脱氢多环芳烃和UV凸块
Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
摘要：
最近的计算表明，消光曲线中约217.5 nm处的UV凸起可以用多环芳烃在几种电荷状态下的复杂混合物来解释。其他研究提出，载体是由中性和单电离脱氢的晕苯分子（C24 Hn，n小于3）组成的有限种群，与星际多环芳烃氢化状态模型预测中等大小的物种高度脱氢一致。为了评估后一种假设的观测结果，我们进行了脱氢多环芳烃的电子光谱的系统研究。我们使用我们的第一个结果，看看这样的光谱是否表现出强烈的一般趋势后脱氢。我们在密度泛函理论（DFT）的框架下使用最先进的技术来获得电子基态几何结构，并使用含时DFT来评估电子激发态性质。计算了C_（24）H_n（n= 12，10，8，6，4，2，0）在中性和阳离子电荷状态下的吸收截面。对其他多环芳烃及其完全脱氢的对应物进行了类似的计算。发现π电子能量总是受到脱氢的强烈影响。在我们研究的所有情况下，渐进脱氢转化为相应的渐进蓝移的主要电子跃迁。特别地，随着脱氢，π-π * 集体共振变得更宽和更蓝。因此，预测其计算的能量位置落在紫外线凸起和消光曲线的远紫外线上升之间的间隙中。由于这种影响似乎是系统性的，它构成了严格的观测限制柱密度强脱氢中型多环芳烃。
英文摘要：
Recent calculations have shown that the UV bump at about 217.5 nm in the extinction curve can be explained by a complex mixture of PAHs in several charge states. Other studies proposed that the carriers are a restricted population made of neutral and singly-ionised dehydrogenated coronene molecules (C24Hn, n less than 3), in line with models of the hydrogenation state of interstellar PAHs predicting that medium-sized species are highly dehydrogenated. To assess the observational consequences of the latter hypothesis we have undertaken a systematic study of the electronic spectra of dehydrogenated PAHs. We use our first results to see whether such spectra show strong general trends upon dehydrogenation. We used state-of-the-art techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state geometries, and of the time- dependent DFT to evaluate the electronic excited-state properties. We computed the absorption cross-section of the species C24Hn (n=12,10,8,6,4,2,0) in their neutral and cationic charge-states. Similar calculations were performed for other PAHs and their fullydehydrogenated counterparts. pi electron energies are always found to be strongly affected by dehydrogenation. In all cases we examined, progressive dehydrogenation translates into a correspondingly progressive blue shift of the main electronic transitions. In particular, the pi-pi* collective resonance becomes broader and bluer with dehydrogenation. Its calculated energy position is therefore predicted to fall in the gap between the UV bump and the far-UV rise of the extinction curve. Since this effect appears to be systematic, it poses a tight observational limit on the column density of strongly dehydrogenated medium-sized PAHs.
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