moment functions. Because the transition dipole moments are much smaller than for the 2 (1)A' state, we conclude that excitation of the (1)A" states has a marginal effect. The dense vibrational spectrum of the quasi-bound 2(1)A" state possibly explains some of the tiny, noise-like structures of the measured absorption spectrum.

The recombination of ozone via the chaperon mechanism, i.e., ArO+O2→Ar+O3 and ArO2+O→Ar+O3, is studied by means of classical trajectories and a pairwise additive Ar-O3 potential energy surface. The recombination rate coefficient has a strong temperature dependence, which approximately can be described by T -n with n≈3. It is negligible for temperatures above 700 K or so, but it becomes important for low temperatures. The calculations unambiguously affirm the conclusions of Hippler et al. [J. Chem. Phys. 93, 6560 (1990)] and Luther et al. [Phys. Chem. Chem. Phys. 7, 2764 (2005)] that the chaperon mechanism makes a sizable contribution to the recombination of O3 at room temperature and below. The dependence of the chaperon recombination rate coefficient on the isotopomer, studied for two different isotope combinations, is only in rough qualitative agreement with the experimental data. The oxygen atom isotope exchange reaction involving ArO and ArO2 van der Waals complexes is also investigated; the weak binding of O or O2 to Ar has only a small effect. [ABSTRACT FROM AUTHOR]

We present the analysis and the semiclassical quantization of the van der Waals states of ozone in the ground electronic state X 1A1. Progressions of these states dominate the spectrum of O3 at threshold. Periodic orbits are used to perform assignment and quantization of the vibrational states. Semiclassical quantization is numerically accurate despite the fact that the classical phase space is chaotic while the nodal patterns of the quantum mechanical wave functions are regular. The lifetimes of recombination of the van der Waals states into the “normal” ozone are also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the analysis and the semiclassical quantization of the van der Waals states of ozone in the ground electronic state X 1A1. Progressions of these states dominate the spectrum of O3 at threshold. Periodic orbits are used to perform assignment and quantization of the vibrational states. Semiclassical quantization is numerically accurate despite the fact that the classical phase space is chaotic while the nodal patterns of the quantum mechanical wave functions are regular. The lifetimes of recombination of the van der Waals states into the “normal” ozone are also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Global ab initio potential energy surfaces for the ground (1 [SUP2]A[SUB1]) and the first excited (1 [SUP2]B[SUB2]) electronic states of NO[SUB2] are presented. The calculations are performed at a multireference configuration interaction level with the correlation consistent quadruple zeta atomic basis set of Dunning (cc-pVQZ). A total of about 5000 ab initio points were calculated. The dissociation energy corresponding to NO[SUB2])→ NO + O and other features of the surfaces are found to be in close agreement with experimental values. The adiabatic ground and the first excited electronic states form a conical intersection and have been diabatized to form two coupled diabatic surfaces. Quantum dynamics calculations are performed to obtain the fundamental frequencies and the energies of low lying bound states. Comparison of bound state energies with experimental values show good agreement. [ABSTRACT FROM AUTHOR]

Examines the photodissociation of water in the first absorption band. Impact of the process on the excitation of the first triplet state; Application of quantum mechanical methods in the calculation of potential-energy surface; Need for a comparison between measurement and absorption spectra levels and water photodissociation.

We present quantum mechanical wave packet calculations for the vibrational predissociation of cis-CH3ONO in the S1 state including three degrees of freedom—the CH3O–NO dissociation bond, the N=O stretching coordinate, and the CH3O–N–O bending angle. We calculate the autocorrelation function, the absorption spectrum, the lifetimes of the excited complex as a function of the internal excitation, and the final vibrational-rotational state distributions of the NO fragment. The lifetimes and the product state distributions are compared with experimental data as well as with previous results obtained from classical trajectory calculations. The calculated vibrational state distributions of the NO product satisfactorily reproduce the systematic variation with the initially prepared quasibound state of the CH3ONO(S1) complex found experimentally; however, they are considerably narrower than the experimental distributions. The theoretical rotational state distributions of NO, all being highly inverted and having the overall shape of a Gaussian, agree well with the experimental data; this is the case for several quasibound vibrational states of CH3ONO(S1) as well as several final vibrational states of the NO product. In general, the classical trajectory calculations parallel the quantum mechanical results. The existing differences have to be attributed to the inability of the purely classical treatment in reproducing subtle quantum effects if the dissociation proceeds through a relatively long-lived complex. While the calculations yield satisfactory agreement with the experimental NO state distributions including the envelope of the absorption spectrum, they disagree with the experiment in that the resonance widths are about one order of magnitude narrower than in the measured spectrum. Additional calculations for which the torsional angle of NO with respect to the intermolecular dissociation vector R is approximately taken into account as a... [ABSTRACT FROM AUTHOR]

