The state multipoles of the n = 2 state of atomic hydrogen

A determination of all the state multipoles of the n = 2 excited states of atomic hydrogen has been made. The experimental technique measures the scattered 10.2 eV energy loss electrons in coincidence with the 10.2 eV photons from either the 2P state or the delayed Stark-quenched 2S-state decay. An external electrostatic field mixes the S and P states, permitting the observation of their coherence by measuring the linear and circular photon polarisations, while in-plane angular correlation for a zero electric field determine the P-state multipoles. The state multipoles were deduced from a least-squares fit of the measured data to the quantum mechanical expressions determined for a model of an S-P(msub(i) = 1/2) superposition excited state with averages over the singlet and triplet spin states. The λ, R and I parameters as well as the 2S and 2P differential cross sections, all determined in a zero external field, are in good agreement with the pseudostate theory of previous authors. The multipole, a partial measure of the coherence between the S and P levels, has also been determined with fair accuracy. The remaining state multipoles, determined in electric fields, have large experimental uncertainties. (author)