Theoretical and experimental study of the extreme ultraviolet photoabsorption spectrum of triply ionized yttrium

The photoabsorption spectrum of Kr-like Y3+ ions in the very soft x-ray/extreme ultraviolet spectral range where single (4s-np) and double (4p4p-nln'l') electron excitations dominate has been measured using a dual laser plasma photoabsorption technique. The corresponding photoionization cross section has been computed within the framework of the configuration interaction Pauli-Fock approach. The calculations confirm that the 4s-5p resonance drops below the 4p ionization threshold so that the first member of the 4s-np photonization resonance series becomes 4s-6p. Also, whereas the first such series member in Rb+ and Sr2+ appears as a window resonance, in Y3+ it switches to a normal, slightly asymmetric absorption peak. In addition, although the 4s-5p(1/2) resonances in the 4p(1/2)(5) and 4p(3/2)(5) continua are mirror images in Rb+ and Sr2+ and hence cancel each other, they exhibit quite different shapes in Y3+ and hence we observe the residual signature of the 4s-6p(1/2) resonance as a weak partner to the main 4s-6p(3/2) resonance.