Multiphoton spectroscopy of doubly excited autoionizing states of Ca

The autoionizing spectrum of Ca has been studied in the wavelength ranges of 490-540 nm and 555-590 nm through multiphoton absorption and the subsequent decay to excited Ca⁺(4p ²P₃2.1/2/) states. The fluorescence signal of Ca⁺(4p) exhibits resonances attributed to the 3d6p and 3dnf (n=4-7) doubly excited autoionizing J=1, 3 states (three-photon absorption) together with resonances due to excited bound states (two-photon absorption) of the neutral atom. Sufficient atomic density was obtained utilizing a vapour in a moderate pressure regime (2-5 mTorr) such that collisional excitation was still negligible. The pulsed approximately 8 ns duration (FWHM) laser radiation was used in linear as well as in circular polarization. The observed J=3 states has been interpreted with the help of theoretical results obtained by a configuration interaction approach.