On the Ionization Tolerance of C20 Fullerene in Ground and Excited Electronic States in Planetary Nebulae

Abstract

<p> As the smallest member of the fullerene family, <span><span>C</span><span>20</span></span> is yet to be discovered in planetary nebulae. In this work, we present a quantum chemical study via density functional theory (DFT) and partially by the MP2 on the ionization tolerance of this molecule in the space environment. Considering that the ionization and excitation phenomena play key roles in demonstrating the lifetime of a molecule, we examined both ground and excited electronic-state potential energy surfaces (PES) of <span><span>C</span><span>20</span></span> and its cations <span><span>C</span><span>20</span><span>𝑞</span><span>+</span></span>. Our theoretical results indicate that the <span><span>C</span><span>20</span></span> cage tolerates a positive charge as high as <span><span>13</span><span>+</span></span> by characterizing local minimum geometries on both the abovementioned electronic states. The results are backed by characterizing both <span><span>C</span><span>20</span><span>12</span><span>+</span></span> and <span><span>C</span><span>20</span><span>13</span><span>+</span></span> as local minimum geometries at the MP2 level of computations. We also explored, theoretically and systematically, scenarios in which the electronic structure of neutral <span><span>C</span><span>20</span></span> is excited to very high spin multiplicity (beyond triplet state), and local minimum molecular geometries with cage structures are well characterized. We anticipate that such structural resistance to excitation and ionization delivers a prolonged lifetime necessary for the spectroscopic detection of this interesting molecule and its cations in space and potentially in planetary nebulae (PN) <br></p>