Excitation of the C 3Πu state of N2 by electron impact

Abstract

The λ=3371 Å spectral band of the C 3Πu→B 3Πg (v′=0, v″=0) second positive system is investigated using time-resolved spectroscopy with electron beam pulse widths of 100 nsec, 260 nsec, and 5 μsec. Lifetime data for the v′=0 vibrational level of the C 3Πu electronic state are obtained for incident electron energies and N2 gas pressures in the range 25–200 eV and 2–380 mtorr, respectively. In the case of the 260 nsec electron pulse, a single radiative decay component is observed which exhibits a dependence upon electron impact energy. However, in the case of a 5 μsec excitation pulse the resulting radiative decay is observed to consist of two decay components. The short-lived component is identified as the primary C 3Πu state excitation, while the long-lived component is identified as the radiative cascade contribution from higher electronic states, including possibly the metastable E 3Σ+g state. When the long-lived cascade contribution is subtracted from the total C 3Πu population, the C state natural radiative lifetime becomes independent of energy. In addition, the lifetime is observed to be independent of N2 gas pressure over the pressure range ∼2–380 mtorr.