NEAR-THRESHOLD PHOTOIONIZATION SPECTRUM OF ALIGNED CA 4S5P1P1

The relative cross section for photoionization from the 4[ital s]5[ital p] [sup 1][ital P][sub 1] 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[sup [minus]1] above the Ca [sup 1][ital S][sub 0] ground state at a resolution of approximately 0.15 cm[sup [minus]1]. The shapes and relative peak heights of resonances corresponding to the previously identified [sup 3][ital S][sub 1] and [sup 3][ital D][sub 1,2] autoionizing levels from the 3[ital d]4[ital d] configuration have been determined. A new, extremely strong resonance is also evident at 51 467 cm[sup [minus]1], and it has been tentatively identified with the 3[ital d]4[ital d] [sup 1][ital P][sub 1] level. This is an experimental identification of a doubly excited even-parity singlet level in calcium.