Polarisation of the OH(A 2Σ+,upsilon' = O → X 2PIsub(i), upsilon'' = O) fluorescence produced electron impact on water vapour

We measured the OH(A 2Σ+ → X 2PIsub(i)) fluorescence polarisation in the vibrational 0-0 band system after dissociative excitation of water by electron impact for energies form 11.9 eV up to 500 eV using a setup of crossed electron and H2O beams. The measured polarisations as a function of the electron energy take a course from being mainly unpolarised in the high energy region to negative polarisation values when the energy of the impining electrons approaches threshold. Quantitatively we find -5.2 +- 1.1% at 11.9 eV. A tentative explanation of our results is made and it appears that the measurement of the OH fluorescence polarisation being equivalent to an anisotropic angular distribution of the emitted photons is a sensitive probe of studying the electron impact induced dissociation mechanism of the water molecule. (orig.)