论文标题
超氢化的多环芳族烃分子：红外的振动光谱
Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared
论文作者
论文摘要
富含H的和紫外线良好的良性区域中可能存在超氢化的多环芳烃（PAHS）。将多余的H原子添加到PAHS中将芳香键转换为脂肪族键，其中最强的键接近3.4 $μ$ m。因此，经常假设超氢化的PAH是3.4 $ $ M发射功能的载体，通常伴随着更强的3.3 $ $ m $ m芳香族C-H拉伸功能。为了评估这一假设，我们使用密度函数理论来计算超氢的PAH的IR振动光谱及其各种尺寸的离子（从苯，萘，佩林烯和冠状动脉）以及各种程度的氢化（从最小的氢化到重型氢化）。对于每个分子，我们得出3.3 $ $ m芳香族C-H伸展的内在振荡器强度（$ a_ {3.3} $）和3.4 $ $ $ m级脂肪型C-h stratch（$ a__ {3.4} $）。通过将$ \ langle a_ a_ a_ a_ a _ {3.4}/a_ {3.3} \ rangle \ sim1.98 $与观察到的强度$ \ langle i_ {3.4}/i_ {3.3} \ rangle frange的平均值进行比较，对于中性PAH的原子，与多余的H原子相关的原子仅为$ \ sim2.2 \％$，主要散发出3.3和3.4 $μ$ m的功能。我们还为每个分子确定6.2 $ $ m芳香族C-c拉伸（$ a_ {6.2} $）和6.85 $ m $ m脂肪族C-H变形（$ a__ {6.85} $）的内在频带强度（$ A_ {6.2} $）。我们从观察到的强度的平均值$ \ langle I_ {6.85}/i_ {6.2} \ rangle \ sim0.10 $和$ \ langle a_ {6.85}/a____2}/a_ {6.2} \ rangle \ sim1.53 a_ {6.85}/a_ {6.2} \ rangle \ sim1.23 $，阳离子为$ \ sillesim 3.1 \％$，中性$ $ $ \％$，阳离子的$ \ sillesim 8.6 \％\％$ $。我们得出的结论是，天体物理PAH主要是芳香的，并且仅略有氢化。
Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) may be present in H-rich and ultraviolet-poor benign regions. The addition of excess H atoms to PAHs converts the aromatic bonds into aliphatic bonds, the strongest of which falls near 3.4 $μ$m. Therefore, superhydrogenated PAHs are often hypothesized as a carrier of the 3.4 $μ$m emission feature which typically accompanies the stronger 3.3 $μ$m aromatic C--H stretching feature. To assess this hypothesis, we use density function theory to compute the IR vibrational spectra of superhydrogenated PAHs and their ions of various sizes (ranging from benzene, naphthalene to perylene and coronene) and of various degrees of hydrogenation (ranging from minimal hydrogenation to heavy hydrogenation). For each molecule, we derive the intrinsic oscillator strengths of the 3.3 $μ$m aromatic C--H stretch ($A_{3.3}$) and the 3.4 $μ$m aliphatic C--H stretch ($A_{3.4}$). By comparing the computationally-derived mean ratio of $\langle A_{3.4}/A_{3.3}\rangle\sim1.98$ with the mean ratio of the observed intensities $\langle I_{3.4}/I_{3.3}\rangle\sim0.12$, we find that the degree of superhydrogenation --- the fraction of C atoms attached with excess H atoms --- is only $\sim2.2\%$ for neutral PAHs which predominantly emit the 3.3 and 3.4 $μ$m features. We also determine for each molecule the intrinsic band strengths of the 6.2 $μ$m aromatic C--C stretch ($A_{6.2}$) and the 6.85 $μ$m aliphatic C--H deformation ($A_{6.85}$). We derive the degree of superhydrogenation from the mean ratio of the observed intensities $\langle I_{6.85}/I_{6.2}\rangle\sim0.10$ and $\langle A_{6.85}/A_{6.2}\rangle\sim1.53$ for neutrals and $\langle A_{6.85}/A_{6.2}\rangle\sim1.23$ for cations to be $\lesssim 3.1\%$ for neutrals and $\lesssim 8.6\%$ for cations. We conclude that astrophysical PAHs are primarily aromatic and are only marginally superhydrogenated.
