Domingos Sérgio R, Tikhonov Denis S, Steber Amanda L, Eschenbach Patrick, Gruet Sebastien, Hrodmarsson Helgi R, Martin Kévin, Garcia Gustavo A, Nahon Laurent, Neugebauer Johannes, Avarvari Narcis, Schnell Melanie
Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607, Hamburg, Germany.
CFisUC, Department of Physics, University of Coimbra, 3004-516, Coimbra, Portugal.
Nat Commun. 2024 Jun 10;15(1):4928. doi: 10.1038/s41467-024-48778-0.
Polycyclic aromatic hydrocarbons (PAHs) are widely established as ubiquitous in the interstellar medium (ISM), but considering their prevalence in harsh vacuum environments, the role of ionisation in the formation of PAH clusters is poorly understood, particularly if a chirality-dependent aggregation route is considered. Here we report on photoelectron spectroscopy experiments on [4]helicene clusters performed with a vacuum ultraviolet synchrotron beamline. Aggregates (up to the heptamer) of [4]helicene, the smallest PAH with helical chirality, were produced and investigated with a combined experimental and theoretical approach using several state-of-the-art quantum-chemical methodologies. The ionisation onsets are extracted for each cluster size from the mass-selected photoelectron spectra and compared with calculations of vertical ionisation energies. We explore the complex aggregation topologies emerging from the multitude of isomers formed through clustering of P and M, the two enantiomers of [4]helicene. The very satisfactory benchmarking between experimental ionisation onsets vs. predicted ionisation energies allows the identification of theoretically predicted potential aggregation motifs and corresponding energetic ordering of chiral clusters. Our structural models suggest that a homochiral aggregation route is energetically favoured over heterochiral arrangements with increasing cluster size, hinting at potential symmetry breaking in PAH cluster formation at the scale of small grains.
多环芳烃（PAHs）在星际介质（ISM）中广泛存在已被充分证实，但考虑到它们在极端真空环境中的普遍性，电离在PAH团簇形成过程中的作用却鲜为人知，特别是当考虑到一种依赖手性的聚集途径时。在此，我们报告了利用真空紫外同步辐射光束线对[4]螺烯团簇进行的光电子能谱实验。我们制备了具有螺旋手性的最小PAH——[4]螺烯的聚集体（直至七聚体），并采用几种最先进的量子化学方法，通过实验与理论相结合的方法对其进行了研究。从质量选择光电子能谱中提取每个团簇尺寸的电离起始点，并与垂直电离能的计算结果进行比较。我们探索了由[4]螺烯的两种对映体P和M聚集形成的众多异构体所产生的复杂聚集拓扑结构。实验电离起始点与预测电离能之间非常令人满意的基准测试，使得我们能够识别理论预测的潜在聚集模式以及手性团簇相应的能量排序。我们的结构模型表明，随着团簇尺寸的增加，同手性聚集途径在能量上比异手性排列更有利，这暗示了在小颗粒尺度上PAH团簇形成过程中可能存在的对称性破缺。
