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

We report measurements of the two linear polarization correlation parameters P, and P2 for excitation of the Ar 4s'[I/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 y and the linear polarization P£n of the angular part of the collisionally induced excited state charge cloud, were extracted from the PL and P2 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. We also carried out P, and P2 measurements for the excitation of the SsC3/2]_r_1 state in Kr at 30 eV and scattering angles up to 55° and extended the previous measurements at 50 eV to 40. The agreement between measured data in Kr and theoretical predictions was found to be very good at both impact energies over the entire range of scattering angles that were studied in the course of this work.