Astrochimistry – Spring 2013 Interstellar PAHs

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III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs Arrhenius law k diss (T) = A diss exp(-E 0 /kT) k diss = dissociation rate coefficient T = vibrational temperature → also kinetic temperature if enough collisions for thermalization A diss = prefactor E 0 = dissociation energy Laplace transform Approximated microcanonical expression k diss = dissociation rate coefficient E = internal (vibrational) energy A diss = prefactor → vibrational frequency → entropy cost (structural changes) → difficult to estimate theoretically, but not impossible E 0 = dissociation energy = vibrational density of state(VDOS) of the parent molecule => one has to know 26 the vibrational modes
III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs Anharmonicity = different from a parabolic potential well → always the case for dissociation → mode frequency vary with internal energy → much more difficult to find the frequencies, and to compute VDOS from frequencies → VDOS directly accessible from molecular dynamics simulations In practice, astronomers still use the harmonic VDOS 27
Page 1 and 2: Astrochimistry - Spring 2013 Lectur
Page 3 and 4: I. From Unidentified to Aromatic In
Page 9 and 10: Related detections I. From Unidenti
Page 11 and 12: II. General properties of “chemic
Page 13 and 14: Electron delocalization (=resonance
Page 21 and 22: In HII regions and reflection nebul
Page 23 and 24: III. Evolution of interstellar PAHs
Page 25: III. Evolution of interstellar PAHs
Page 37 and 38: IV. Influence of PAHs on the ISM ev
Page 39 and 40: IV. Influence of PAHs on the ISM ev
Page 41: VI. Some bibliographic references

Astrochimistry – Spring 2013 Interstellar PAHs

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