Abstract
Results from spectroscopic studies of the vibrational levels of dissociating molecules and from state-selected, state-resolved photofragmentation spectroscopy are presented. The extent of energy flow among the modes of a molecule is explored through the couplings, or lack thereof, revealed by high-resolution spectroscopy. The dynamics of energy flow during bond breaking are revealed by photofragment excitation spectroscopy and by product energy state distributions. These completely resolved data provide sensitive tests of dynamical constraints such as vibrational or rotational adiabaticity and thus of theoretical models for unimolecular reaction dynamics.
References
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- add some brief comments on the experiments of the Moore group on ketene
- dissociation and their interpretation in terms of statistical models for lifetimes and
- product state distributions. For lifetimes one may use any of the statistical models
- whereas originally RRKM theory did not predict energy distributions in products as
- pointed out by R. A. Marcus a number of years ago. More recently theories rather
- similar (but not identical) to the adiabatic channel model for product state
- distributions have been called RRKM theory but I shall not address the question of
- nomenclature rather the logical interpretation of the ketene experiments. The first finding is the close agreement of measured product state distributions
- with predictions from phase space theory. This has been taken as evidence against
- the validity of the adiabatic channel model by the Moore group in some places in the
- literature. Such a conclusion is logically impossible as phase space theory can be
- obtained as a limiting case of the adiabatic channel model by simple approximations
- (setting a = co corresponding to an isotropic potential and approximating
- centrifugal energy by the two-particle expression (see Quack 1979; Quack & Troe
- 1981)). If there is thus agreement of experiment with phase space theory this implies
- agreement with a limiting situation in the adiabatic channel model which is the
- inclusive theory (the reverse is not true in general). Thus the claimed disagreement
- of experiment with the adiabatic channel model on the basis of product state
- distributions is a logically inconsistent conclusion and totally unjustified. The second finding concerns specific dissociation-rate constants which are
- measured to be about a factor of two smaller than would seem to result from a
- calculation in the `loose complex ' phase space theory or similarly the adiabatic
- channel model in th at limit. The interpretation of this result is ambiguous for several
- reasons. In principle it contradicts the simple one parameter adiabatic channel
- model because if k{E J)defines a parameter a < oo then the product state
- distribution corresponding to a = oo (phase space theory) should be impossible. In
- reality this reasoning is a bit too simple and I shall summarize here several possible
- interpretations of the combined experimental results on and product state
- distributions. 1. The discrepancy may be only apparent and disappear if quantitative
- calculations are done. It may well be th a t oo applies but th at the resulting
- product energy distribution is too similar to a phase space theory result to be
- distinguished from it within experimental error quantitatively. 2. The product energy distribution depends upon the number of open adiabatic
- channels W{E J) whereas the specific rate constants k{E J) depend upon the
- density of states p(E J) as well :
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Dates
Type | When |
---|---|
Created | 18 years, 8 months ago (Dec. 15, 2006, 3:41 p.m.) |
Deposited | 4 years, 6 months ago (Feb. 14, 2021, 8:20 p.m.) |
Indexed | 1 year, 6 months ago (Feb. 10, 2024, 8:47 a.m.) |
Issued | 35 years ago (Aug. 15, 1990) |
Published | 35 years ago (Aug. 15, 1990) |
Published Online | 28 years, 7 months ago (Jan. 1, 1997) |
Published Print | 35 years ago (Aug. 15, 1990) |
@article{1990, volume={332}, ISSN={2054-0299}, url={http://dx.doi.org/10.1098/rsta.1990.0116}, DOI={10.1098/rsta.1990.0116}, number={1625}, journal={Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences}, publisher={The Royal Society}, year={1990}, month=aug, pages={297–307} }