State-selective predissociation dynamics of methylamines: The vibronic and H∕D effects on the conical intersection dynamics

The photodissociation dynamics of methylamines (CH3NH2 and CD3ND2) on the first electronically excited state has been investigated using the velocity map ion imaging technique probing the H or D fragment. Two distinct velocity components are found in the H(D) translational energy distribution, implying the existence of two different reaction pathways for the bond dissociation. The high H(D) velocity component with the small internal energy of the radical fragment is ascribed to the N–H(D) fragmentation via the coupling of S1 to the upper-lying S2 repulsive potential energy surface along the N–H(D) bond elongation axis. Dissociation on the ground S0 state prepared via the nonadiabatic dynamics at the conical intersection should be responsible for the slow H(D) fragment. Several S1 vibronic states of methylamines including the zero-point level and nν9 states (n=1, 2, or 3) are exclusively chosen in order to explore the effect of the initial quantum content on the chemical reaction dynamics. The branching ra...