Coherent excitation of Ar 4s'[1/2]10and 4s[3/2]10by low energy electrons

Electron-photon coincidence measurements are reported of the excitation of the 4s'[ 1 / 2 ] 1 0 state of Ar at incident energies of 20 and 30 eV and scattering angles up to 80 degrees . All values of the rho 00 parameter obtained are very small and not distinguishable from zero within experimental uncertainty. The level of agreement with available perturbative theories is quite good but significant discrepancies are noted at some angles particularly at the lower impact energy. Previous measurements from this laboratory of the 4s[ 3 / 2 ] 1 0 state are reanalyzed in terms of the ratio of the exchange to direct excitation cross sections.