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. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 31]
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机译：未知红外波段（UIB）的发射已归因于通过吸收单个高能光子激发多环芳烃（PAH）。 Williams and Leone（1995）实验表明，分子（萘）在此过程中受到很大干扰，因此通常的简化（谐波）假设和相关分析处理均不适用。另一方面，单光子机制无法在辐射场不强或UV光子不够硬的经常遇到的环境中运行。在这种情况下，本文将探索化学发光过程的“可行性”。在光子激发和化学激发中，能量重新分配的问题都可以通过数值更好地解决。在此，使用最新的数字代码来模拟萘，18个原子的碳氢化合物颗粒（出于目前的目的，大致代表了实际的载体材料）及其与H原子，在太空中最丰富的物种。因此，沉积的化学能将颗粒激发成复杂的振动状态。因此，该代码遵循所有原子的动力学并计算每一步的电荷分布，从中得出电偶极矩随时间的变化。 FFT最终给出了振动能量的频谱密度，发现该频谱能量与同一粒子的吸收频谱有很大不同，并且由不同宽度和变化宽度的几个波段组成。这是我们的主要结果之一，是由于模式非谐和耦合导致模式相互作用的证据。该发射过程的能量效率很高，并被证明足以用于天体物理学目的。此机制的其他属性也显示与观察结果一致。讨论了当前理论和数值处理的假设和不足，以期进一步研究。 （C）2001 Elsevier Science B.V.保留所有权利。 [参考：31]
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