Clarifying Photodetachment in Al ₁₃ ^– Anions and Photoionization in Al ₁₃ Neutrals Using Diffusion Monte Carlo

Aluminum clusters (Al_<i>n</i>) represent prototypical superatomic behaviors at <i>n</i> = 13, where their molecular electronic states mimic atomic shells. Specifically, Al₁₃ has been regarded as the "superhalogen archetype", yet its fundamental properties remain controversial. Reported energies for the photodetachment of Al₁₃^- anions and the photoionization of Al₁₃ neutrals vary widely across density functional and correlated wave function methods, and even the underlying structural assignments remain disputed. Here, we employ diffusion Monte Carlo (DMC) with multideterminant trial wave functions to resolve these discrepancies. DMC reproduces the experimental adiabatic detachment energy of Al₁₃^- and reconciles the vertical detachment energy with experiment once vibronic effects are included. For the Al₁₃ neutrals, we show that the measured ionization energy of 6.42 eV corresponds not to the photoionization into the ground state cap cation but to a strongly distorted cation of an oblate isomer, consistent with experimental photoionization mass spectra. These results settle long-standing benchmark controversies, and demonstrate the power of quantum Monte Carlo as a reference method for superatomic Al₁₃ clusters.