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

Abstract

A determination of all the state multipoles of the n=2 excited state of atomic hydrogen has been made. The kinematical conditions were a crossed electron-atom-beam geometry in which the incident electron energy was 350 eV and the scattering angle 3'. 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 correlations 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 (mj=1/2) superposition excited state with averages over the singlet and triplet spin states.