Dissociative Photoionisation of Methane and its Deuterated Compounds in theAState Region

Dissociative photoionisation of methane is studied in a wide range of internal energy (12.6–28 eV photon energy) by a threshold electron–ion coincidence technique. Whereas the CH 4 + X ground state is statistical, the A (2 a 1 ) -1 state (22.4–25 eV) and the doubly excited states (26–32 eV) show direct dissociation mechanisms and inefficient internal conversion to the CH 4 + ground state. In the Franck–Condon gap between the X and A states, we observe a 1.7 eV shift in the sequential dissociation of CH 3 + into CH 2 + + H. This is interpreted as the result of dissociation of Rydberg states into neutrals followed by resonant autoionisation of the CH 3 ** fragment into a stable excited electronic state of CH 3 + .