The predissociation dynamics of ammonia: A theoretical study

A quantum mechanical model of the ammonia predissociation dynamics is presented. The effects of an excited state barrier and well, a deep conical intersection between the ground and excited states, and strong angular dependence of the potential energy surfaces are explored. The predissociation rate is sensitive to both the vibrational (v′₂) and the rotational (J′,K′) level of the initially excited metastable state. The product state distribution following excitation within the v′₂ = 0 band reflects the shape of the ground bend resonance and is broad, extending to the energetic limit. The photofragment angular distributions following excitation of magnetic-rovibronic state-selected parent molecules depend sensitively on the photon frequency. Their structure varies markedly with the internal state of the accompanying NH₂ fragment. The results are traced to the complex excitation dynamics, which prepare a frequency-dependent superposition of rotational states, and to the strong forces exerted on the dissociating system in the region of the conical intersection.