Numerical simulation of the infrared emission of interstellar dust

The emission of the unidentified infrared bands (UIBs) has been attributed to excitation of polycyclic aromatic hydrocarbons (PAHs) by absorption of single energetic photons. Williams and Leone (1995) showed experimentally that a molecule (naphtalene) is considerably perturbed in this process so that the usual simplifying (harmonic) assumptions and associated analytical treatment do not apply. On the other hand, the single photon mechanism cannot operate in the frequently encountered environments where the radiation field is not strong, or the UV photons not hard, enough. This paper explores the 'feasibility' of a chemiluminescent process instead, in such cases. In both photonic and chemical excitation, the problem of energy redistribution is better tackled numerically. Here, a state-of-the-art numerical code is used to simulate naphtalene, a hydrocarbon particle of 18 atoms (assumed for the present purposes to be roughly representative of the real carrier material) and its chemical reaction with an H atom, a species known to be most abundant everywhere in space. The chemical energy deposited thus excites the particle into a complicated state of vibration. The code thereupon follows the dynamics of all the atoms and calculates the electric charge distribution at every step, from which the electric dipole moment is derived as a function of time. The FFT of this finally gives the spectral density of vibrational energy, which is found to be very different from the absorption spectrum of the same particle and to consist of several bands of different and varying widths. This--one of our main results--is the evidence of mode interactions due to mode anharmonicity and coupling. The energetic efficiency of this emission process is high and was proven to be adequate for astrophysical purposes. Other properties of this mechanism are also shown to be in agreement with observations. The assumptions and weaknesses of the present theoretical and numerical treatments are discussed with a view to further research.