We demonstrate what is to our knowledge the first atomic optical filter that uses velocity selection to achieve a passband width that is less than the Doppler width of the filtering transition. A narrow-linewidth pump laser is used to induce circular birefringence in a narrow velocity class of atoms in a dense potassium vapor for 694-nm light resonant with the 4P(3/2)-6S(1/2) transition. The filter displays a single 170-MHz passband at a peak transmittance of 9.5%. The bandwidth is an order of magnitude lower than that of previously demonstrated atomic optical filters.

We present results from a joint theoretical and experimental investigation of electronimpact-induced 2S → 2P lithium transition at 7 and 14 eV. The experiment employs the superelastic technique to measure the Stokes parameters P 1 , P 2 and P 3 over the angular range of 0 • to 140 • , yielding statistical errors of only a few per cent. The convergent close-coupling theory obtains complete agreement with the full set of measurements.

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Measurements of the orientation and alignment parameters have been performed on potassium atoms using a superelastic scattering technique. In these experiments a laser beam is tuned to the 4 2 S 1/2 -4 2 P 3/2 transition in potassium. Electrons are scattered from these excited atoms and those electrons that gain energy in the collision are detected. These superelastic electrons carry information about the orientation and alignment parameters which describe the scattering process. Orientation and alignment parameters have been measured at an electron energy of 10 eV referred to the ground state and over an angular range from 0 • to 150 • . These parameters are in excellent agreement with those predicted using the convergent close-coupling theory.

A series of superelastic scattering experiments is described in which electrons are scattered from lithium atoms optically pumped to the 2 P excited state. The incident electron energies were 5.2, 12.2, and 20 eV and superelastically scattered electrons with energies of 7, 14, and 21.8 eV were detected over the angular range from 0°to 140°. Both differential cross sections and the orientation and alignment parameters were deduced from the same measurements. A complete set of the four spin-averaged parameters, which fully characterize the electron impact excitation of the 2S-2 P transition in lithium, is presented. The experimental results are compared with those predicted from convergent-close-coupling ͑CCC͒ calculations and two distorted-wave calculations. Excellent agreement is found with the CCC calculations. ͓S1050-2947͑99͒06406-9͔