Theoretical study of predissociation dynamics of HCN/DCN in their first absorption bands

Predissociation dynamics of HCN and DCN in the alpha absorption band is studied using a newly calculated ab initio potential energy surface of the first excited singlet (1 (1)A(')) state and the corresponding transition dipole surface. The recently proposed complex-symmetric single Lanczos propagation method is applied to generate absorption spectra and fragment internal state distributions for the photodissociation of both HCN and DCN. The absorption spectra of both molecules are dominated by bending progressions, thanks to the linear-to-bent transition. For most low-lying resonances, the CN fragment is found predominantly in its ground vibrational state even when significant energy is available. The absence of fragment vibrational excitation is attributed to the coincidence of the C-N distance at the exit barrier and the dissociation asymptote, and to the lack of final-state interaction outside the barrier. On the other hand, the CN rotational distribution is found to be highly oscillatory and depend on the vibrational quanta of the resonance. The exit barrier plays a role in restricting rotational excitation in the CN fragment. The calculated results provide detailed information about the dissociation dynamics. (C) 2002 American Institute of Physics.