Vibrational structure in the vinylidene anion photoelectron spectrum: Closing the gap between theory and experiment

Photoelectron spectra of the vinylidene anion exhibit rich vibrational structure, including several transitions of the neutral that lie well above the barrier for rearrangement to acetylene. Most of these features have been assigned, and are largely consistent with vibrational energy levels estimated by ab initio calculations. Notable exceptions include the 602 and 611 vibronic transitions which involve the CH2 rocking coordinate that ultimately leads to isomerization. Very poor agreement between theory and experiment has been attributed to a neglect of vibrational anharmonicity in the former, a hypothesis that has not been verified computationally. This issue forms the focus of the present work, in which high-level calculations and an explicit determination of the anharmonic force fields of both the anion and neutral are used to predict the vibrational energy levels. Excellent agreement is achieved for both 602 and 611, as both calculated values fall within the experimental error bars.