Excitation of the C 3Pi/subu/ state of N2 by electron impact

The lambda=3371 A spectral band of the C 3PI/sub u/→B 3PI/sub g/ (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 3PI/sub u/ 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 3PI/sub u/ state excitation, while the long-lived component is identified as the radiative cascade contribution from higher electronic states, including possibly the metastable E 3Σ+/sub g/ state. When the long-lived cascade contribution is subtracted from the total C 3PI/sub u/ 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 approx.2--380 mtorr