Radiative Lifetime of N2(a 1Πg) and the Formation of Metastable N2(a′ 1Σu−)

A model calculation has been carried out on the intrasystem cascading between the a 1Πg, a′ 1Σu−, and w 1Δu states of N2. Input data to the calculation are spectroscopically determined energy levels and Franck-Condon factors, experimentally determined radiative transition probabilities from each of the three excited states to the ground state, and reasonable estimates of the transition moments between excited states. A set of 21 coupled radiative decay equations is then solved for the populations of the excited state levels. The main conclusions are (1) the a′ 1Σu− metastable state is efficiently populated by intrasystem cascade, (2) the a 1Πg state decays nonexponentially, with an ``apparent'' lifetime shorter than that derived from the oscillator strength for the a 1Πg− X 1Σg+ transition, (3) lifetimes of individual vibrational levels of the a 1Πg state decrease with increasing vibrational quantum number, and (4) decay of a metastable molecular beam is nonexponential and shows an ``apparent'' lifetime which differs from that of the a 1Πg state, due to formation of metastable a′ 1Σu− molecules.