Abstract
Two-photon ionization of ground state calcium is investigated both experimentally and theoretically in the 374 - 323 nm spectral range. The spectra are dominated by the presence of the 4p(2) S-1(0) perturber and by members of even parity 3dnd J = 0, 2 (4 <= n <= 10) and 3dns J = 2 (6 <= n <= 12) autoionizing Rydberg series, the latter appearing as window resonances. No single-photon resonances are observed. The majority of recorded lines are identified using older stepwise spectroscopic data while there is a number of newly discovered low-lying members. The two previously unobserved 3dns, n <= 12, series members are treated using a semi-empirical multichannel quantum defect theory (MQDT) model. Moreover, the low-lying part of the observed predominantly singlet spectrum is fairly well reproduced by theoretical calculations, combining a configuration interaction approach with B-splines basis functions and neglecting relativistic effects. The agreement between theory and experiment becomes less satisfactory as we move towards higher lying resonances dominated by singlet-triplet mixing effects, which are evident in the experimental data while neglected in the calculations.