Photofragment imaging and electronic spectroscopy of Al₂

A combination of photodissociation spectroscopy, ion imaging, and high-level theory is employed to refine the bond strength of the aluminum dimer cation (Al₂⁺) and elucidate the electronic structure and photodissociation dynamics between 38 500 and 42 000 cm^-1. Above 40 400 cm^-1, structured photodissociation is observed from an extremely anharmonic excited state, which calculations identify as the double minimum G ²Σ⁺_u state. The photodissociation spectrum of the G ²Σ⁺_u ← X ²Σ⁺_g transition in Al₂⁺ gives an average vibrational spacing of 170 cm^-1 for the G ²Σ⁺_u state and ν₀ = 172 cm^-1 for the ground state. Photofragment images of G ²Σ⁺_u ← X ²Σ⁺_g transitions indicate that once the Al (⁴P) + Al⁺ (¹S) product channel is energetically accessible, it dominates the lower energy, spin-allowed pathways despite being spin-forbidden. This is explained by a proposed competition between radiative and non-radiative decay pathways from the G ²Σ⁺_u state. The photofragment images also yield D₀ (Al⁺-Al) = 136.6 ± 1.8 kJ/mol, the most precise measurement to date, highlighting the improved resolution achieved from imaging at near-threshold energies. Additionally, combining D₀ (Al⁺-Al) with IE (Al) and IE (Al₂) gives an improved neutral D₀ (Al-Al) = 136.9 ± 1.8 kJ/mol.