Electron excitation of the (0,0) second positive band of nitrogen from threshold to 1000 eV

Abstract

The excitation of the 2P G (0,0) band of nitrogen (λ3371) by electron impact was studied from threshold (11 eV) to 1000 eV. In view of the serious discrepancies in the existing cross section measurements for this band at higher energies (> 30 eV), the effects due to high pressures and secondary electrons were carefully studied. Secondary electron effects were monitored by scanning the luminosity profiles in the collision chamber with a moveable optical detector assembly. The signal count rates were also monitored as a function of pressure. As a result of these checks, the present measurements were carried out at pressures in the 10−5 torr range. In order to increase the signal count rates at these pressures and high electron energies, the collision chamber was given the shape of an ellipsoid with highly polished interior surfaces. The electron gun was of the electrostatically focussed type. The signal count rates were compared with those from the 1N G (0,0) band at λ3914, for which the cross section is well established. As a result, it was found that the cross section for the 2P G (0,0) band decreases rapidly with increasing energy, but not rapidly enough to yield the expected E−3 dependence. The cross section has a value of 1.3×10−19 cm2 at 100 eV, which is about an order of magnitude smaller than most existing measurements.