Intersystem crossing in the molecular channel of ammonia photodissociation manipulated by an external laser field

Abstract

Conical intersections (CIs) play an important role in photochemistry, allowing for ultrafast radiationless decay in processes such as photodissociation. In addition to these natural CIs, an external electric field can create light-induced conical intersections (LICIs), as the dipole–field interaction shifts the coupled potential energy surfaces. This work explores the effect of LICIs on the minor molecular (NH + H2) channel of ammonia photodissociation, building on prior work that studied the major radical (NH2 + H) channel. The molecular channel can involve intersystem crossing to the low-lying triplet state. A Floquet Hamiltonian was used to simulate the dynamics of ammonia photodissociation in the presence of an external laser field. A total of 230 000 quasi-classical trajectory surface-hopping calculations were performed with SHARC using recently reported diabatic potential energy matrices, dipole matrices, and spin–orbit coupling matrices, which were fitted using neural networks. With the field off, 0.04% of trajectories with a total energy of 8.4 eV yielded triplet NH. With an electric field strength of 0.02 Ha/ea0, over 0.3% of trajectories yielded triplet NH (an eightfold increase), and this value was even larger for higher field strengths.