Polarised emission from highly rotational excited OH(A²Σ ⁺) radicals produced by dissociative electron-impact excitation of H₂O

The polarisation of the hydroxyl fragment radiation arising from highly rotational excited OH radicals produced in the electron-impact dissociation of H₂O has been measured for incident energies ranging from just above threshold (9.3 eV) up to 500 eV. Whereas the OH fluorescence is essentially unpolarised for energies greater than about 150 eV, a significantly non-zero polarisation occurs when the impact energy approaches threshold; by monitoring either pure P-branch fluorescence or a mixture of P- and predominantly Q-branch radiation extrapolated threshold polarisations of +7 and -9%, respectively, are obtained. The sign of the polarisation at threshold implies that rapidly rotating (abnormal) hydroxyl radicals arise from a parent H₂O state of symmetry A₁.