## Comment on "Photodestruction of PAHs in the interstellar medium I. Photodissociation rates for the loss of an acetylenic group"
In the beginning of this paper (Allain et al. 1996), the authors make a comparison between their results concerning the photofragmentation of PAHs and the predictions of an older paper on the same topic "Photo-thermo-dissociation..." (Léger et al. 1989) hereafter referred as PTD. However, it appears that the basis of this comparison is not clearly defined, which can induce some confusion for a reader not acquainted with the formalism of unimolecular reactions. A rapid glance in Fig. 2 of Allain et al., and in the text related to it, would lead to the conclusion that the PTD model, based on the Inverse Laplace Transform (ILT) method (Forst 1972) is definitely not a good one, since it gives numerical results up to ten orders of magnitude too low. But, what is not stated clearly, neither in the caption, nor in the text, is that the different methods of calculation of the rate are applied to different fragmentation pathways: the ILT method is used to calculate the rate of loss of a single carbon atom, while the RRK formalism is used for the loss of an acetylenic group. This point deserves two comments. On the one hand, concerning the fragmentation pathway, the paper
PTD was "not directly based on reactions with PAHs" merely because no
experimental data concerning the photofragmentation of PAHs was
published at the time it was written. Besides the dehydrogenation
process, the only pathways for which data could be available, by
extrapolation of properties of bulk graphite, were the losses of
On the other hand, if one wants to compare the accuracy of different calculations, the comparison has to be done on the same physical process. In the present case, this could have been done for the dehydrogenation process. All the coefficients needed to calculate the rate of hydrogen atom loss from an arbitrary PAH are summarised in the paper Allain et al. As an example, Fig. 1 compares, for naphthalene and coronene, the dehydrogenation rates obtained using the RRK and ILT formalism. As can be seen, the discrepancy between the two methods is now not larger than two orders of magnitude. This uncertainty does not exceed the range that can be expected for any model only using two parameters (, ) to deal with the whole PAH family, without accounting for the particularity of each molecule. Moreover, it can be noticed that, if the experimental data of Jochims et al. (1994) is closer to the RRK result for coronene, the opposite is true in the case of naphthalene.
Then the ILT calculation appears to work at least as well as the
RRK one when applied to the same process. But, it has to be reminded
that, while the RRK parameters have been fitted to reproduce
experimental data, those used in the ILT calculation are only based on
the knowledge of the binding energy and on statistical physics
considerations. The PTD model has then a predictive nature. This is
possible because, in the ILT formula, is the
dissociation energy, the same which appears in the Arrhenius law
Therefore, the criticisms of the ILT calculation are not founded.
This formalism, based on parameters with a clear physical meaning,
gives dissociation rates with at least the same accuracy than the RRK
one. Moreover, if both calculations are ## Contents© European Southern Observatory (ESO) 1997 Online publication: May 26, 1998 |