The umbrella motion of core-excited CH3 and CD3 methyl radicals

An accurate experimental and theoretical study of the lowest core excitation of CH3 and CD3 methyl radicals is presented. The complex vibrational structure of the lowest band of the x-ray absorption spectrum (XAS) is due to the large variation of the molecular geometry, which is planar in the ground state and pyramidal in the core-excited state. The XAS spectra of the two radicals were recorded at high resolution and assigned by theoretical simulations of the spectra, taking into account the coupling of symmetrical stretching and symmetrical bending (umbrellalike) deformations of the radicals. An excellent agreement between experimental and theoretical spectral profiles allowed us to accurately characterize the vibrational structure of the electronic transition. The similarities, as well as the differences, of the peculiar vibrational progression observed for the two radicals are explained by the strong anharmonicity along the umbrella coordinate and by the isotopic variation, leading to a different probing of the double-well potential energy surface of the core excited state during the nuclear motion.