The cross section for excitation of Lyman-alpha radiation in electron-hydrogen atom collisions has been measured as a function of electron energy. The measurement is made by observing Lyman-alpha photons with an iodine-vapor-filled photon counter. A relative-cross-section curve is normalized to fit the Born approximation for high electron energies. The experiment utilizes a 100-cps-modulated atomic beam whose purity in hydrogen-atom content is determined by ionization and mass analysis. A dc electron beam crosses the modulated ground-state atomic beam. The photon counter, which looks at the interaction region, has its output passed through a tuned amplifier and phase-sensitive detection system. Although this treatment of the counter output as an ac current (in which the quantum of charge is that of a Geiger-Müller pulse) introduces some unique noise problems, it satisfactorily separates the photons which arise from the interaction under study from those which arise from other processes (e.g., electron collisions with the residual gas in the high-vacuum chamber).

Some implications of the measured excitation cross section upon scattering theory are discussed.

• Received 12 May 1958

DOI:https://doi.org/10.1103/PhysRev.112.1151