The photodissociation dynamics of chlorine azide (ClN3) at the 157 nm region was studied theoretically using the multireference configuration interaction method and the complete active space self-consistent field direct dynamics method. The excitation at the 157 nm region was assigned to the 4 A1 ′ (S7) X̃ A1 ′ (S0) transition. A likely pathway for the formation of cyclic CN3 after this transition was identified by direct dynamics as follows: ClN3 excited to 4 A1 ′ (S7) dissociates after about 40 fs to excited CN3 (2 A2 ′, with about 44 kcal/mol internal energy) +Cl (P2). This vibrationally hot CN 3 (2 A2 ′) goes diabatically through a conical intersection with CN3 (1 A2 ′) at 44 fs onto 1 A2 ′. At 19 fs later and repeatedly after every 55 fs, CN3 (1 A2 ′) crosses and trickles down via Coriolis coupling to CN3 (2 A2 ″ / B2 1) state, which has a potential minimum at the cyclic- CN3 structure. Some fraction of CN3 (2 A2 ″ / B2 1) produced will survive dissociation and will be found as the cyclic CN3, and some other fraction will eventually dissociate to N (D2) + N2 over a high barrier found previously. © 2008 American Institute of Physics.