Spin Torque on Molecular Rotation Induced by Polarized Electrons

We have measured the linear and circular polarization of $391.4$ nm N$_{2}^{\:+}$ $B\:^{2}\Sigma_{u}^{+}(v'=0)$~$\rightarrow$~$X\:^{2}\Sigma_{g}^{+}(v''=0)$ $P$-branch fluorescence produced by spin-polarized electron impact excitation of N$_{2}$ $X\:^{1}\Sigma_{g}^{+}(v=0)$ ground states. We find the linear polarization to be ${\sim}0.03$ for an energy range of $20$ eV above threshold. Of particular note, however, is that the resulting non-zero values of the spin-normalized circular polarization (energy-averaged value of P$_3$/P$_e=0.0133(8)$) indicate that spin-rotation coupling, in the absence of well defined electron orbital angular momentum $(\Lambda=0)$, acts to produce oriented rotational angular momenta in the excited molecular state prior to decay.