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 S-1 to the upper-lying S-2 repulsive potential energy surface along the N-H(D) bond elongation axis. Dissociation on the ground S-0 state prepared via the nonadiabatic dynamics at the conical intersection should be responsible for the slow H(D) fragment. Several S-1 vibronic states of methylamines including the zero-point level and n nu(9) 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 ratio of the fast and slow components is found to be sensitive to the initial vibronic state for the N-H bond dissociation of CH3NH2, whereas it is little affected in the N-D dissociation event of CD3ND2. The fast component is found to be more dominant in the translational distribution of D from CD3ND2 than it is in that of H from CH3NH2. The experimental result is discussed with a plausible mechanism of the conical intersection dynamics. (C) 2008 American Institute of Physics.