GEC Student Award for Excellence Finalist: Fluorescence Polarization of H$_{2}$, D$_{2}$, and N$_{2}$ Molecules Excited by Polarized Electron Impact

We report relative Stokes parameters for both molecular and dissociated atomic transitions in H$_{2}$, D$_{2}$, and N$_{2}$ excited by spin-polarized electrons. In the case of molecular transitions, we find that values of the normalized circular polarization fraction, P$_{3}$/P$_{e}$, for light emitted in the direction of electron spin polarization are non-zero for H$_{2}$ and D$_{2}$, but consistent with zero for N$_{2}$. Specifically, we have succeeded in rotationally isolating the H$_{2}\,d\,{ }^3\Pi _u \to a\,{ }^3\Sigma _g^+ $Q(1) 601.8 nm transition. It exhibits relatively large near-threshold values for P$_{3}$/P$_{e}$ ($\sim $17{\%}). This is in contrast to the null measurements for the N$_{2}\,C\,{ }^3\Pi _u \to B\,{ }^3\Pi _g $ 380.5 nm and N$_{2}^{+}\,B\,{ }^2\Sigma _u^+ \to X\,{ }^2\Sigma _g^+ $ 391.4 nm transitions. For the case of fluorescence from dissociated atomic fragments we observe non-zero circular polarization values for H, D, and N. The near-threshold P$_{3}$/P$_{e}$ values observed for the Balmer-alpha H and D 656.3 nm transitions are nearly equivalent ($\sim $8{\%}). For the N$\,({ }^3P)3p\to ({ }^3P)3s$ 824.2 nm transition we find P$_{3}$/P$_{e}$ values of approximately -7{\%}. This surprising negative sign indicates that the atomic fragment is spinning the ``wrong way,'' i.e. in a direction opposite that of the incident electron responsible for the molecular dissociation.