A somewhat special role is played by tetrahedral systems like CH 4 and P 4. Here threefold degenerate electronic states and vibrational modes come into play. Nevertheless, also twofold degeneracies continue to be important. The dynamics of Jahn-Teller distortion in CH 4 has been characterized by transient X-ray absorption spectroscopy, revealing that symmetry breaking occurs within ten femtoseconds in this prototypical system.