Threshold Alignment Reversal and Circularly Polarized Fluorescence in Rotationally ResolvedH2

Using H2 and D2 targets, we have measured the polarization of Fulcher-band fluorescence resulting from spin-polarized electron-impact excitation of vibrationally and rotationally resolved d 3Π(u) → a 3Σ(g)+ transitions for incident electron energies from 14.3 to 28.5 eV. Near threshold, the linear polarization P1 descends from positive values through zero to negative values, indicating a dynamic production of M(N)=0 states. The circular polarization P3 is measured to be nonzero, indicating the orientation of rotationally resolved molecular states. For Q-branch transitions, P3 is consistent with theory based on Hund's case (b) coupling. The R-branch P3 values do not agree with theory equally well, indicating the effect of Σ-symmetry perturbation of the parent d 3Π(u)+ state.