Evidencing an elusive conical intersection in the dissociative photoionization of methyl iodide.

The valence-shell dissociative photoionization of methyl iodide (CH 3 I) is studied using double imaging photoelectron photoion coincidence (i 2 PEPICO) spectroscopy in combination with highly-tunable synchrotron radiation from synchrotron SOLEIL. The experimental results are complemented by new high-level ab initio calculations of the potential energy curves of the relevant electronic states of the methyl iodide cation (CH 3 I + ). An elusive conical intersection is found to mediate internal conversion from the initially populated first excited state, CH 3 I + ( Ã 2 A 1 ), into the ground cationic state, leading to the formation of methyl ions (CH 3 + ). The reported threshold photoelectron spectrum for CH 3 + reveals that the ν 5 scissors vibrational mode promotes the access to this conical intersection and hence, the transfer of population. An intramolecular charge transfer takes place simultaneously, prior to dissociation. Upon photoionization into the second excited cationic state, CH 3 I + ( B̃ 2 E ), a predissociative mechanism is shown to lead to the formation of atomic I + .