Abstract
The excitation of the 2P and 2S states of atomic hydrogen by electron impact has been studied by observing separately the prompt ( approximately 10-9 s) Lyman- alpha photons from the decay of the 2P state, the delayed Lyman- alpha photons from the quenched 2S state and the 10.2 eV energy-loss scattered electrons. Observations of these channels indicate the presence of at least four resonant states at 11.74, 11.85, 11.94 and 12.05 eV (+or-0.080 eV). These energies agree within the experimental error with the positions of the 1S, 1D, 1P and 1S(3P) resonance states predicted by six-state close-coupling theory. Over the first 0.5 eV above threshold, the measured values of the absolute cross section for separate excitation of the 2S and 2P states agree, within the experimental errors of +17% and -15%, with the three-state (1s,2s,2p) plus twenty-correlation-term close-coupling predictions of Taylor and Burke (1967). At incident electron energies of 13.87, 16.46 and 19.58 eV the first low-energy measurements are reported of the differential cross sections for exciting the n=2 states.