Electron-polarised-photon coincidence study of the excitation of the C1Πustate in hydrogen and deuterium

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Abstract
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.