HIGH-RESOLUTION PHOTOABSORPTION MEASUREMENT AND MULTICHANNEL QUANTUM-DEFECT-THEORY ANALYSIS OF THE 2P(5)3S((1)P(1)) NS,ND AUTOIONIZING SERIES OF SODIUM

We measured the photoabsorption spectrum of sodium in the 2p subshell excitation region using synchrotron radiation and a 3-m spectrograph a t a resolution of +/-0.008 angstrom. In spite of the large number of o verlapping structure due to the four ionization limits 2p(5)3s(P-3(210 ) and P-1(1)), well separated autoionizing Rydberg series are detected converging to the 2p(5)3s(P-1(1)) threshold. A three-channel quantum- defect-theory analysis is performed for the 2p(5)3s(P-1(1)ns and 2p(5) 3s(P-1(1)nd autoionizing resonances. A simple n-channel multichannel q uantum-defect-theory formula in terms of cofactors is presented for th e calculation of the photoionization cross section for one open and (n -1) bound channels.