Polarisation and Angular Correlations of Excited P States of Helium and Hydrogen

The techniques associated with angular and polarisation correlations have been used to study the electron impact excitation of the 21 P state of helium and the 2P state of hydrogen. The circular, and linear, polarisations of the 58�4 and 121�6 nm radiation have been measured with a double reflection analyser. The measurements have better statistical accuracy than previous work and, for helium at 81�2 eV and a large scattering angle of 108�, confirm that theoretical values are still outside the statistical accuracy of measurement. In hydrogen, the first complete determination of the elements of the 2P density matrix elements is reported. The parameters fl., Re, Im and the absolute value of 0'(2p) have been measured. No single theory is able to predict the scattering angle dependence above 70�.