Dutta Saurav, Behera Nihar Ranjan, Barik Saroj, Kushawaha Rajesh Kumar, Sajeev Y, Aravind G
Indian Institute of Technology Madras, Chennai 600036, India.
Physical Research Laboratory, Ahmedabad 380009, India.
J Chem Phys. 2024 Sep 28;161(12). doi: 10.1063/5.0226386.
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in space and govern the interstellar chemistry. The two isomers of cyanonaphthalene (1-CNN and 2-CNN) were the first PAHs to be recently identified in the Taurus Molecular Cloud (TMC-1). Their large abundance is attributed to high photostability with nearly no photofragmentation at photon energies above the ionization potential. Here, we show that at ambient light and at densities akin to dense molecular clouds and the upper atmosphere of planets and moons, 1-CNN could undergo extensive fragmentation through a new mechanism leading to daughter cations. On UV photoexcitation, at a photon energy way below the ionization threshold, 1-CNN monomers form photoexcited dimer units. Intermolecular Coulombic decay between the two photoexcited units of the dimer leads to ionization, and the subsequent molecular rearrangements form new daughter cations. These daughter cations could react further, contributing to rich bottom-up astrochemistry, and could play a pivotal role in developmental astrobiology. Photofragmentation in atmospheric and astrophysical environments is hitherto known to be unimolecular, while the present results point a pathway involving bimolecular photofragmentation.
多环芳烃（PAHs）在宇宙空间中无处不在，并主导着星际化学。氰基萘的两种异构体（1 - CNN和2 - CNN）是最近在金牛座分子云（TMC - 1）中首次被发现的PAHs。它们的高丰度归因于高光稳定性，在高于电离势的光子能量下几乎没有光碎片化现象。在此，我们表明，在环境光以及类似于致密分子云、行星和卫星高层大气的密度条件下，1 - CNN可能通过一种导致子阳离子的新机制发生广泛的碎片化。在紫外光激发下，当光子能量远低于电离阈值时，1 - CNN单体形成光激发二聚体单元。二聚体的两个光激发单元之间的分子间库仑衰变导致电离，随后的分子重排形成新的子阳离子。这些子阳离子可能进一步反应，促成丰富的自下而上的天体化学过程，并可能在发展天体生物学中发挥关键作用。迄今为止，大气和天体物理环境中的光碎片化被认为是单分子过程，而目前的结果指出了一条涉及双分子光碎片化的途径。
