Electron-photon polarization correlation studies in neon and argon

We studied the electron impact excitation of the (²P_1/2) ns [1/2]_J=1 states in Ne (n=3) and Ar (n=4) using the electron-photon polarization correlation technique. The two linear Stokes' parameters P₁ and P₂ have been measured in Ne at an impact energy of 50 eV and in Ar at 30 eV and 40 eV for electron scattering angles up to 40° (up to 50° in Ar at 40 eV). In addition, previous measurements in Ar at 50 eV have been extended to scattering angles of 40° in an effort to facilitate a more meaningful and broader comparison of the Ar results obtained at different impact energies. Two of the four independent parameters required for a complete description of the excitation process, the alignment angle γ and the linear polarization P_lin⁺ of the angular part of the collisionally induced excited state charge cloud were extracted from the P₁ and P₂ measurements. A comparison with theoretical predictions from first-order perturbative theories such as a distorted-wave Born approximation (DWBA) and a first-order many-body theory (FOMBT) revealed a generally good agreement between experiment and theory in Ne and Ar at 50 eV except for some discrepancies at scattering angles above 30°, whereas the agreement between experiment and theory was much less satisfactory in Ar at 30 eV and at 40 eV over the entire range of scattering angles studies in this work. We found that the measured coherence parameters appear to be shifted systematically towards larger scattering angles compared to the theoretical predictions as the impact energy is decreased.