Two-color zero kinetic energy-pulsed field ionization spectroscopy of the acetone n-radical cation: The a₂ torsional vibration

Two-color, (2+1′) resonance multiphoton ionization via the ¹A₂ 3p_x Rydberg state is used in combination with high-resolution threshold photoelectron spectroscopy (ZEKE-PFI) to measure the a₂ (ν₁₂) anti-gearing torsional frequencies in the n-radical cation of acetone (-h₆ and -d₆). The photoelectron band intensities and measured torsional frequencies show that the anti-gearing torsional potentials for the cation and the 3p_x Rydberg state are very similar. The cation torsional frequencies also indicate that, despite the localization of positive charge in the carbonyl group, the barrier for synchronous methyl rotation in the cation is higher than that for the neutral acetone ground state.