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lecture notes3

4 microwave emission we have now discussed at length

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Unformatted text preview: out ­of ­plane bend Some weaker features can be observed in some sources; these are sometimes attributed to other modes involving the carbon skeleton, overtones, and non ­PAH carriers. The shorter ­wavelength bands (3.3 and 6.2 μm) will be enhanced for the smallest dust grains because they get hotter during thermal spikes. However, the interpretation is slightly complicated by charging effects: remember that the strength of an emission feature depends on whether that vibrational mode of the molecule results in an oscillating dipole moment. The C−H modes generally have larger dipole moments than modes of the carbon skeleton (C−C stretch) in neutral PAHs. For ionized PAHs, the C−C stretch modes radiate more efficiently. Thus some ratios (e.g. 6.2/7.7) are very sensitive to sizes, while others (e.g. 11.3/7.7) are very sensitive to charge. PAH charge is determined by a balance between photoionization and collisions with charges (electrons or ions), just as for the ionization states of atoms; but of course the actual mechanisms are different and the ionization potential is lower (~5 eV for large PAHs). PAHs also have large electron affinities and in regions of high electron/UV ratio may form anions. A full analysis of size distribution fitting of PAHs (as done in the series by Draine & Li) must therefore simultaneously model the charging of PAHs. The Li & Draine (2001) model uses an abundance of carbon C:H = 6×10−5 in small carbonaceous grains and PAHs down to a minimum size of a ~ 0.35 nm (20 carbon atoms!). The smallest grains, as we have already discussed, are really molecules, and a variety of effects may become important. Since they reach very high temperatures during thermal spikes, they are susceptible to hydrogen atoms evaporating off, 7 leaving only the carbon skeletons, or even being destroyed by evaporation of C2, C2H2, etc. units. This sets a lower limit of the order of 20 carbons. Small PAHs (at least in the laboratory) can actually form specific molecular structures, often based on interlocking benzene rings. Specific examples include naphthalene (C10H8), pyrene (C16H10), coronene (C24H12),...
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