Quantum Manifestation of Roaming in H + MgH → Mg + H2: the Birth of Roaming Resonances

The dynamics of the H + MgH → Mg + H2 reaction at low collision energies is analyzed with both quasi-classical trajectory and quantum wave packet methods on an improved potential-energy surface for the ground electronic state of MgH2. Three microscopic reaction channels, namely, direct abstraction, roaming via a loose roaming transition state, and complex decaying via a tight transition state, are identified. It is shown that the reaction is dominated at low collision energies by the direct abstraction channel, whereas the roaming channel is responsible for about 20% of the reaction flux. The pathway via the tight transition state plays almost no role at the energy of study. The two dominant channels produce similar highly excited vibrational distributions for the H2 product. Finally, it is shown that roaming is manifested quantum-mechanically by a large-amplitude vibration that emerges just below the reaction threshold and is guided by the roaming transition state. Its continuation into the continuum leads to roaming resonances.