Excitation of resonance levels of alkali metal atoms by electron impact

The purpose of the present study was: (1) to obtain most reliable data on effective cross sections of resonance level excitation of alkali metals in the electron energy range E = 0-300 eV and (2) to compare experimental results with calculations in the Born approximation and to see how they agree with available theoretical estimates for small energies. To ensure the purest possible conditions of modelling the excitation of alkali atoms by an electron impact, use was made of the method of intersecting electron and atomic beams. Both beams were formed and intersected at a right angle in a metal vacuum chamber where the residual gas pressure in the operating regime did not exceed 5x10-7 mm Hg. The electron beam intensity was varied in the range from 0.05 to 1 mcA and the energy spread was o.1-0.5 eV. The cross sections for the excitation maximum were 4.9x10-15 cm2 for Li, 8.15x10-15 cm2 for Na, and 14.2x10-15 . cm2 for Cs. For all the atoms a good fit was obtained of the experimental results to those calculated in the first Born approximation, starting from an electron energy of about 20 threshold units, and to those calculated by the strong coupling method for small energies (including excitation maximum)