Electron-polarized-photon coincidence study of argon and krypton excitation by electron impact

The authors report measurements of the two linear polarization correlation parameters P 1 and P 2 for excitation of the Ar 4s'(1/2) J=1 state by 25 eV electrons and scattering angles up to 55'. Two of the four independent natural parameters required for a complete description of the excitation process, the alignment angle gamma and the linear polarization P lin + of the angular part of the collisionally induced excited state charge cloud, were extracted from the P 1 and P 2 parameters. A comparison with theoretical predictions and with similar data obtained at impact energies in the range 50 eV to 30 eV support the previously observed discrepancy between measured data and theoretical predictions which increases systematically and gradually with decreasing impact energy. The authors also carried out P 1 and P 2 measurements for the excitation of the 5s(3/2) J=1 state in Kr at 30 eV and scattering angles up to 55' and extended the previous measurements at 50 eV to 40'.