VUV Photoionization Study of the Gas Formation of the Indene-A Molecular Building Block of Nonplanar PAHs

Indene is contemplated as the elementary moiety in molecular mass growth processes, leading to nonplanar polycyclic aromatic hydrocarbons (PAHs), such as buckminsterfullerene (C₆₀). Chemical reaction routes involving resonantly stabilized free radicals are recognized as fundamental and efficient pathways to form indene. Exploiting a chemical microreactor in combination with tunable synchrotron photoionization and molecular beam mass spectrometry techniques, the present experiment provides compelling evidence on the formation of indene in the presence of cyclopentadienyl, the simplest C₅-ring molecule, and the unsaturated hydrocarbon vinylacetylene. Theoretical calculations on the potential energy surfaces on C₉H₈ constants reveal that indene can be efficiently produced through two direct addition channels of cyclopentadienyl and vinylacetylene, along with multistep isomerization. RRKM-Master Equation simulations demonstrate that indene formation is kinetically favored at elevated temperatures and reduced pressures. Furthermore, H-assisted isomerization mechanisms on C₉H₈ intermediates are proposed to significantly enhance indene yields under thermally activated conditions. These findings establish a fundamental framework for PAH growth in hydrogen-abundant systems, including combustion flames and interstellar ones, thereby advancing our understanding of the knowledge of PAH propagation and even the formation of carbonaceous nanoparticles in interstellar and terrestrial environments.