We present a theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, X 1 AЈ, 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 Ϸ2 eV above the HϩCO threshold, corresponding to the eleventh overtone in the CO stretching mode (v 2 ϭ11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. ͓J. Chem. Phys. 103, 1448 ͑1995͔͒ is pleasing. The root-mean-square deviation from the experimental energies is only 17 cm Ϫ1 over a range of about 20 000 cm Ϫ1 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.

The unimolecular dissociation of the formyl radical HCO in the electronic ground state is investigated using a completely new ab initio potential energy surface. The dynamics calculations are performed in the time-independent picture by employing a variant of the log-derivative Kohn variational principle. The full resonance spectrum up to energies more than 2 eV above the vibrational ground state is explored. The three fundamental frequencies ͑in cm Ϫ1 ͒ for the H-CO and CO stretches, and the bending mode are 2446 ͑2435͒, 1844 ͑1868͒, and 1081 ͑1087͒, where the numbers in parentheses are the measured values of Sappey and Crosley obtained from dispersed fluorescence excitation spectra ͓J. Chem. Phys. 93, 7601 ͑1990͔͒. In the present work we primarily emphasize the dissociation of the pure CO stretching resonances (0v 2 0) and their decay mechanisms. The excitation energies, dissociation rates, and final vibrational-rotational state distributions of CO agree well with recent experimental data obtained from stimulated emission pumping. Similarities with and differences from previous time-independent and time-dependent calculations employing the widely used Bowman-Bittman-Harding potential energy surface are also discussed. Most intriguing are the pronounced oscillations of the dissociation rates for vibrational states v 2 у7 which are discussed in the framework of internal vibrational energy redistribution.

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.

We present quantum mechanical bound-state calculations for HCP(X) using an ab initio potential energy surface. The wave functions of the first 700 states, corresponding to energies roughly 23 000 cm Ϫ1 above the ground vibrational state, are visually inspected and it is found that the majority can be uniquely assigned by three quantum numbers. The energy spectrum is governed, from the lowest excited states up to very high states, by a pronounced Fermi resonance between the CP stretching and the HCP bending mode leading to a clear polyad structure. At an energy of about 15 000 cm Ϫ1 above the origin, the states at the lower end of the polyads rather suddenly change their bending character. While all states below this critical energy avoid the isomerization pathway, the states with the new behaviour develop nodes along the minimum energy path and show large-amplitude motion with H swinging from the C-to the P-end of the diatomic entity. How this structural change can be understood in terms of periodic classical orbits and saddle-node bifurcations and how this transition evolves with increasing energy is the focal point of this article. The two different types of bending motion are clearly reflected by the rotational constants. The relationship of our results with recent spectroscopic experiments is discussed.

A quantitative test of unimolecular rate theory in the multiphoton dissociation of CF2CFCl