Vibrational conical intersections as a mechanism of ultrafast vibrational relaxation.

Presenting true crossings of adiabatic potential energy surfaces, conical intersections are a paradigm of ultrafast and efficient electronic relaxation dynamics. The same mechanism is shown to apply also for vibrational conical intersections, which may occur when two high-frequency modes (such as OH stretch vibrations) are coupled to low-frequency modes (such as hydrogen bonding modes). By derivation of a model Hamiltonian and its parametrization for a concrete example, malonaldehyde, the conditions that such conical intersections occur are identified and the consequences for the vibrational dynamics and spectra are demonstrated.