Absolute differential electron-impact cross sections of atomic hydrogen: Elastic and n = 2 excitation scattering.

Abstract

By means of a modulated crossed-beam technique, we have measured electron energy-loss spectra for atomic hydrogen. The scattering angles of the measurements were from 12 through 156$\sp\circ$, in 12$\sp\circ$ increments. A variety of electron impact energies in the range 15 through 200 eV were employed. The results of our measurements are differential cross sections for elastic scattering and for excitation of the n = 2 state. Using these, we have computed integrated cross sections for the elastic and $n=2$ excitation scattering processes and momentum-transfer cross sections for the elastic scattering process. The elastic scattering cross sections show agreement with previous measurements for scattering angles less than 90$\sp\circ$, but it is found that the present results exhibit a stronger backward scattering than the previous measurements and theoretical calculations by up to a factor of 3. The $n=2$ excitation scattering cross sections show agreement with the previous measurements of other researchers and with theoretical calculations. The significance of the agreement of the $n=2$ excitation cross sections despite the disagreement of the elastic cross sections is discussed.