We investigated the photodissociation of ClNO via the S1 electronic state using a three-dimensional (3D) ab initio potential-energy surface (PES). The dissociation is found to be fast and direct. In the Franck–Condon (FC) region the slope of the potential along the dissociation path is relatively small giving rise to narrow partial absorption peaks. The total absorption spectrum therefore exhibits a broad vibrational structure which is in perfect agreement with recent measurements. The vibrational excitation of the NO fragment is small and can be qualitatively described within the adiabatic approximation. It is found to be very sensitive to the vibrational FC factor in the transition region. The rotational state distribution of NO is highly inverted with a peak around j=30. It is readily explained by the rotational reflection principle. The experimental results are satisfactorily reproduced by our calculations which underlines the overall quality of the calculated PES. Minor adjustments are necessary, however, to quantitatively reproduce the vibrational branching ratio. [ABSTRACT FROM AUTHOR]

We report exact quantum mechanical close-coupling calculations for the photodissociation of Cl–CN using the ab initio potential of Waite and Dunlap. We find that the simple classical calculations of Barts and Halpern based on the rotational reflection principle agree very nicely with the exact rotational state distributions. Furthermore, it is shown that the impulsive model fails severely to account for the high degree of rotational excitation in the photodissociation of Cl–CN. [ABSTRACT FROM AUTHOR]

We use time-dependent quantum theory to interpret the absorption spectrum of CH3ONO in terms of the nuclear motion on the upper potential surface. The model uses one excited potential energy surface and two nuclear coordinates: the NO stretch and the CH3O–NO bond. The latter bond breaks upon excitation leading to dissociation. The spectrum consists of a broad band and two progressions corresponding to predissociation resonances. The band width is inverse proportional to the time scale on which the NO bond length increases to adjust to the longer equilibrium bond length of the upper potential energy surface. The progression of intense narrow resonances corresponds to the NO stretching motion. The other progression is due to oscillations along the reaction coordinate of the wave function temporarily trapped in the predissociation well. Our calculations show that important dynamic information can be obtained by ‘‘smearing off’’ the high resolution spectrum to generate a series of low resolution versions which reveal the time scales on which various spectral features develop in the spectrum. We also show that time-dependent theory can be used efficiently to calculate the lifetime of relatively long lived resonances. [ABSTRACT FROM AUTHOR]

Collision cross sections for rotational and vibrational excitation of CO by fast H atoms are calculated for two potential energy surfaces, the older Bowman–Bitman–Harding potential and the recently constructed surface of Werner, Keller, and Schinke. Both quantum mechanical and classical calculations are performed. The results obtained with the new potential energy surface are very similar to those obtained with the older potential; in particular, they do not rectify the discrepancies between the experimental and theoretical cross sections for vibrationally elastic transitions into small rotational states of CO. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present exact quantum mechanical bound-state calculations for HCP using an ab initio potential energy surface. The main result is the existence of two distinct families of bending states: one with normal-mode-type behavior and wave functions confined to small bending angles and the other one sampling the isomerization path all the way from H–CP to CP–H. Stable periodic orbits provide a clear-cut assignment. Possible relations to recent spectroscopic observations are discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Comparisons, for J=0, are made between HO2→H+O2 reaction path anharmonic energy levels, variational transition states, and unimolecular rate constants determined by three different semiclassical models and a quantum mechanical model. The semiclassical models are based on the reaction path Hamiltonian. However, to determine anharmonic energy levels, the harmonic potential of this Hamiltonian is replaced by the actual anharmonic DMBE IV potential for the HO2 system. Two of the semiclassical models use Einstein–Brillouin–Keller (EBK) quantization to determine energy levels for motion orthogonal to the reaction path; i.e., one model neglects anharmonic coupling between modes, while the other retains all the coupling. The third semiclassical model is based on a quartic expansion of the potential and second-order perturbation theory to determine the energy levels. A comparison of the results of these three semiclassical models shows that anharmonic coupling between modes orthogonal to the reaction path is unimportant for HO2 dissociation. The separable EBK model gives a RRKM rate constant versus energy in very good agreement with that obtained from a quantum mechanical calculation which retains full coupling between modes in determining the reaction path energy levels. If anharmonicity is treated, the reaction path Hamiltonian and its vibrator transition state give accurate RRKM rate constants for HO2 dissociation. Rate constants calculated with the flexible transition states model are in very good agreement with those of the semiclassical and quantum vibrator transition state models, if the O2 stretch conserved mode is treated as an anharmonic oscillator in the flexible model. However, in contrast to the vibrator transition state models, ‘‘steps’’ are not observed in the rate constants for the flexible model, since the transitional mode is treated classically. Harmonic and anharmonic rate constants are compared for... [ABSTRACT FROM AUTHOR]

