1s-2p excitation in hydrogen has been studied by observing the angular correlation of Lyman a photons detected in coincidence with inelastically scattered electrons at 54.4 eV incident energy. The dectron scattering angles ranged from 10' to 133'; the results at scattering angles larger than 20' cannot be explained by currently available theories,where p(z) is the target atom number density at point z, I,/e is the number of incident electrons passing through the interaction region per second, &, and E"are, respectively, the overall efficiencies for detecting inelastically scattered electrons and photons, and dA, and dQ"are the respective differential solid angles. Furthermore, the angular correlation K is described by If(k, ', Q"Q") =~» [11+SX+ (1 -3X) cos'8" + 6M2R sin(28") cos(Q, -P") -3(1 -X) sin'P"cos2(g,g")],We have recently studied electron-photon coincidences in atomic hydrogen" at small electron scattering angles (8, = 10-20') at incident energies of 40, 54.4, 70, 100, and 200 eV. The salient feature of this communication is that we have now extended the electron scattering angle to 133 for 54.4-eV incident electrons and still obtained a satisfactory coincidence signal-to-noise ratio. Electron excitation of atomic hydrogen presents the simplest inelastic collision process. Nevertheless, theoretical amplitudes for excitation of the 2p level of hydrogen exhibit significant differences for 8, & 20', as will be discussed later.The major objective of this work has, therefore, been to obtain reliable electron-photon angular correlations which can be used to test the validity of the theories at large momentum transfer.Specifically, we have studied the process e+ H(ls)-H(2p)+ e', in which the excited hydrogen atoms (lifetime 1.6 nsec) decay by emission of Lyman n photons (121.6 nm), by detecting the uv photons in coincidence with inelastieally scattered electrons that have suffered a characteristic energy loss of 10.2 eV.For s-p transitions in hydrogen, the electronphoton coincidence count rate is given by' which in our coplanar geometry (Q, -Q"=m) re duces to BK = [SA.+ 4+ 3(1 -2X) cos'8"-Sv 2 R sin28"] .(4)From the experimental angular correlations, we can therefore determine the important parameters and R =-Re(a, a,)/g, , where g, =(a,a, ) is the differential cross section for excitation of the m, = 0 substate, and g, =(a,a,) for the m, =+1 substates, a, and a, being the corresponding amplitudes, and we define 0~= o, + 20, .The general layout of the apparatus and the techniques used are discussed in detail by Hood et al. '4 In brief, hydrogen atoms were produced in a direct-current electrical discharge' and allowed to effuse through a small aperture into the interaction region with an atom number density of about 10" m '. The divergence of the electron beam was less than 0.5 and the opening angle of the electron analyzer less than 2'. 'The major improvement over the previous apparatus is that we used chevron-mounted multichannel plates to detect the Lyman n photons instead of a channel electron multiplier. The...

The total ionization due to the passage of an electron beam in cesium vapor has be~n measured with a Tate and Smith-type apparatus. The retarding potential difference method was used III the electron gun to obtain energy resolution better than 0.1 eV. The density of Cs atoms was determined from the Taylor and Langmuir formula and also measured with a surface-ionization d:tecto; .. T~e tW? methods gave 0e same density to within ±3% when the apparatus was in thermodynanuc eqUlhbnum wIth the Cs :eservOlr. The cross section for production of Cs+ has been determined from threshold to 100 eV. For energIes above 28 eV the results agree closely with those of McFarland and Kinney. At threshold the slope of the cross-sectio~-vs-energy curve was measured to be 2.7 A.'/eV.

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Absolute experimental cross sections are presented for elastic differential scattering of electrons by molecular hydrogen at impact energies between 100 and 2000 eV and at scattering angles between 5 and 50 degrees . The results are compared with the relative experimental cross sections of previous experiments and with the data of different theoretical calculations. Total elastic cross sections are considered as well. The present absolute molecular hydrogen cross sections, together with the 100 and 200 eV ratio measurements of the differential elastic cross sections of H2 and H of Lloyd et al. (1974) were used to obtain a new set of absolute elastic cross sections for atomic hydrogen. These experimental data are compared with the absolute experimental sections obtained by Williams (1975) and with theory.

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