Atomic hydrogen revisited: electron impact excitation to H(2p) at 54 eV

New measurements of the three reduced Stokes parameters for 54.4 eV electron impact excitation of the 2p state of atomic hydrogen support convergent close coupling and propagating exterior complex scaling theoretical values within two standard deviations and over a larger angular range of 2 degrees-140 degrees. than earlier work. Whereas the present measurements and theoretical values of the circular polarization Stokes parameter P-3 agree, they differ by up to about four standard deviations from the measurements by Gradziel and O'Neill ( 2004 J. Phys. B: At. Mol. Opt. Phys. 37 1893) in the vicinity of 30 degrees. electron scattering angle and so do not support their explanation of an increased significance of electron exchange for this excitation process at 54.4 eV. We explore some variations in experimental methods and their implications for the measurements.