Based on the potential energy surface study of HNCO photolysis, we deduced the formula to compute the state-to-state accumulated transition probability at the crossing point of different energy potential surfaces. We computed their transition probability for the two kinds of transition S_1 → T_1 and S_1 → S_0 respectively at the energy minimum crossing points S_1/T_1 and S_1/S_0. The results indicate that their orders of magnitude are considerably high. Therefore, the molecule can transfer from the higher energy electronic state to the lower state directly at the surface crossing point. According to the theory of monomolecular microcanonical variational theory, we calculated the photolysis rate corresponding to the state S_1 and T_1 and the reaction rate of isomerizing from trans-form to cis-form of HNCO (S_1) under varying wavelength. The results indicate that at low energy the main reaction pathway of the photolysis reaction is that the ground state of HNCO molecule is excited to the Frank-Condon region after absorbing photons. Then through relaxation along the minimum pathway of S_1, the molecule ultimately falls on the trans-equilibrium geometry of S_1. Aftwards, it passes through the minimum energy crossing S_1/T_1 and becomes the first triplet excited state (T_1). Then through relaxation again the molecule falls on the equilibrium geometry of T_1. Finally the reaction is complete along the T_1 IRC, reacting (X~3∑~-) and (X~1∑~+). This result is consistent with the experiment.

Key words: PHOTOLYSIS, ISOCYANIC ACID, REACTION RATE, TRANSITION PROBABILITY