High-resolution photoelectron spectra of cryogenically cooled acetyl anions (CH₃CO^-) obtained using slow photoelectron velocity-map imaging are reported. The high resolution of the photoelectron spectrum yields a refined electron affinity of 0.4352 ± 0.0012 eV for the acetyl radical as well as the observation of a new vibronic structure that is assigned based on ab initio calculations. Three vibrational frequencies of the neutral radical are measured to be 1047 ± 3 cm^-1 (ν₆), 834 ± 2 cm^-1 (ν₇), and 471 ± 1 cm^-1 (ν₈). This work represents the first experimental measurement of the ν₆ frequency of the neutral. The measured electron affinity is used to calculate a refined value of 1641.35 ± 0.42 kJ mol^-1 for the gas-phase acidity of acetaldehyde. Analysis of the photoelectron angular distributions provides insight into the character of the highest occupied molecular orbital of the anion, revealing a molecular orbital with strong d-character. Additionally, details of a new centroiding algorithm based on finite differences, which has the potential to decrease data acquisition times by an order of magnitude at no cost to accuracy, are provided.

High-resolution anion photoelectron spectra of cryogenically cooled NiO₂^- anions, obtained using slow photoelectron velocity-map imaging (cryo-SEVI), are presented in tandem with coupled cluster electronic structure calculations including relativistic effects. The experimental spectra encompass the X̃¹Σ_g⁺ ← X̃²Π_g, ã³Π_g ← X̃²Π_g, and ùΠ_g ← X̃²Π_g photodetachment transitions of linear ONiO^0/-, revealing previously unobserved vibrational structure in all three electronic bands. The high-resolution afforded by cryo-SEVI allows for the extraction of vibrational frequencies for each state, consistent with those previously measured in the ground state and in good agreement with scalar-relativistic coupled-cluster calculations. Previously unobserved vibrational structure is observed in the ã³Π_g and ùΠ_g states and is tentatively assigned. Further, a refined electron affinity of 3.0464(7) eV for NiO₂ is obtained as well as precise term energies for the ã and à states of NiO₂ of 0.3982(7) and 0.7422(10) eV, respectively. Numerous Franck-Condon forbidden transitions involving the doubly degenerate ν₂ bending mode are observed and ascribed to Herzberg-Teller coupling to an excited electronic state.