Probing the potential energy surface by high-resolution x-ray absorption spectrum: The umbrella motion of the core-excited CH3 free radical

A detailed study of the umbrella-like vibration in inner-shell spectroscopy is presented. The high-resolution X-ray absorption spectrum (XAS) for the lowest lying core excitation of the CH$_3$ free radical was recorded. High quality potential energy surfaces (PES) for the initial and final states of the transition were calculated as a function of the symmetrical stretching and the umbrella deformation coordinates. The strong anharmonicity along the umbrella coordinate in the double-well region of the PES of the core excited state has a strong effect on the bending vibrational progressions. The excellent agreement between the experiment and theory allows an accurate spectroscopic characterization of the vibrational structure of the electronic transition, and the estimation of the umbrella inversion-time of 148 fs.