Formation of c-C5H7+ and C5H9+ through sequential reactions of C2H5+ with C2H4.

Polycyclic aromatic hydrocarbons (PAHs) have been the subject of extensive investigation by astrochemists over the past two decades. This interest has been spurred by recent observations of multiple PAH species throughout the interstellar medium. Although many PAHs have been observed, the process by which these molecules formed is not yet fully understood. Two categories of mechanisms have been proposed to explain the production of PAHs, "top-down" and "bottom-up." Top-down mechanisms describe PAH formation through fragmentation of carbonaceous particles by high-energy particles and photons, whereas bottom-up mechanisms describe PAH formation through sequential reactions of smaller molecules that progressively increase in molecular complexity. One proposed bottom-up pathway to PAHs is through the formation of c-C5H7+; however, bottom-up pathways to c-C5H7+ are not yet known. Here, we demonstrate efficient, barrierless pathways to c-C5X7+ and C5X9+ (X = H, D) through the sequential ion-molecule reactions beginning with C2X5+ with C2X4 under interstellar conditions.