Three-dimensional quantum mechanical calculations are carried out, in a time-independent scattering approach, to study the unimolecular dissociation HO2→H+O2. The dissociation cross section is governed by narrow resonances with widths that vary over five orders of magnitude. The unimolecular dissociation rates strongly fluctuate about an average which agrees surprisingly well with the Rampsberger–Rice–Kassel–Marcus (RRKM) theory. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a quantum mechanical wave packet study for the unimolecular dissociation of a triatomic molecule into an atom and a diatom. The 3D potential energy surface used in the dynamics calculations is that of the B state of water corresponding to the second absorption band. Both OH stretching coordinates and the bending angle are included. What is not taken into account is the strong nonadiabatic coupling to the lower-lying A and X states which in reality drastically shortens the lifetime in the B state. For this reason the present study is not a realistic account of the dissociation dynamics of water in the 122 nm band. It is, however, a representational investigation of a unimolecular reaction evolving on a realistic potential energy surface without barrier. The main focus is the resonance structure of the absorption spectrum and the final rotational state distributions of the OH fragment. The total absorption spectrum as well as the partial dissociation cross sections for individual rotational states of OH show drastic fluctuations caused by overlapping resonances. The widths of the individual resonances increase, on average, with the excess energy which has the consequence that the cross sections become gradually smoother. Although the low-energy part of the spectrum is rather irregular, it shows ‘‘clumps’’ of resonances with an uniform spacing of ∼0.1 eV. They are discussed in the context of IVR and a particular unstable periodic orbit. In accordance with the fluctuations in the partial dissociation cross sections as functions of the excess energy the final rotational state distributions show pronounced, randomlike fluctuations which are extremely sensitive on the energy.The average is given by the statistical limit (PST), in which all levels are populated with equal probability. With increasing excess energy the distributions more and more exhibit dynamical features which are reminiscent of direct dissociation like... [ABSTRACT FROM AUTHOR]

The photodissociation of H2S through excitation in the first absorption band (λ≊195 nm) is investigated by means of extensive ab initio calculations. Employing the MRD-CI method we calculate the potential energy surfaces for the lowest two electronic states of 1A‘ symmetry varying both HS bond distances as well as the HSH bending angle. (In the C2v point group these states have electronic symmetry 1B1 and 1A2, respectively.) The lower adiabatic potential energy surface is dissociative when one H atom is pulled away whereas the upper one is binding. For the equilibrium angle of 92° in the electronic ground state they have two conical intersections, one occurring near the Franck–Condon point. Because of the very small energy separation between these two states nonadiabatic coupling induced by the kinetic energy operator in the nuclear degrees of freedom are substantial and must be incorporated in order to describe the absorption and subsequent dissociation process in a realistic way. In the present work we treat the coupling between the two electronic states in a diabatic representation extracting the coordinate-dependent mixing angle from the CI coefficients of the electronic wave functions. The nuclear motion is treated in three dimensions in an exact quantum mechanical approach by propagation of a two-component time-dependent wave packet. The calculated absorption spectra for H2S and D2S satisfactorily agree with the measured spectra. In particular, the calculations reproduce the diffuse structures with energy spacing of about 1200 and 850 cm-1 for H2S and D2S, respectively. Furthermore, the calculated rotational- and vibrational-state distributions of the HS and DS fragments reproduce recent measurements in a convincing way. The photodissociation of H2S is a prototype for very fast electronic predissociation. The photon preferentially excites the binding (diabatic) state. This state, however, is quickly depleted by strong... [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]