Abstract
The 2S and 2P states of atomic hydrogen are excited by 350 eV electrons in the presence of a parallel electric field of 250 V cm-1. The time decay of the Stark-perturbed states has been observed through the detection of the 10.2 eV energy loss scattered electrons in coincidence with the 121.6 nm circularly polarised photons. The time development of the eta 2 Stokes parameter shows well defined Lamb shift beats with a period of 0.7 ns; the sign of eta 2 changes as the sign of the electric field changes. With a negative electric field eta 2 changes from negative to positive values after 3 ns indicating a relative change in the decay of the 2S and 2P states. This behaviour is explained in terms of the time development coefficients and their dependence on the electric field. These measurements are the most sensitive test of scattering theory and are in good agreement with the prediction of van Wyngaarden and Walters (1985) at 350 eV and 3 degrees scattering angle.