Excitation-ionization of the calcium atom by electron impact

Double differential cross sections, measured using an electron-photon coincidence method, are reported for the 4p(2)P(3/2) excited state of Ca+ produced by electron interaction with the 4s(2) S-1 ground state of the calcium atom. The cross section at an incident electron energy of 400 eV is dominated by indirect processes involving 3p excitation to states lying 31.4-31.6 eV above the atomic ground state. These then autoionize to the observed 4p P-2(3/2) ion state. Other indirect processes involving states with binding energies in the range 27-41 eV, including triply excited states, are also observed. Preliminary measurements of the linear Stokes parameters at the peak of the main resonance give P-1 = -0.09 +/- 0.01 and P-2 = 0.01 +/- 0.01.