Electron-polarised-photon coincidence study of the excitation of the C ¹Π_u state in hydrogen and deuterium

The electron-polarised-photon coincidence technique has been used to study the X¹ Sigma _g⁺ to C¹ Pi _U to X¹ Sigma _g⁺ excitation/de-excitation process in H₂ and D₂. Polarisation correlations for integrated vibrational bands have been measured for electron scattering angles up to 25 degrees and incident electron energies up to 80 eV. It was found that the P(0) parameter displays a pronounced dip to zero at a scattering angle of theta _c-12.5 degrees in H₂, independent of the actual impact energy, and at theta _c=10 degrees in D₂. Characteristic features in the P(¹/₄ pi ) angular dependence are a negative value at a small scattering angle, P( ¹/₄ pi )=-0.36 at theta _c=10 degrees in hydrogen and P(¹/₄ pi )=-0.23 at theta _e=7.5 degrees in deuterium, and a rapid variation of P(¹/₄ pi ) back to zero. The region of the steepest gradient in the P(¹/ ₄ pi ) curve was found to coincide with the dip in P(0). The decline in the level of coherence when going from H₂ to D₂ can be attributed to the more developed rotational envelope in deuterium. An attempt is made to relate the observed systematic trends in the data to the properties of the respective target molecule.