Slice imaging of the photodissociation of acetaldehyde at 248 nm.: Evidence of a roaming mechanism

The photodissociation of acetaldehyde in the molecular channel yielding CO and CH4 at 248 nm has been studied, probing different rotational states of the CO(v = 0) fragment by slice ion imaging using a 2+1 REMPI scheme at around 230 nm. From the slice images, clear evidence of the co-existence of two different mechanisms has been obtained. One of the mechanisms is consistent with the well-studied conventional transition state in which CO products appear rotationally excited, and the second is consistent with a roaming mechanism. This roaming mechanism is characterized by a low rotational energy disposal into the CO fragment as well as by a very low kinetic energy release, corresponding to a high internal energy in the CH4 counter-fragment.