Vibrationally mediated photodissociation of ammonia: the influence of N-H stretching vibrations on passage through conical intersections.

Velocity map ion imaging of the H atoms formed in the photodissociation of vibrationally excited ammonia molecules measures the extent of adiabatic and nonadiabatic dissociation for different vibrations in the electronically excited state. Decomposition of molecules with an excited symmetric N-H stretch produces primarily ground state NH(2) along with a H atom. The kinetic energy release distribution is qualitatively similar to the ones from dissociation of ammonia excited to the electronic origin or to several different levels of the bending vibration and umbrella vibration. The situation is very different for electronically excited molecules containing a quantum of antisymmetric N-H stretch. Decomposition from that state produces almost solely electronically excited NH(2), avoiding the conical intersection between the excited state and ground state surfaces. These rotationally resolved measurements agree with our previous inferences from lower resolution Doppler profile measurements. The production of NH(2) suggests that the antisymmetric stretching excitation in the electronically excited molecule carries it away from the conical intersection that other vibrational states access.