Journal of Molecular Spectroscopy ( IF 1.4 ) Pub Date : 2021-04-15 , DOI: 10.1016/j.jms.2021.111458 Jordy Bouwman , Harold Linnartz , Alexander G.G.M. Tielens
In this work, we characterize – for the first time – in the gas phase infrared spectra of three isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations of C24H14 composition that belong to distinctly different symmetry groups (, and ). Mid-infrared (Mid-IR) spectra are recorded by means of infrared multiple photon dissociation (IRMPD) spectroscopy at the free electron laser for infrared experiments (FELIX) laboratory. The measured infrared (IR) band positions compare reasonably well with density functional theory (DFT) calculated values. The number of IR active bands increases as the symmetry of the molecule lowers. The IRMPD spectra of irregular PAHs are found to be dense and do not resemble the sharp signatures typical of astronomical IR bands, but rather look like the broad plateau on which these are perched. This lends credit to the GrandPAH hypothesis that suggests that small and irregular PAHs are weeded out by the strong interstellar radiation field and only large regular PAHs remain.
中文翻译:
二苯并py阳离子的中红外光谱特征–对称性对PAH红外光谱的影响
在这项工作中,我们首次在气相中表征了C 24 H 14组成的三个异构多环芳烃(PAH)阳离子的红外光谱,这些阳离子属于明显不同的对称基团(, 和 )。在用于红外实验的自由电子激光器(FELIX)实验室中,通过红外多光子离解(IRMPD)光谱记录了中红外(Mid-IR)光谱。测得的红外(IR)波段位置与密度泛函理论(DFT)的计算值相当合理。随着分子对称性降低,IR活性带的数量增加。发现不规则PAH的IRMPD光谱很稠密,与天文IR波段典型的清晰特征不相似,而看上去像是栖息在其上的广阔高原。这归功于GrandPAH假说,该假说表明,小而不规则的PAHs被强的星际辐射场淘汰掉了,只剩下大的常规PAHs。