We report on an H(D)-atom Rydberg tagging experiment for H(D)N(3) photolysis providing detailed dynamical information on the wavelength dependence of the H(D) + N(3) channel. We observe subtle yet striking changes in the photochemical dynamics as the photolysis energy passes through approximately 5.6 eV. In addition to producing linear azide with an average of approximately 40% of available energy appearing as translation, a second H(D)-atom producing channel grows in above this energy releasing only about 15%. An observed (inverse) isotope effect suggests that statistical decomposition on S(0) is unimportant. High level ab initio quantum chemical calculations reveal a transition state to cyclization of the N(3) moiety in H(D)N(3) on the first excited singlet (S(1)) surface that is close in energy to the experimentally observed threshold energy for this "slow channel". Furthermore, the translational energy release of the "slow channel" is energetically consistent with cyclic-N(3) formation. This work provides the clearest presently available insights into how ring closure can occur in azide photochemistry.