Search

Three-dimensional quantum mechanical calculations have been carried out, using a modification of the log-derivative version of Kohn’s variational principle, to study the dissociation of HO2 into H and O2. In a previous paper, over 360 bound states were found for each parity, and these are shown to extend into the continuum, forming many resonant states. Analysis of the bound states close to the dissociation threshold have revealed that HO2 is a mainly irregular system and in this paper it is demonstrated how this irregularity persists in the continuum. At low energies above the threshold, these resonances are isolated and have widths that fluctuate strongly over more than two orders of magnitude. At higher energies, the resonances begin to overlap, while the fluctuations in the widths decrease. The fluctuations in the lifetimes and the intensities in an absorption-type spectrum are compared to the predictions of random matrix theory, and are found to be in fair agreement. The Rampsberger–Rice–Kassel–Marcus (RRKM) rates, calculated using variational transition state theory, compare well to the average of the quantum mechanical rates. The vibrational/rotational state distributions of O2 show strong fluctuations in the same way as the dissociation rates. However, their averages do not agree well with the predictions of statistical models, neither phase space theory (PST) nor the statistical adiabatic channel model (SACM), as these are dependent on the dynamical features of the exit channel. The results of classical trajectory calculations agree well on average with those of the quantum calculations. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This is the first of a series of papers in which we investigate the unimolecular dissociation of hydroperoxyl. Using the DMBE IV potential energy surface [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)], in the present study 726 bound states of HO2(X) up to the H+O2 dissociation threshold are calculated in an attempt to access the extent of the coupling between the modes of the system. The first approach involves an analysis of the nodal structure of the wave functions. While the wave functions for the lowest states are regular and assignable, the degree of mixing and complexity rapidly increases with energy. The wave functions close to the dissociation threshold are mostly irregular without any clear cut nodal structure and fill the entire coordinate space available. Nevertheless, a small number of regular states, that are associated with large excitation in the O2 stretching coordinate and no or only little excitation in the other modes, are found even at high energies. The second approach used to study the degree of intramolecular coupling is an analysis of the energy spectrum. The nearest neighbor level spacing distribution, which probes the short-range correlation, as well as the Σ2 and Δ3 statistics, which are sensitive to the long-range correlations in the spectrum, are investigated and compared to the distributions predicted for regular and irregular spectra. Both of these approaches indicate that the system is almost totally irregular with a Brody parameter of about 92%. In addition, the sum of states at a particular energy, which is extremely important in all statistical models for unimolecular dissociation, is approximately calculated from the volume of classical phase space and found to be in excellent agreement with the exact quantum mechanical result. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present three-dimensional wave packet calculations for the photodissociation of FNO in the first excited singlet state S1 using a new ab initio potential surface. While the calculated absorption spectrum agrees satisfactorily with the measured spectrum, the energy dependence of the partial cross sections for particular NO product states is only in fair agreement with experiment. The same is true for the vibrational and rotational state distributions of NO for selected energies. Because of the interference between direct and indirect dissociation, details of the cross sections are highly sensitive to subtleties of the potential surface. Altogether, the new calculation reproduces the available experimental data more satisfactorily than a previous one. © 1995 American Institute of Physics. [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 theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, X1A′, using a new ab initio potential energy surface and a modification of the log-derivative version of the Kohn variational principle for the dynamics calculations. Altogether we have analyzed about 120 resonances up to an energy of ≊2 eV above the H+CO threshold, corresponding to the eleventh overtone in the CO stretching mode (v2=11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. [J. Chem. Phys. 103, 1448 (1995)] is pleasing. The root-mean-square deviation from the experimental energies is only 17 cm-1 over a range of about 20 000 cm-1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. The assignment of the states is discussed in terms of the resonance wave functions. In addition, we compare the quantum mechanical state-resolved dissociation rates with the results of classical trajectory calculations and with the predictions of the statistical model. © 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]