Spectroscopic investigation of Al ₂N and its anion via negative ion photoelectron spectroscopy

Negative ion photoelectron spectroscopy was used to elucidate the electronic and geometric structure of the gaseous Al ₂N/Al ₂N ^- molecules, using photodetachment wavelengths of 416 nm (2.977 eV), 355 nm (3.493 eV), and 266 nm (4.661 eV). Three electronic bands are observed and assigned to the X ²Σ ⁺ _u ← X ¹Σ ⁺ _g, A ²∏ _u ← X ¹Σ ⁺ _g, and B ²Σ ⁺ _g ← X ¹Σ ⁺ _g electronic transitions, with the caveat that one or both excited states may be slightly bent. With the aid of density functional theory calculations and Franck-Condon spectral simulations, we determine the adiabatic electron affinity of Al ₂N, 2.571 ± 0.008 eV, along with geometry changes upon photodetachment, vibrational frequencies, and excited-state term energies. Observation of excitation of the odd vibrational levels of the antisymmetric stretch (ν ₃) suggests a breakdown of the Franck-Condon approximation, caused by the vibronic coupling between the X ²Σ ⁺ _u and B ²Σ ⁺ _g electronic states through the ν ₃ mode.