Near-threshold photoionization spectrum of aligned Ca 4s5p 1P1

The relative cross section for photoionization from the 4s5p 1P1 excited level of neutral calcium has been measured using pulsed laser excitation and ionization of Ca atoms in an atomic beam. Photoelectrons are counted as a function of ionizing photon energy and polarization. Measurements cover the energy range from 51 300 to 51 700 cm-1 above the Ca 1S0 ground state at a resolution of approximately 0.15 cm-1. The shapes and relative peak heights of resonances corresponding to the previously identified 3S1 and 3D1,2 autoionizing levels from the 3d4d configuration have been determined. A new, extremely strong resonance is also evident at 51 467 cm-1, and it has been tentatively identified with the 3d4d 1P1 level. This is an experimental identification of a doubly excited even-parity singlet level in calcium