Stokes Parameter Measurements of Fluorescence from N$_2$, N$_2^+$, and NI Resulting from Polarized-Electron Impact on N$_2$

We have measured the Stokes parameters of the N$_2^+$ B $^2\Sigma_u^+$ $\rightarrow$ X $^2\Sigma_g^+$ 391.4 nm transition, the N$_2$ C $^3\Pi_u$ $\rightarrow$ B $^3\Pi_g$ 380.5 nm transition, and the NI ($^3$P)3p $\rightarrow$ ($^3$P)3s 824.2 nm transition, all resulting from spin-polarized electron impact on N$_2$ gas. Incident electron energies ranged from threshold to 100 eV; the scattered electrons were not detected. We find the circular polarization fraction P$_3$ for light emitted in the direction of the incident electron spin polarization is consistent with zero in the region within 10 eV above threshold for these transitions. This stands in contrast to the linear polarization fraction P$_1$, which is $\sim$0.05 or greater over a broad range of energies in all three cases. These results are considered in light of the fact that H$_2$ molecular triplet transitions [1] exhibit large values of P$_3$ as do the excited atomic fragments resulting from electron-impact induced dissociation [1,2]. [1] A.S. Green, G.A. Gallup, M.A. Rosenberry, and T.J. Gay, Phys. Rev. Lett. \textbf{92}, 093201 (2004). [2] J.F. Williams and D.H. Yu, Phys. Rev. Lett. \textbf{93}, 073201 (2004).